Practical Temperature Measurements

Keysight Technologies Practical Temperature Measurements

Application Note Contents Introduction

Introduction 2 The purpose of this application note is with barometric pressure, and the to explore the more common tempera- tube had no scale. Vast improvements The Thermocouple 4 ture measurement techniques, and in- were made in temperature measure- Practical Thermocouple troduce procedures for improving their ment accuracy with the development Measurement 12 accuracy. It will focus on the four most of the Florentine thermometer, which The RTD 21 common temperature transducers: the incorporated sealed construction and thermocouple, the RTD (Resistance a graduated scale. The Thermistor 26 Temperature Detector), the thermistor The IC Sensor 27 and the IC (Integrated Circuit) sensor. In the ensuing decades, many ther- Despite the widespread popularity mometric scales were conceived, all The Measurement System 28 of the thermocouple, it is frequently based on two or more ixed points. Appendix A 31 misused. For this reason, we will One scale, however, wasn’t universally concentrate primarily on thermocouple recognized until the early 1700’s when Appendix B 32 measurement techniques. Gabriel Fahrenheit, a Dutch instru- Thermocouple Hardware 34 ment maker, produced accurate and Bibliography 35 Appendix A contains the empirical repeatable mercury thermometers. For laws of thermocouples which are the the ixed point on the low end of his basis for all derivations used herein. temperature scale, Fahrenheit used a Readers wishing a more thorough mixture of ice water and salt (or discussion of thermocouple theory ammonium chloride). This was the are invited to read reference 3 in the lowest temperature he could repro- Bibliography. duce, and he labeled it “zero degrees.” For the high end of his scale, he chose For those with a speciic thermo-cou- human blood temperature and called it ple application, Appendix B may aid 96 degrees. in choosing the best type of thermocouple. Why 96 and not 100 degrees? Earlier scales had been divided into twelve Throughout this application note we parts. Fahrenheit, in an apparent quest will emphasize the practical consider- for more resolution divided his scale ations of transducer placement, signal into 24, then 48 and eventually 96 conditioning and instrumentation. parts.

Early measuring devices The Fahrenheit scale gained popularity primarily because of the repeatability Galileo is credited with inventing the and quality of the thermometers that 1, 2 thermometer, circa 1592. In an open Fahrenheit built. container illed with colored alcohol, he suspended a long narrow-throated Around 1742, Anders Celsius proposed glass tube, at the upper end of which that the melting point of ice and the was a hollow sphere. When heated, boiling point of water be used for the the air in the sphere expanded and two benchmarks. Celsius selected bubbled through the liquid. Cooling zero degrees as the boiling point and the sphere caused the liquid to move 100 degrees as the melting point. 1 up the tube. Fluctuations in the Later, the end points were reversed temperature of the sphere could then and the centigrade scale was born. In be observed by noting the position 1948 the name was oficially changed of the liquid inside the tube. This to the Celsius scale. “upside-down” thermometer was a poor indicator since the level changed

2 In the early 1800’s William Thomson Thermocouple RTD Thermistor I. C. Sensor (Lord Kelvin), developed a universal thermodynamic scale based upon the coeficient of expansion of an ideal gas. Kelvin established the concept of absolute zero, and his scale remains the standard for modern thermometry. T

The conversion equations for the four Figure 1. Four common temperature measurement sensors modern temperature scales are: °C = 5/9 (°F - 32) °F = 9/5°C + 32 Advantages k = °C + 273.15 °R = °F + 459.67 – Self-powered – Most stable – High output – Most linear – Simple – Most accurate – Fast – Highest output The Rankine Scale (°R) is simply the – Rugged – More linear than – Two-wire ohms – Inexpensive Fahrenheit equivalent of the Kelvin thermocouple measurement scale, and was named after an early – Inexpensive pioneer in the ield of thermodynamics, – Wide variety of physical forms W. J. M. Rankine. Notice the oficial Kelvin scale does not carry a degree – Wide temperature range sign. The units are epressed in “kelvins,” not degrees Kelvin. Disadvantages – Non-linear – Expensive – Non-linear – T<250ºC Reference temperatures – Low voltage – Slow – Limited temperature – Power supply range required We cannot build a temperature divider – Reference required – Current source required – Fragile – Slow as we can a voltage divider, nor can – Least stable we add temperatures as we would – Small resistance – Current source – Self-heating – Least sensitive change required add lengths to measure distance. We – Limited – Four-wire – Self-heating conigurations must rely upon temperatures estab- measurement lished by physical phenomena which are easily observed and consistent in nature. Table 1. ITS-90 ﬁxed points Temperature The International Temperature Scale Element Type K ºC (ITS) is based on such phenomena. Revised in 1990, it establishes seven- (H2) Hydrogen Triple Point 13.8033 K –259.3467 °C teen ixed points and corresponding (Ne) Neon Triple Point 24.5561 K –248.5939 °C temperatures. A sampling is given in (02) Oxygen Triple Point 54.3584 K –218.7916 °C Table 1. (Ar) Argon Triple Point 83.8058 K –189.3442 °C (Hg) Mercury Triple Point 234.315 K –38.8344 °C (H2O) Water Triple Point 273.16 K +0.01 °C (Ga) Gallium Melting Point 302.9146 K 29.7646 °C (In) Indium Freezing Point 429.7485 K 156.5985 °C (Sn) Tin Freezing Point 505.078 K 231.928 °C (Zn) Zinc Freezing Point 692.677 K 419.527 °C (Al) Aluminum Freezing Point 933.473 K 660.323 °C (Ag) Silver Freezing Point 1234.93 K 961.78 °C (Au) Gold Freezing Point 1337.33 K 1064.18 °C

3 Since we have only these ixed tem- The Thermocouple Measuring thermocouple peratures to use as a reference, we voltage must use instruments to interpolate When two wires composed of dis- between them. But accurately interpo- similar metals are joined at both ends lating between these temperatures can We can’t measure the Seebeck volt- and one of the ends is heated, there age directly because we must irst require some fairly exotic transducers, is a continuous current which flows connect a voltmeter to the thermo- many of which are too complicated or in the thermoelectric circuit. Thomas expensive to use in a practical situa- Seebeck made this discovery in 1821 couple, and the voltmeter leads, tion. We shall limit our discussion (Figure 2). themselves, create a new thermoelec- to the four most common temperature tric circuit. transducers: thermocouples, resis- If this circuit is broken at the center, tance-temperature detectors (RTD’s), the net open circuit voltage (the Let’s connect a voltmeter across a thermistors, and integrated circuit Seebeck voltage) is a function of the copper-constantan (Type T) thermo- sensors. junction temperature and the compo- couple and look at the voltage output sition of the two metals (Figure 3). (Figure 4).

All dissimilar metals exhibit this ef- We would like the voltmeter to read fect. The most common combinations only V1, but by connecting the of two metals are listed on page 32 of voltmeter in an attempt to measure this application note, along with their the output of Junction J we have important characteristics. For small 1 changes in temperature the Seebeck created two more metallic junctions: voltage is linearly proportional to J2 and J3. Since J3 is a copper-to- copper junction, it creates no thermal temperature: eAB = αT e.m.f. (V3 = 0) but J2 is a copper-to- Figure 2. The Seebeck effect Where α, the Seebeck coeficient, is constantan junction which will add the constant of proportionality. (For an e.m.f. (V2) in opposition to V1. The real world thermocouples, α is not resultant voltmeter reading V will be constant but varies with tempera- proportional to the temperature dif- ture. This factor is discussed under ference between J1 and J2. This says “Voltage-to-Temperature Conversion” that we can’t ind the temperature at on page 9.) J1 unless we irst ind the temperature of J2.

Figure 3. Seebeck voltage proportional to temperature change

Figure 4. Measuring junction voltage with a DVM

4 The reference junction We use this protracted derivation to The copper-constantan thermocouple emphasize that the ice bath junction shown in Figure 5 is a unique example One way to determine the tempera- output V2 is not zero volts. It is a because the copper wire is the same ture J2 is to physically put the junction function of absolute temperature. metal as the voltmeter terminals. into an ice bath, forcing its tempera- Let’s use an iron-constantan (Type J) ture to be 0°C and establishing J2 as By adding the voltage of the ice point thermocouple instead of the copper- the Reference Junction. Since both reference junction, we have now constantan. The iron wire (Figure 6) voltmeter terminal junctions are now referenced the reading V to 0°C. This increases the number of dissimilar copper-copper, they create no thermal method is very accurate because the metal junctions in the circuit, as both e.m.f. and the reading V on the volt- ice point temperature can be precisely voltmeter terminals become Cu-Fe meter is proportional to the tempera- controlled. The ice point is used by thermocouple junctions. ture difference between J1 and J2. the National Institute of Standards and Technology (NIST) as the fun- Now the voltmeter reading is (See damental reference point for their Figure 5): ~ thermo-couple tables, so we can now V = (V – V ) = α(t – t ) 1 2 J1 J2 look at the NIST tables and directly convert from voltage V to temperature If we specify T in degrees Celsius: T . J1 J1 T (°C) + 273.15 = t (K) J1 J1 then V becomes: V = V – V = α[(T + 273.15) – 1 2 J1 (T + 273.15)] J2 = α(T – T ) = (T – 0) J1 J2 J1 V = αT J1

Figure 5. External reference junction

Figure 6. Iron Constantan couple

5 This circuit will still provide moderate- This is still a rather inconvenient We can do this by irst joining the two ly accurate measurements as long as circuit because we have to connect isothermal blocks (Figure 9b). the voltmeter high and low terminals two thermocouples. Let’s eliminate We haven’t changed the output volt- (J3 and J4) act in opposition (Figure 7). the extra Fe wire in the negative (LO) lead by combining the Cu-Fe junction age V. It is still: (J ) and the Fe-C junction (J ). If both front panel terminals are not at 4 REF V = α(T – T ) the same temperature, there will be an J1 REF error. For more precise measurement, Now we call upon the law of inter- the copper voltmeter leads should be mediate metals (see Appendix A) extended so the copper-to-iron junc- to eliminate the extra junction. This tions are made on an isothermal (same empirical law states that a third metal temperature) block (Figure 8). (in this case, iron) The isothermal block is an electrical insulator but a good heat conductor and it serves to hold J3 and J4 at the same temperature. The absolute block temperature is unimportant because the two Cu-Fe junctions act in opposi- Figure 7. Junction voltage cancellation tion. We still have: V = α(T – T ) J1 REF

Reference circuit The circuit in Figure 8 will give us accurate readings, but it would be nice to eliminate the ice bath if possible.

Let’s replace the ice bath with another isothermal block (Figures 9a and 9b). Figure 8. Removing junctions from DVM terminals The new block is at Reference Tem- perature TREF, and because J3 and J4 are still at the same temperature we can again show that:

V = α(T1 – TREF)

Figure 9a. Eliminating the ice bath

Figure 9b. Joining the isothermal blocks 6 inserted between the two dissimilar A thermistor, whose resistance RT is metals of a thermo-couple junction a function of temperature, provides us will have no effect upon the output with a way to measure the absolute voltage as long as the two junctions temperature of the reference junction. formed by the additional metal are at Junctions J3 and J4 and the thermis- the same temperature (Figure 10). tor are all assumed to be at the same temperature, due to the design of the This is a useful conclusion, as it com- isothermal block. Using a digital multi- pletely eliminates the need for the iron meter (DMM), we simply: (Fe) wire in the LO lead (Figure 11). 1. Measure RT to ind TREF and convert Again V = α(T1 – TREF) where α is TREF to its equivalent the Seebeck coeficient for an Fe-C reference junction voltage, thermocouple. VREF . Figure 10. Law of intermediate metals

Junctions J3 and J4 take the place of 2. Measure V and add VREF the ice bath. These two junctions now to ind V1 and convert V1 to become the reference junction. temperature T . J1 Now we can proceed to the next This procedure is known as sofware logical step: Directly measure the compensation because it relies upon temperature of the isothermal block software in the instrument or a (the reference junction) and use that computer to compensate for the information to compute the unknown effect of the reference junction. The temperature, T (Figure 12). isothermal terminal block temperature J1 sensor can be any device which has a characteristic proportional to absolute temperature: an RTD, a thermistor, or an integrated circuit sensor.

Figure 11. Equivalent circuit

Figure 12. External reference junction - no ice bath

7 It seems logical to ask: If we This is accomplished by using Software compensation is the most already have a device that will the isothermal reference junction for versatile technique we have for mea- measure absolute temperature (like more than one thermocouple ele- suring thermocouples. Many thermo- an RTD or thermistor), why do we ment (Figure 13). A relay scanner couples are connected on the same even bother with a thermocouple that connects the voltmeter to the various block, copper leads are used through- requires reference junction compen- thermocouples in sequence. All of out the scanner, and the technique is sation? The single most important the voltmeter and scanner wires are independent of the types of thermo- answer to this question is that the copper, independent of the type of couples chosen. In addition, when thermistor, the RTD, and the inte- thermocouple chosen. In fact, as using a data acquisition system with grated circuit transducer are only long as we know what each thermo- a built-in zone box, we simply connect useful over a certain temperature couple is, we can mix thermocouple the thermocouple as we would a pair range. Thermocouples, on the other types on the same isothermal junction of test leads. All of the conversions hand, can be used over a range of block (often called a zone box) and are performed by the instrument’s temperatures, and optimized for make the appropriate modiications in software. The one disadvantage is that various atmospheres. They are much software. The junction block tempera- it requires a small amount of addition- more rugged than thermistors, as ture sensor, RT is located at the center al time to calculate the reference junc- evidenced by the fact that thermo- of the block to minimize errors due to tion temperature. For maximum speed couples are often welded to a metal thermal gradients. we can use hardware compensation. part or clamped under a screw. They can be manufactured on the spot, either by soldering or welding. In short, thermocouples are the most versatile temperature transducers available and since the measurement system performs the entire task of reference compensation and software voltage- to-temperature conversion, using a thermocouple becomes as easy as connecting a pair of wires.

Thermocouple measurement becomes especially convenient when we are required to monitor a large number of data points.

Figure 13. Switching multiple thermocouple types

8 Figure 14. Hardware compensation circuit

Hardware compensation Rather than measuring the temperature of the reference junction and computing its equivalent voltage as we did with software compensation, we could insert a battery to cancel the offset voltage of the reference junction. The combination of this hardware compensation voltage and the reference junction voltage is equal to that of a 0°C junction (Figure 14). Figure 15. Practical hardware compensation The compensation voltage, e, is a function of the temperature sensing resistor, RT. The voltage V is now referenced to 0°C, and may be read The advantage of the hardware Table 2. Hardware versus software directly and converted to temperature compensation circuit or electronic ice compensation by using the NIST tables. point reference is that we eliminate Hardware Software the need to compute the reference compensation compensation Another name for this circuit is the temperature. This saves us two electronic ice point reference 6. These computation steps and makes a Fast Requires more circuits are commercially available hardware compensation temperature software for use with any voltmeter and with measurement somewhat faster than a manipulation a wide variety of thermocouples. software compensation measurement. time The major drawback is that a unique However, today's faster microproces- Restricted to one Versatile - ice point reference circuit is usually sors and advanced data aquisition thermocouple type accepts any needed for each individual thermo- designs continue to blur the line per reference junction thermocouple couple type. between the two methods, with soft- Hard to reconigure Easy to ware compensation speeds challeng- - requires hardware reconigure Figure 15 shows a practical ice point ing those of hardware compensation change for new reference circuit that can be used in in practical applications (Table 2). thermocouple type conjunction with a relay scanner to compensate an entire block of thermocouple inpuits. All the thermocouples in the block must be of the same type, but each block of inputs can accomo- date a different thermocouple type by simply changing gain resistors.

9 Voltage-to-temperature conversion We have used hardware and software compensation to synthesize an ice-point reference. Now all we have to do is to read the digital voltmeter and convert the voltage reading to a temperature. Unfortunately, the temperature-versus-voltage relation- ship of a thermocouple is not linear. Output voltages for some popular thermocouples are plotted as a function of temperature in Figure 16. If the slope of the curve (the Seebeck coef- Type Metals icient) is plotted vs. temperature, as + - Figure 17. Seebeck coefﬁcient vs. temperature in Figure 17, it becomes quite obvious E Chromel vs. Constantan that the thermocouple is a non-linear J Iron vs. Constantan device. K Chromel vs. Alumel R Platinum vs. Platinum 13% Rhodium A horizontal line in Figure 17 would S Platinum vs. Platinum 10% Rhodium indicate a constant α, in other words, T Copper vs Constantan a linear device. We notice that the slope of the type K thermocouple approaches a constant over a tem- Figure 16. Thermocouple temperature perature range from 0°C to 1000°C. vs. voltage graph Consequently, the type K can be used with a multiplying voltmeter and an external ice point reference to Table 3. Type E thermocouple obtain a moderately accurate direct Temperatures in ºC (ITS-90) readout of temperature. That is, the temperature display involves only mV .00 .01 .02 .03 .04 .05 .06 .07 .08 .09 .10 mV a scale factor. 0.00 0.00 0.17 0.34 0.51 0.68 0.85 1.02 1.19 1.36 1.53 1.70 0.00 0.10 1.70 1.87 2.04 2.21 2.38 2.55 2.72 2.89 3.06 3.23 3.40 0.10 By examining the variations in 0.20 3.40 3.57 3.74 3.91 4.08 4.25 4.42 4.59 4.76 4.92 5.09 0.20 Seebeck coeficient, we can easily see that using one constant scale 0.30 5.09 5.26 5.43 5.60 5.77 5.94 6.11 6.28 6.45 6.61 6.78 0.30 factor would limit the temperature 0.40 6.78 6.95 7.12 7.29 7.46 7.63 7.79 7.96 8.13 8.30 8.47 0.40 range of the system and restrict the 0.50 8.47 8.64 8.80 8.97 9.14 9.31 9.48 9.64 9.81 9.98 10.15 0.50 system accuracy. Better conversion accuracy can be obtained by reading 0.60 10.l5 10.32 10.48 10.65 10.82 10.99 11.15 11.32 11.49 11.66 11.82 0.60 the voltmeter and consulting the NIST 0.70 11.82 11.99 12.16 12.33 12.49 12.66 12.83 12.99 13.16 13.33 13.50 0.70 Thermocouple Tables4 (NIST Mono- 0.80 13.50 13.66 13.83 14.00 14.16 14.33 14.50 14.66 14.83 15.00 15.16 0.80 graph 175 - see Table 3). 0.90 15.16 15.33 15.50 15.66 15.83 16.00 16.16 16.33 16.49 16.66 16.83 0.90

1.00 16.83 l6.99 17.16 17.32 17.49 17.66 17.82 17.99 18.15 18.32 18.49 1.00 1.10 18.49 18.65 18.82 18.98 19.15 19.31 19.48 19.64 19.81 19.98 20.14 1.10 1.20 20.14 20.31 20.47 20.64 20.80 20.97 21.13 21.30 21.46 21.63 21.79 1.20 1.30 21.79 21.96 22.12 22.29 22.45 22.61 22.78 22.94 23.11 23.27 23.44 1.30 1.40 23.44 23.60 23.77 23.93 24.10 24.26 24.42 24.59 24.75 24.92 25.08 1.40

10 We could store these look-up table Table 4. NIST ITS-90 polynomial coefﬁcients values in a computer, but they would Thermocouple consume an inordinate amount of type Type J Type K memory. A more viable approach is to Temperature range –210 to O °C 0 to 760 ºC –200 to 0 ºC 0 to 500 ºC approximate the table values using a Error range ± 0.05 °C ±0.04 ºC ±0.04 ºC ±0.05 ºC power series polynomial: Polynomial order 8th order 7th order 8th order 9th order t = c + c x + c x2 + c x3 + ... + c xn 90 0 1 2 3 n C0 0 0 0 0 -2 -2 -2 -2 where C2 1.9528268 x 10 1.978425 x 10 2.5173462 x 10 2.508355 x 10 -6 -7 -6 -8 C1 –1.2286185 x 10 –2.001204 x 10 –.1662878 x 10 7.860106 x 10 t90 = Temperature C –1.0752178 x 10-9 1.036969 x 10-11 1.0833638 x 10-9 –2.503131 x 10-10 x = Thermocouple voltage 3 -13 -16 -13 -14 c = Polynomial coeficients unique to C4 –5.9086933 x 10 –2.549687 x 10 –8.9773540 x 10 8.315270 x 10 -16 -21 -16 -17 each thermocouple C5 –1.7256713 x 10 3.585153 x 10 –3.7342377 x 10 –1.228034 x 10 -20 -26 -20 -22 n = Maximum order of the polynomial C6 –2.8131513 x 10 –5.344285 x 10 –8.6632643 x 10 9.804036 x 10 -24 -31 -23 -26 C7 –2.3963370 x 10 5.099890 x 10 –1.0450598 x 10 –4.413030 x 10 As n increases, the accuracy of the C –8.3823321 x 10-29 –5.1920577 x 10-28 –1.057734 x 10-30 polynomial improves. Lower order poly- 8 -35 nomials may be used over a narrow C9 –1.052755 x 10 2 9 temperature range to obtain higher Temperature conversion equation: t90 = c0 + c1x + c2x + . . . + c9x system speed. Table 4 is an example Nested polynomial form (4th order example): t90 = c0 + x(c1 + x(c2 + x(c3 + c4x))) of the polynomials used in conjunction with software compensation for a data acquisition system. Rather than directly calculating the exponentials, Table 5. Required voltmeter sensitivity the software is programmed to use the nested polynomial form to save execu- Seebeck DVM coeficient sensitivity tion time. The poly-nomial it rapidly Thermocouple at 25 °C for 0.1 °C degrades outside the temperature range type (µV/°C) (µV) shown in Table 4 and should not be E 61 6.1 extrapolated outside those limits. J 52 5.2 The calculation of high-order polynomi- K 40 4.0 als is a time consuming task, even for R 6 0.6 today’s high-powered microprocessors. As we mentioned before, we can save S 6 0.6 time by using a lower order polynomial T 41 4.1 Figure 18. Curve divided into sectors for a smaller temperature range. In the software for one data acquisition system, the thermocouple character- The data acquisition system mea- All the foregoing procedures assume istic curve is divided into eight sectors sures the output voltage, categorizes the thermocouple voltage can be and each sector is approximated by a it into one of the eight sectors, and measured accurately and easily; third-order polynomial (Figure 18). chooses the appropriate coeficients however, a quick glance at Table 5 for that sector. This technique is shows us that thermocouple output both faster and more accurate than voltages are very small indeed. the higher-order polynomial. Examine the requirements of the system voltmeter. An even faster algorithm is used in many new data acquisition systems. Using many more sectors and a series of irst order equations, they can make hundreds, even thousands, of internal calculations per second.

11 Even for the common type K ther- Practical Thermocouple Analog ﬁlter - A ilter may be used mocouple, the voltmeter must be directly at the input of a voltmeter to able to resolve 4 µV to detect a 0.1°C Meaurement reduce noise. It reduces interference change. This demands both excellent dramatically, but causes the volt- resolution (the more bits, the better) Noise rejection meter to respond more slowly to step and measurement accuracy from inputs (Figure 20). the DMM. The magnitude of this Tree switching - Tree switching signal is an open invitation for noise Integration - Integration is an A/D is a method of organizing the chan- to creep into any system. For this technique which essentially averages nels of a scanner into groups, each reason instrument designers utilize noise over a full line cycle, thus with its own main switch. several fundamental noise rejection power line-related noise and its techniques, including tree switching, harmonics are virtually eliminated. If Without tree switching, every chan- normal mode iltering, integration the integration period is chosen to nel can contribute noise directly be less than an integer line cycle, and isolation. through its stray capacitance. With its noise rejection properties are es- tree switching, groups of paral- sentially negated. lel channel capacitances are in series with a single tree switch capacitance.

The result is greatly reduced cross- talk in a large data acquisition system, due to the reduced interchannel capacitance (Figure 19).

Figure 19. Tree switching

Figure 20. Analog ﬁlter

12 Since thermocouple circuits that cover long distances are especially susceptible to power line related noise, it is advisable to use an integrating analog-to-digital converter to measure the thermocouple voltage. Integration is an especially attrac- tive A/D technique in light of recent innovations have brought the cost in line with historically less expensive A/D technologies. Figure 21. Isolation minimizes common mode current

Isolation - A noise source that is common to both high and low measurement leads is called common mode noise. Isolated inputs help to reduce this noise as well as protect the measurement system from ground loops and transients (Figure 21).

Let’s assume a thermocouple wire Figure 22. Thermocouple in molten metal Figure 23. Thermocouple shorts to liquid has been pulled through the same bath conduit as a 220 V AC supply line. The capacitance between the power This error is reduced by isolating lines and the thermocouple lines will the input terminals from safety create an AC signal of approximately ground with a careful design that equal magnitude on both thermo- minimizes the low-earth capacitance. couple wires. This is not a problem Non-isolated or ground-referenced in an ideal circuit, but the voltmeter inputs (“single-ended” inputs are is not ideal. It has some capacitance often ground-referenced) don’t have between its low terminal and safety the ability to reject common mode Figure 24. Noise path through thermo- ground (earth). Current lows through noise. Instead, the common mode couple this capacitance and through the current lows through the low lead thermocouple lead resistance, creating directly to ground, causing poten- Isolated inputs reject the noise a normal mode signal which appears tially large reading errors. current by maintaining a high as measurement error. impedance between LO and Earth. Isolated inputs are particularly useful A non-isolated system, represented in eliminating ground loops created in Figure 24, completes the path to when the thermocouple junction earth resulting in a ground loop. comes into direct contact with a The resulting currents can be dan- common mode noise source. gerously high and can be harmful to both instrument and operator. In Figure 22 we want to measure the Isolated inputs are required for temperature at the center of a mol- making measurements with high ten metal bath that is being heated common mode noise. by electric current. The potential at the center of the bath is 120 VRMS. The equivalent circuit is shown in Figure 23.

13 Sometimes having isolated inputs isn’t Practical precautions enough. In Figure 23, the voltmeter inputs are loating on a 120 VRMS We have discussed the concepts common mode noise source. They of the reference junction, how to use must withstand a peak offset of a polynomial to extract absolute tem- ±170 V from ground and still make perature data and what to look for in accurate measurements. An a data acquisition system to mini- isolated system with electronic FET mize the effects of noise. Now let’s look at the thermocouple wire itself. switches typically can only handle Figure 25. Soldering a thermocouple ±12 V of offset from earth; if used in The polynomial curve it relies upon this application, the inputs would be the thermocouple wire being perfect; damaged. that is, it must not become decali- brated during the act of making a Poor junction connection The solution is to use commercially temperature measurement. We shall available external signal conditioning now discuss some of the pitfalls of There are a number of accept- (isolation transformers and amplii- thermocouple thermometry. able ways to connect two thermo- ers) that buffer the inputs and reject couple wires: soldering, silver-solder- the common mode voltage. Another Aside from the speciied accuracies ing, welding, etc. When the thermo- easy alternative is to use a data of the data acquisition system and its couple wires are soldered together, acquisition system that can loat isothermal reference junction, most we introduce a third metal into several hundred volts. measurement error may be traced to the thermocouple circuit. As long as one of these primary sources: the temperatures on both sides of Notice that we can also minimize the the thermocouple are the same, the noise by minimizing RS. We do this 1. Poor junction connection solder should not introduce an error. by using larger thermocouple wire The solder does limit the maximum 2. Decalibration of that has a smaller series resistance. temperature to which we can subject Also, to reduce the possibility of thermocouple wire this junction (Figure 25). To reach a magnetically induced noise, the 3. Shunt impedance high measurement temperature, the thermocouple should be twisted in a and galvanic action joint must be welded. But weld uniform manner. Thermocouple exten- ing is not a process to be taken sion wires are available commercially 4. Thermal shunting lightly.5 Overheating can degrade in a twisted pair coniguration. the wire, and the welding gas and 5. Noise and leakage currents the atmosphere in which the wire 6. Thermocouple speciications is welded can both diffuse into the thermocouple metal, changing its 7. Documentation characteristics. The dificulty is com- pounded by the very different nature of the two metals being joined.

14 Commercial thermocouples are Robert Moffat in his Gradient Ap- Thermocouple wire obviously can’t welded on expensive machinery us- proach to Thermocouple Thermom- be manufactured perfectly; there ing a capacitive-discharge technique etry explains that the thermocouple will be some defects which will to insure uniformity. voltage is actually generated by the cause output voltage errors. These section of wire that contains a tem- inhomogeneities can be especially A poor weld can, of course, result in perature gradient, and not neces- disruptive if they occur in a region an open connection, which can be sarily by the junction.9 For example, of steep temperature gradient. detected in a measurement situation if we have a thermal probe located by performing an open thermocouple in a molten metal bath, there will Since we don’t know where an im- check. This is a common test func- be two regions that are virtually perfection will occur within a wire, tion available with many data loggers isothermal and one that has a large the best thing we can do is to avoid and data acquisition systems. gradient. creating a steep gradient. Gradi- ents can be reduced by using me- In Figure 26, the thermocouple junc- tallic sleeving or by careful placement tion will not produce any part of the of the thermocouple wire. Decalibration output voltage. The shaded section Decalibration is a far more serious will be the one producing virtually fault condition than the open ther- the entire thermocouple output volt- mocouple because it can result in age. If, due to aging or annealing, the temperature readings that appears to output of this thermocouple was be correct. Decalibration describes found to be drifting, replacing only the process of unintentionally the thermocouple junction would altering the physical makeup of not solve the problem. We would the thermocouple wire so that it have to replace the entire shaded no longer conforms to the NIST section, since it is the source of the polynomial within speciied limits. thermocouple voltage. Decalibration can result from diffusion of atmospheric particles into the metal, caused by temperature extremes. It can be caused by high temperature annealing or by cold- working the metal, an effect that Figure 26. Gradient produces voltage can occur when the wire is drawn through a conduit or strained by rough handling or vibration. Anneal- ing can occur within the section of wire that undergoes a temperature gradient.

15 Shunt impedance High temperatures can also take their toll on thermocouple wire insulators. Insulation resistance decreases exponentially with increasing temperature, even to the point that it creates a virtual junction. Assume we have a completely open Figure 27. Leakage resistance Figure 28. Virtual junction thermocouple operating at a high temperature (Figure 27).

The leakage resistance, RL can be suficiently low to complete the Galvanic action circuit path and give us an improper The dyes used in some thermo- Extension wire is commercially voltage reading. Now let’s assume couple insulation will form an available wire primarily intended to the thermocouple is not open, but electrolyte in the presence of water. cover long distances between the we are using a very long section of This creates a galvanic action, with measuring thermocouple and the small diameter wire (Figure 28). a resultant output hundreds of times voltmeter. Extension wire is made of greater than the Seebeck effect. metals having Seebeck coeficients If the thermocouple wire is small, its Precautions should be taken to very similar to a particular thermo- series resistance, RS, will be quite shield the thermocouple wires from couple type. It is generally larger high and under extreme conditions all harsh atmospheres and liquids. in size so that its series resistance RL << RS. This means that the ther- does not become a factor when mocouple junction will appear to be traversing long distances. It can at R and the output will be propor- L Thermal shunting also be pulled more readily through tional to T1, not T2. conduit than very small thermocouple No thermocouple can be made wire. It generally is speciied over a High temperatures have other detri- without mass. Since it takes energy much lower temperature range than mental effects on thermocouple to heat any mass, the thermocouple premium-grade thermocouple wire. wire. The impurities and chemicals will slightly alter the temperature it In addition to offering a practical within the insulation can actually was meant to measure. If the mass size advantage, extension wire is diffuse into the thermocouple metal to be measured is small, the ther- less expensive than standard ther- causing the temperature-voltage mocouple must naturally be small. mocouple wire. This is especially dependence to deviate from the But a thermocouple made with true in the case of platinum-based published values. When using small wire is far more susceptible thermocouples. thermocouples at high temperatures, to the problems of contamination, the insulation should be chosen annealing, strain, and shunt imped- carefully. Atmospheric effects can ance.7 To minimize these effects, be minimized by choosing the proper thermocouple extension wire can protective metallic or ceramic be used. sheath.

16 Since the extension wire is speci- Wire calibration Since channel numbers invariably ied over a narrower temperature change, data should be categorized range and it is more likely to receive Thermocouple wire is manu- by measurement, not just channel mechanical stress, the temperature factured to a certain speciication number.10 Information about any gradient across the extension wire signifying its conformance with the given measurand, such as transduc- should be kept to a minimum. This, NIST tables. The speciication can er type, output voltage, typical according to the gradient theory, sometimes be enhanced by calibrat- value, and location can be main- assures that virtually none of the ing the wire (testing it at known tained in a data ile. This can be output signal will be affected by the temperatures). Consecutive pieces done under PC control or simply extension wire. of wire on a continuous spool will by illing out a preprinted form. No generally track each other more matter how the data is maintained, Noise - We have already discussed closely than the speciied tolerance, the importance of a concise system the line-related noise as it per- although their output voltages may should not be underestimated, es- tains to the data acquisition system. be slightly removed from the center pecially at the outset of a complex The techniques of integration, of the absolute speciication. data gathering project. tree switching and isolation serve to cancel most line-related inter- If the wire is calibrated in an effort to improve its fundamental speciica- ference. Broadband noise can be Diagnostics rejected with an analog ilter. tions, it becomes even more impera- tive that all of the aforementioned Most of the sources of error that The one type of noise the data conditions be heeded in order to we have mentioned are aggravated acquisition system cannot reject is avoid decalibration. by using the thermocouple near its a DC offset caused by a DC leak- temperature limits. These conditions age current in the system. While it will be encountered infrequently in is less common to see DC leakage Documentation most applications. But what about currents of suficient magnitude to the situation where we are using cause appreciable error, the possibil- It may seem incongruous to speak small thermocouples in a harsh ity of their presence should be noted of documentation as being a source atmosphere at high temperatures? and prevented, especially if the ther- of voltage measurement error, but How can we tell when the thermo- mocouple wire is very small and the the fact is that thermocouple couple is producing erroneous re- related series impedance is high. systems, by their very ease of use, sults? We need to develop a reliable invite a large number of data points. set of diagnostic procedures. The sheer magnitude of the data can become quite unwieldy. When a large Through the use of diagnostic amount of data is taken, there is an techniques, R.P. Reed has developed increased probability of error due to an excellent system for detecting a mislabeling of lines, using the faulty thermocouple and data chan- wrong NIST curve, etc. nels.10 Three components of this system are the event record, the zone box test and the thermocouple resistance history.

17 Event record - The irst diagnostic Zone box test - The zone box is an Thus, for a DVM reading of V = 0, the is not a test at all, but a recording isothermal terminal block with a system will indicate the zone box of all pertinent events that could known temperature used in place of temperature. First we observe even remotely affect the measure- an ice bath reference. If we tempo- the temperature TJ (forced to be ments. rarily short-circuit the thermocouple different from TREF), then we short directly at the zone box, the system the thermocouple with a copper An example is: should read a temperature very wire and make sure that the system close to that of the zone box, i.e., indicates the zone box temperature March 18 event record close to room temperature (Fig- instead of TJ. ure 30). 10:43 Power failure This simple test veriies that the 10:47 System power returned If the thermocouple lead resistance controller, scanner, voltmeter and 11:05 Changed M821 to type K is much greater than the shunting zone box compensation are all op- thermocouple resistance, the copper wire shunt erating correctly. In fact, this simple 13:51 New data acquisition program forces V = 0. In the normal unshort- procedure tests everything but the 16:07 M821 appears to be bad reading ed case, we want to measure TJ, thermocouple wire itself. and the system reads: Figure 29. Sample test event record V = α(TJ – TREF)

We look at our program listing and But, for the functional test, ind that measurand #M821 uses we have shorted the terminals so a type J thermocouple and that that V = 0. The indicated tempera- our new data acquisition program ture T is thus: interprets it as type J. But from the J event record, apparently thermocou- 0 = α(TJ – TREF ) ple #M821 was changed to a type K, and the change was not entered into TJ = TREF the program. While most anomalies are not discovered this easily, the event record can provide valuable insight into the reason for an unex- plained change in a system measurement. This is especially true in a system conigured to measure hundreds of data points.

Figure 30. Shorting the thermocouples at the terminals

18 Thermocouple resistance - A sudden change in the resistance of a thermocouple circuit can act as a warn- ing indicator. If we plot resistance vs. time for each set of thermocouple wires, we can immediately spot a sudden resistance change, which could be an indication of an open wire, a wire shorted due to insulation failure, changes due to vibration fatigue or one of many failure mechanisms. Figure 31. Buring coal seam Figure 32. Thermocouple resistance vs. time For example, assume we have the thermocouple measurement shown in Figure 31. The resistance of the thermocouple will naturally change with time as We want to measure the temper- the resistivity of the wire changes ature proile of an underground due to varying temperatures. But a seam of coal that has been sudden change in resistance is an ignited. The wire passes through indication that something is wrong. a high temperature region, into a In this case, the resistance has Figure 33. Cause of the resistance change cooler region. Suddenly, the tem- dropped abruptly, indicating that perature we measure rises from the insulation has failed, effectively Measuring resistance - We have 300°C to 1200°C. Has the burning shortening the thermocouple loop casually mentioned checking the section of the coal seam migrated (Figure 33). resistance of the thermocouple to a different location, or has the wire, as if it were a straightforward thermocouple insulation failed, thus The new junction will measure tem- measurement. But keep in mind that causing a short circuit between the when the thermocouple is producing perature TS, not T1. The resistance two wires at the point of a hot spot? measurement has given us addi- a voltage, this voltage can cause tional information to help interpret a large resistance measurement If we have a continuous history of the physical phenomenon that has error. Measuring the resistance of a the thermocouple wire resistance, occurred. This failure would not thermocouple is akin to measuring we can deduce what has actually have been detected by a standard the internal resistance of a battery. happened (Figure 32). open-thermocouple check. We can attack this problem with a technique known as offset compensated ohms measurement.

19 As the name implies, the data acqui- – When using long thermocouple sition unit irst measures the ther- wires, use shielded, twisted mocouple offset voltage without the pair extension wire. ohms current source applied. Then the ohms current source is switched – Avoid steep temperature on and the voltage across the gradients. resistance is again measured. The instrument irmware compensates – Try to use the thermocouple for the offset voltage of the ther- wire well within its tempera- mocouple and calculates the actual ture rating. thermocouple source resistance. – Use an integrating A/D converter with high resolution Special thermocouples - Under and good accuracy. extreme conditions, we can even Figure 34. Four-wire thermocouples use diagnostic thermocouple circuit – Use isolated inputs with conigurations. Tip-branched and ample offset capability. leg-branched thermocouples are four-wire thermocouple circuits that Summary – Use the proper sheathing allow redundant measurement of material in hostile envi- In summary, the integrity of a temperature, noise voltage and re- ronments to protect the thermocouple system may be sistance for checking wire integrity thermocouple wire. (Figure 34). Their respective merits improved by following these are discussed in detail in Bibliogra- precautions: – Use extension wire only at phy 8. – Use the largest wire possible low temperatures and only that will not shunt heat away in regions of small gradients. Only severe thermocouple ap- from the measurement area. plications require such extensive – Keep an event log and a diagnostics, but it is comforting to – If small wire is required, use continuous record of know that there are procedures that it only in the region of the thermocouple resistance. can be used to verify the integrity of measurement and use exten- an important thermocouple mea- sion wire for the region with surement. no temperature gradient. – Avoid mechanical stress and vibration, which could strain the wires.

20 The RTD Although this construction produces a very stable element, the thermal contact between the platinum History and the measured point is quite The same year that Seebeck made poor. This results in a slow thermal his discovery about thermoelectricity, response time. The fragility of the Sir Humphrey Davy announced that structure limits its use today primar- the resistivity of metals showed a ily to that of a laboratory standard. marked temperature dependence. Fifty years later, Sir William Siemens Another laboratory standard has proffered the use of platinum as the taken the place of the Meyer’s design. element in a resistance thermom- This is the bird-cage element pro- 16 eter. His choice proved most propi- posed by Evans and Burns. The tious, as platinum is used to this platinum element remains largely Figure 35. Meyers RTD construction day as the primary element in all unsupported, which allows it to high-accuracy resistance thermome- move freely when expanded or con- ters. In fact, the platinum resistance tracted by temperature variations temperature detector, or PRTD, is (Figure 36). used today as an interpolation standard from the triple point of equilib- Strain-induced resistance changes rium hydrogen (–259.3467°C) to the caused by time and temperature freezing point of silver (961.78°C). are thus minimized and the bird-cage becomes the ultimate laboratory Platinum is especially suited to this standard. Due to the unsupported Figure 36. Bird-caged PRTD purpose, as it can withstand high structure and subsequent suscepti- temperatures while maintaining bility to vibration, this coniguration excellent stability. As a noble metal, is still a bit too fragile for industrial it shows limited susceptibility to environments. contamination. A more rugged construction The classical resistance tempera- technique is shown in Figure 37. The ture detector (RTD) construction platinum wire is biilar wound on using platinum was proposed by a glass or ceramic bobbin. The C.H. Meyers in 1932.12 He wound a biilar winding reduces the effective helical coil of platinum on a crossed enclosed area of the coil to mini- mica web and mounted the as- mize magnetic pickup and its related sembly inside a glass tube. This noise. Once the wire is wound onto construction minimized strain on the the bobbin, the assembly is then wire while maximizing resistance sealed with a coating of molten (Figure 35). glass. The sealing process assures that the RTD will maintain its integrity under extreme vibration, but it also limits the expansion of the platinum metal at high temperatures. Unless the coefficients of expansion of the platinum and the bobbin match perfectly, stress will be placed on the wire as the temperature changes, resulting in a strain- induced resistance change. This may result in a permanent change in the resistance of the wire. Figure 37. Ruggedized RTDs

21 There are partially supported ver- size itself is small, which means it Table 6. Resistivity of different metals sions of the RTD which offer a can respond quickly to step changes Resistivity Ω/CMF compromise between the bird-cage in temperature. Film RTD’s are less Metal (cmf = circular mil foot) approach and the sealed helix. One stable than their wire-wound coun- such approach uses a platinum helix terparts, but they are more popular Gold Au 13.00 threaded through a ceramic cylinder because of their decided advantages Silver Ag 8.8 and afixed via glass-frit. These de- in size, production cost and Copper Cu 9.26 vices will maintain excellent stability ruggedness. in moderately rugged vibrational Platinum Pt 59.00 applications. Metals - All metals produce a Tungsten W 30.00 positive change in resistance for Nickel Ni 36.00 a positive change in temperature. Metal ilm RTD’s This, of course, is the main function of an RTD. As we shall soon see, Copper is used occasionally as an In the newest construction tech- system error is minimized when the RTD element. Its low resistivity nique, a platinum or metal-glass nominal value of the RTD resistance forces the element to be longer than slurry ilm is deposited or screened is large. This implies a metal wire a platinum element, but its linear- onto a small lat ceramic substrate, with a high resistivity. The lower ity and very low cost make it an etched with a laser-trimming sys- the resistivity of the metal, the more economical alternative. Its upper tem, and sealed. The ilm RTD offers material we will have to use. temperature limit is only about substantial reduction in assembly 120°C. time and has the further advantage Table 6 lists the resistivities of com- of increased resistance for a given mon RTD materials. The most common RTD’s are made size. Due to the manufacturing of either platinum, nickel, or nickel technology, the device Because of their lower resistivities, alloys. The economical nickel de- gold and silver are rarely used as rivative wires are used over a limited RTD elements. Tungsten has a rela- temperature range. They are quite tively high resistivity, but is reserved non-linear and tend to drift with for very high temperature applica- time. For measurement integrity, tions because it is extremely brittle platinum is the obvious choice. and dificult to work.

22 Resistance measurement The common values of resistance for a platinum RTD range from 10 ohms for the bird-cage model to several thousand ohms for the ilm RTD. The single most common value is 100 ohms at 0°C. The DIN 43760 standard temperature coeficient of platinum wire is α = .00385. For a Figure 38. Effect of load resistance Figure 39. Wheatstone bridge 100 ohm wire this corresponds to +0.385 Ω/°C at 0°C. This value for α is actually the average slope from 0°C to 100°C. The more chemically pure platinum wire used in platinum resistance standards has an α of +.00392 ohms/ohm/°C.

Both the slope and the absolute value are small numbers, especially when we consider the fact that the measurement wires leading to the Figure 40. RTD separated by extension Figure 41. 3-wire bridge sensor may be several ohms or even wires tens of ohms. A small lead imped- The bridge output voltage is an If wires A and B are perfectly ance can contribute a signiicant indirect indication of the RTD resis- matched in length, their impedance error to our temperature measure- tance. The bridge requires four con- effects will cancel because each is ment (Figure 38). nection wires, an external source, in an opposite leg of the bridge. The and three resistors that have a zero third wire, C, acts as a sense lead A 10 ohm lead impedance implies ~ temperature coeficient. To avoid and carries no current. 10/.385 = 26°C error in our mea- subjecting the three bridge-comple- surement. Even the temperature tion resistors to the same temperature The Wheatstone bridge shown in coeficient of the lead wire can as the RTD, the RTD is separated Figure 41 creates a non-linear rela- contribute a measurable error. The from the bridge by a pair of exten- tionship between resistance change classical method of avoiding this ~ sion wires (Figure 40). and bridge output voltage change. problem has been the use of a This compounds the already non- bridge (Figure 39). These extension wires recreate the linear temperature-resistance char- problem that we had initially: The acteristic of the RTD by requiring impedance of the extension wires an additional equation to convert affects the temperature reading. bridge output voltage to equivalent This effect can be minimized by RTD impedance. using a 3-wire bridge coniguration (Figure 41).

23 4-Wire ohms - The technique of Resistance to temperature The exact values for coeficients α, using a current source along with δ and β are determined by testing a remotely sensed digital voltmeter conversion the RTD at four temperatures and alleviates many problems associ- The RTD is a more linear device solving the resultant equations. This ated with the bridge. Since no cur- than the thermocouple, but it still familiar equation was replaced in rent lows through the voltage sense requires curve-itting. The Callendar- 1968 by a 20th order polynomial in leads, there is no IR drop in these Van Dusen equation has been used order to provide a more accurate leads and thus no lead resistance for years to approximate the RTD curve it. error in the measurement. curve.11, 13 The plot of this equation shows the The output voltage read by the T T T T3 RTD to be a more linear device than R = R + R α[T–δ(– -1)(–)-β(– -1)(–)] DVM is directly proportional to RTD T 0 0 100 100 100 100 the thermocouple (Figure 43). resistance, so only one conversion Where: equation is necessary. The three bridge-completion resistors are RT = resistance at temperature T replaced by one reference resistor. R0 = resistance at T = 0°C The digital voltmeter measures only α = temperature coeficient at T = 0°C the voltage dropped across the RTD (typically + 0.00392 Ω/Ω/°C) and is insensitive to the length of δ = 1.49 (typical value for .00392 platinum) the lead wires (Figure 42). β = 0 T > 0 0.11 (typical) T < 0 The one disadvantage of using 4-wire ohms is that we need one more extension wire than the 3-wire bridge. This is a small price to pay if we are at all concerned with the accuracy of the temperature measurement.

Figure 43. Plot of RTD versus Thermocouple

Figure 42. 4-Wire Ohms measurement

24 Practical precautions Construction - Due to its construc- To reduce self-heating errors, use tion, the RTD is somewhat more the minimum ohms measurement The same practical precautions fragile than the thermocouple, and current that will still give the resolu- that apply to thermocouples also precautions must be taken to tion you require, and use the largest apply to RTD’s, i.e., use shields protect it. RTD you can that will still give good and twisted-pair wire, use proper response time. Obviously, there are sheathing, avoid stress and steep- Self-heating - Unlike the thermocouple, compromises to be considered. gradients, use large extension wire, the RTD is not self-powered. keep good documentation and use A current must be passed through the Thermal shunting - Thermal shunting an integrating DMM. In addition, device to provide a voltage that can is the act of altering the measure- the following precautions should be be measured. The current causes ment temperature by inserting a observed. Joule (I2R) heating within the RTD, measurement transducer. Thermal changing its temperature. This shunting is more a problem with self-heating appears as a measure- RTD’s than with thermocouples, ment error. Consequently, attention as the physical bulk of an RTD is Small RTD Large RTD must be paid to the magnitude of greater than that of a thermocouple. the measurement current supplied Thermal EMF Fast Response Slow Response by the ohmmeter. A typical value for - The platinum-to-cop- Time Time self-heating error is ½°C per mil- per connection that is made when liwatt in free air. Obviously, an RTD the RTD is measured can cause a Low Thermal Poor Thermal immersed in a thermally conductive thermal offset voltage. The offset- Shunting Shunting medium will distribute its Joule compensated ohms technique can heat to the medium and the error be used to eliminate this effect. High Self-heating Low Self-heating Error Error due to self-heating will be smaller. The same RTD that rises 1°C per milliwatt in free air will rise only 1/10°C per milliwatt in air which is lowing at the rate of one meter per second.6

25 The Thermistor more than 100°C within the nominal center of the thermistor’s tempera- Like the RTD, the thermistor ture range, this equation approaches is also a temperature-sensitive resis- a rather remarkable ± .02°C curve it. tor. While the thermocouple is the most versatile temperature Somewhat faster computer execu- transducer and the PRTD is the most tion time is achieved through a stable, the word that best describes simpler equation: 1 the thermistor is sensitive. Of the T = -C three major categories of sen- (In R) – A sors, the thermistor exhibits by far the largest parameter change with where A, B, and C are again found temperature. by selecting three (R,T) data points Figure 44. Sensor sensitivities and solving the three resultant Thermistors are generally com- simultaneous equations. This equa- posed of semiconductor materials. not standardized thermistor curves tion must be applied over a nar- Although positive temperature to the extent that RTD and thermo- rower temperature range in order coeficient units are available, couple curves have been standard- to approach the accuracy of the most thermistors have a negative ized (Figure 44). Steinhart-Hart equation. temperature coeficient (TC); that is, their resistance decreases with An individual thermistor curve can be very closely approximated through increasing temperature. The nega- 18 Measurement tive TC can be as large as several use of the Steinhart-Hart equation: The high resistivity of the thermistor percent per degree C, allowing the 1 3 affords it a distinct measurement thermistor circuit to detect minute = A + B(ln R) + C (ln R) T advantage. The four-wire resistance changes in temperature which could measurement may not be required not be observed with an RTD or where: as it is with RTD’s. For example, a thermocouple circuit. T = Kelvins R = Resistance of the thermistor common thermistor value is 5000 Ω at 25°C. With a typical TC of 4%/°C, The price we pay for this increased A,B,C = Curve-itting constants a measurement lead resistance of sensitivity is loss of linearity. The 10 Ω produces only .05°C error. This thermistor is an extremely non- A, B, and C are found by selecting error is a factor of 500 times less linear device which is highly de three data points on the published than the equivalent RTD error. pendent upon process parameters. data curve and solving the three Consequently, manufacturers have simultaneous equations. When the data points are chosen to span no

26 Disadvantages - Because they are The IC Sensor semiconductors, thermistors are more susceptible to permanent An innovation in thermometry decalibration at high temperatures is the IC (Integrated Circuit) than are RTD’s or thermocouples. temperature transducer. These are The use of thermistors is generally available in both voltage and current- limited to a few hundred degrees output conigurations. Both supply Celsius, and manufacturers warn an output that is linearly proportion- that extended exposures even well al to absolute temperature. Typical below maximum operating limits will values are 1 µA/K and 10 mV/K F cause the thermistor to drift out of its (Figure 45). speciied tolerance. Some integrated sensors even Thermistors can be made very small represent temperature in a digital which means they will respond output format that can be read quickly to temperature changes. It directly by a microprocessor. also means that their small thermal mass makes them especially suscep- Except that they offer a very linear tible to self-heating errors. output with temperature, these IC Thermistors are a good deal more sensors share all the disadvantages fragile than RTD’s or thermocouples of thermistors. They are semicon- and they must be carefully mounted ductor devices and thus have a to avoid crushing or bond separation. limited temperature range. The same problems of self-heating and fragility are evident and they require an external power source. Figure 45. Different IC sensor outputs

These devices provide a convenient way to produce an easy- to-read output that is proportional to temperature. Such a need arises in thermo-couple reference junction hardware, and in fact these devices are increasingly used for thermocouple compensation.

27 The Measurement System

Figure 46 shows a practical method of implementing a thermocouple reference junction. The arrow points to an IC sensor which is used to perform software thermocouple compensation.

Conversion routines built into the Figure 46. General purpose multiplexer module for the Keysight 34970A or 34972A data Keysight Technologies, Inc. 34970A acquisition/switch unit. and 34972A irmware accept B, E, J, K, N, R, S and T type thermocouples, 2.2 kΩ, 5 kΩ and 10 kΩ thermistors, as well as a wide range of RTD’s. Results are displayed directly in degrees C, F or kelvins.

The Keysight 34970A and 34972A data acquisition systems incorpo- rates all of the desirable features mentioned in this application note:

– Internal 6½ digit DMM – Integrating A/D for noise rejection – Low-thermal scanning with built-in thermocouple reference junctions – Open thermocouple check – Built-in thermocouple, thermistor, and RTD linearization routines Figure 47. Keysight 34970A and 34972A data acquisition/switch units. with ITS-90 conformity

– Four-wire Ohms function For automated testing applications, The lash memory provides extend- with offset compensation the 34970A comes standard with ed non-volatile memory for readings GPIB and RS-232 interfaces and the and can be removed to transport – Isolated inputs that loat measurement setups and data. up to 300 V 34972A includes standard, easy to use USB and LAN interfaces. Both support up to 50,000-measurement Use the Keysight Benchlink Data non-volatile memory for stand-alone Logger software for easy PC-based data logging. The 34972A also sup- testing. Plus, the 34970A and ports an external USB lash memory 34972A support up to three modules drive, so scan data can be logged making it easy to add channels for directly to the lash memory. The various applications. 34972A also has an internal web server, making it easy to set up and monitor the 34972A via LAN.

28 The Keysight 34980A Multifunction Switch/Measure Unit (Figure 48), another example solution, provides high-speed temperature measurements where point count is high. The system offers the same desirable features as the Keysight 34970A data acquisition system. Some features include:

– Up to 560 (2-wire) or 640 (1-wire) temperture measurement channels per system with a scanning rate of up to 1000 reading/s. – B, E, J, K, N, R, S, T thermocouples, 2.2 kΩ, 5 kΩ, 10 kΩ, thermistors and a wide range Figure 48. Keysight 34980A System of RTD’s are supported. – External terminal block with built-in thermocouple reference junction and terminal connections to the application. This LXI-based system offers much In addition to the 34980A 8-slot more than temperature measure- LXI mainframe, there are also the – Four-wire Ohms SCP with ments. It provides a wide variety Keysight L4400 instruments. There offset compensation for RTD of analog/digital input and output are single-slot LXI instruments with and thermistor measurements. capability required by designers of the same features and capabilities electro-mechanical products and as the 34980A. – Built-in engineering unit manufacturers needing stringent conversions for thermocouple, monitoring and control of physi- thermistor, and RTD mea- cal processes. The 34980A consists surements. of an 8-slot LXI mainframe, GPIB, USB and LAN interfaces, alarms, an analog bus and Keysight BenchLink Data Logger software for the PC. Keysight Benchlink Data Logger software is a powerful time-saving software used to setup, verify, and monitor data logging routines.

29 Summary

Reliable temperature measurements require a great deal of care in both selecting and using the transducer, as well as choosing the right measurement system. With proper precautions observed for self-heating, thermal shunting, transducer decalibration, speciications and noise reduction, even the most complex temperature monitoring project will produce repeatable, reliable data. Today’s data acquisition system assumes a great deal of this burden, allowing us to concentrate on meaningful test results.

30 Appendix A The law of intermediate metals

The empirical laws of thermocouples

The following examples illustrate the empirically derived laws of thermocouples which are useful in understanding and diagnosing thermocouple circuits. Inserting the copper lead between the iron and constantan leads will not change the All of the examples assume the output voltage V, regardless of the temperature of the copper lead. The voltage V is measurement wires are homoge- that of an Fe-C thermocouple at temperature T1. neous; that is, free of defects and impurities. The law of interior temperatures

The output voltage V will be that of an Fe-C thermocouple at temperature T, regardless of the external heat source applied to either measurement lead.

The law of inserted metals

The voltage V will be that of an Fe-C thermocouple at temperature T, provided both ends of the platinum wire are at the same temperature. The two thermocouples created by the platinum wire (Fe-Pt and Pt -Fe) act in opposition.

All of the above examples assume the measurement wires are homogeneous; that is, free of defects and impurities.

31 Appendix B Stability - The platinum-based Base metal thermocouples couples are by far the most stable of all the common thermocouples. Unlike the noble metal thermocou- Thermocouple Type S is so stable that it is speci- ples, the base metal couples have characteristics ied as the standard for temperature no speciied chemical composition. calibration between the antimony Any combination of metals may be Over the years speciic pairs of point (630.74°C) and the gold point used which results in a voltage vs. thermocouple alloys have been (1064.43°C). temperature curve it that is within developed to solve unique measure- the standard wire errors. This leads ment problems. Idiosyncrasies of the to some rather interesting metal com- more common thermocouples are binations. Constantan, for example, is discussed here. not a speciic metal alloy at all, but a generic name for a whole series of We will use the term “standard copper-nickel alloys. Incredibly, the wire error” to refer to the common Constantan used in a type T commercial speciication pub- (copper-Constantan) thermocouple lished in the Annual Book of ASTM is not the same as the Constantan Standards. It represents the allow- used in the type J (iron-Constantan) able deviation between the actual thermocouple.3 thermocouple output voltage and Type B. the voltage predicted by the tables Type E - Although Type E standard in NIST Monograph 175. wire errors are not speciied below 0°C, the type E thermocouple is Type B - The B couple is the only Noble metal thermocouples - ideally suited for low tempera- common thermocouple that exhibits The noble metal thermocouples, types ture measurements because of a double-valued ambiguity. B, R, and S, are all platinum or its high Seebeck coefficient (58 platinum-rhodium thermocouples V/°C), low thermal conductivity Due to the double-valued curve and and hence share many of the same and corrosion resistance. characteristics. the extremely low Seebeck coeficient at low temperatures, Type B is The Seebeck coeficient for Type E virtually useless below 50°C. Since is greater than all other standard Diffusion - Metallic vapor diffusion the output is nearly zero from 0°C couples, which makes it useful at high temperatures can readily to 42°C, Type B has the unique ad- for detecting small temperature change the platinum wire calibra- vantage that the reference junction changes. tion, hence platinum wires should temperature is almost immaterial, as only be used inside a non-metallic long as it is between 0° and 40°C. sheath such as high-purity alumina. Of course, the measuring junction The one exception to this rule is a temperature is typically very high. sheath made of platinum, and this

option is prohibitively expensive.

32 Type J - Iron, the positive element in a J thermocouple is an inexpensive metal rarely manufactured in pure form. J thermocouples are subject to poor conformance characteristics because of impurities in the iron. Even so, the J thermocouple is popular because of its high Seebeck coeficient and low price.

The J thermocouple should never be used above 760°C due to an Type T abrupt magnetic transformation that can cause decalibration even when returned to lower temperatures. Types K & N - Type K has long been a Tungsten - There are three common popular thermocouple. It is espe- types of tungsten thermocouples. Type T - This is the only thermo- cially suited to higher temperature All are alloyed with rhenium to couple with published standard applications due to its resistance to make the metal more malleable. wire errors for the temperature oxidation. region below 0°C; however, type E Type G* W vs W–26% Re is actually more suitable at very low The type N thermocouple is gain- Type C* W–5% Re vs W–26% Re temperatures because of its higher ing popularity as a replacement for Type D* W–3% Re vs W–25% Re Seebeck coeficient and lower ther- type K. It has a slightly lower output mal conductivity. (smaller Seebeck coefficient) than Tungsten thermocouples are used type K, but an even higher resis- for measuring very high tempera- Type T has the unique distinction of tance to oxidation. The type N ther- tures in either a vacuum or an inert having one copper lead. This can be mocouple output curve is dependent atmosphere. an advantage in a specialized moni- upon wire size, and there are two toring situation where a tempera- distinct Nicrosil-Nisil characteristic ture difference is all that is desired. curves published in NIST Mono- graph 175, the differences being The advantage is that the copper wire size and temperature range.14 * Not ANSI symbols thermocouple leads are the same metal as the DVM terminals, making lead compensation unnecessary.

33 Thermocouple Hardware

Connector Thermocouple Underground junction Composed of same well – best protection metals as thermo- – lower gradient – electrically isolated couple, for minimum – protects wire connection error. – change thermocouple without interrupting process.

Grounded junction Exposed junction Thermocouple washers – wires protected – wires unprotected – couple built into washer – faster response – fastest response – convenient mounting

Standard U.S. Ω/double Seeback NIST speciied Type Metal color code foot @ 20ºC coeficient Wire error in °C material range* + – + – 20 AWG S(µV/°C)@T(°C) Range Standard Special (°C) B Platinum- Platinum- Gray Red 0.22 5.96 600 870 to 1700 ± 0.5% ± 0.25% 0 to 1820 30% Rhodium 6% Rhodium E Nickel- Constantan Violet Red 0.71 58.67 0 0 to 900 ± 1.7 or ± 1 or –270 to 1000 10% Chromium ± 0.5% ± 0.4% J Iron Constantan White Red 0.36 50.38 0 0 to 750 ± 2.2 or ± 1.1 or –210 to 1200 ± 0.75% ± 0.4% K Nickel- Nickel Yellow Red 0.59 39.45 0 0 to 1250 ± 2.2 or ± 1.1 or –270 to 1372 10% Chromium ± 0.75% ± 0.4% N Nicrosil Nisil Orange Red 0.78 25.93 0 0 to 1250 ± 2.2 or ± 1.1 or –270 to 1300 ± 0.75% ± 0.4% R Platinum- Platinum Black Red 0.19 11.36 600 0 to 1450 ± 1.5 or ± 0.6 or –50 to 1768 13% Rhodium ± 0.25% ± 0.1% S Platinum- Platinum Black Red 0.19 10.21 600 0 to 1450 ± 1.5 or ± 0.6 or –50 to 1768 10% Rhodium ± 0.25% ± 0.1% T Copper Constantan Blue Red 0.30 38.75 0 0 to 350 ± 1 or ± 0.5 or –270 to 400 ± 0.75% ± 0.4%

* Material range is for 8 AWG wire and decreases with decreasing wire size.

34 Bibliography

1. Charles Herzfeld, F.G. 7. R.L. Anderson: Accuracy of Small 14. Burley, Powell, Burns, & Scroger: Brickwedde: Temperature - Its Diameter Sheathed Thermocou- The Nicrosil vs. Nisil Thermocou- Measurement and Control in Sci- ples for the Core Flow Test Loop, ple: Properties and Thermoelectric ence and Industry , Vol. 3, Part 1, Oak Ridge National Laboratories, Reference Data, NBS Monograph Reinhold, New York, 1962. ORNL-5401, (available from Na- 161, U.S. Dept. of Commerce, tional Information Service), April, Washington, D.C., 1978. 2. Robert P. Benedict: Fundamentals 1979. of Temperature, Pressure and 15. J.P. Tavener: Platinum Resistance Flow Measurements, John Wiley 8. R.P. Reed: Branched Thermo- Temperature Detectors - State of & Sons, Inc., New York, 1969. couple Circuits in Underground the Art, Measurements & Control, Coal Gasiﬁcation Experiments, Measurements & Data Corpora- 3. Manual on the Use of Thermo- Proceedings of the 22nd ISA tion, Pittsburgh, PA., April 1974. couples in Temperature Measure- International Instrumentation ment, Fourth Edition , Revision of Symposium, Instrument Society 16. J.P. Evans and G.W. Burns: A ASTM Special Publication 470B, of America, 1976. Study of Stability of High Tem- Philadelphia, PA., 1993. perature Platinum Resistance 9. R.J. Moffat: The Gradient Ap- Thermometers, in Temperature 4. Temperature-Electromotive Force proach to Thermocouple Circuitry, - Its Measurement and Control on Reference Functions and Tables from Temperature- Its Measure- Science and Industry, for the Letter-Designated Ther- ment and Control in Science and Reinhold, New York, 1962. mocouple Types Based on the Industry, Reinhold, New York, ITS-90 , NIST Monograph 175, 1962. 17. D.D. Pollock: The Theory and National Institute of Standards Properties of Thermocouple Ele- and Technology, 10. R.P. Reed: A Diagnostics- ments, ASTM STP 492, Omega Washington, D.C., 1993 oriented System for Thermocouple Press, Ithaca, New York, 1979. Thermometry, Proceedings of 24th 5. H. Dean Baker, E.A. Ryder, N.H. ISA International Instrumentation 18. YSI Precision Thermistors, Yellow Temperature Measure- Baker: Symposium, Instrument Society Springs Instruments, Inc., Yellow ment in Engineering , Omega of America, 1978. Springs, Ohio, 1977. Press, Division of Omega Engineering Inc. 11. Harry R. Norton: Handbook of 19. R.P. Reed: Thermoelectric Ther- Transducers for Electronic Mea- mometry. A Functional Model, 6. Temperature Measurement Hand- suring Systems, Prentice-Hall, from Temperature - Its Measure- book 1983, Omega Englewood Cliffs, New Jersey. ment and Control in Science Engineering, Stamford, and Industry, Vol. 5, American Connecticut, 06907. 12. C.H. Meyers: Coiled Filament Institute of Physics, N.Y. 1982. Resistance Thermometers, NBS Journal of Research, Vol. 9, 1932.

13. Bulletin 9612, Rev. B: Platinum Resistance Temperature Sensors, Rosemount Engineering Co., 1962.

35 36 | Keysight | Practical Temperature Measurements - Application Note

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Note: this document was formerly known as Application Note 290

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