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Thermocouple Introduction and Theory

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Thermocouple Introduction and Theory Reference Temperatures We cannot build a temperature divider as we can a Metal A voltage divider, nor can we add temperatures as we + would add lengths to measure distance. We must rely eAB upon temperatures established by physical phenomena – which are easily observed and consistent in nature. The Metal B International Practical Temperature Scale (IPTS) is based on such phenomena. Revised in 1968, it eAB = SEEBECK VOLTAGE establishes eleven reference temperatures. Figure 3 Since we have only these fixed temperatures to use All dissimilar metals exhibit this effect. The most as a reference, we must use instruments to interpolate common combinations of two metals are listed in between them. But accurately interpolating between Appendix B of this application note, along with their these temperatures can require some fairly exotic important characteristics. For small changes in transducers, many of which are too complicated or temperature the Seebeck voltage is linearly proportional expensive to use in a practical situation. We shall limit to temperature: our discussion to the four most common temperature ∆ α∆ transducers: thermocouples, resistance-temperature eAB = T detector’s (RTD’s), thermistors, and integrated Where α, the Seebeck coefficient, is the constant of circuit sensors. proportionality. Measuring Thermocouple Voltage - We can’t measure the Seebeck voltage directly because we must IPTS-68 REFERENCE TEMPERATURES first connect a voltmeter to the thermocouple, and the 0 EQUILIBRIUM POINT K C voltmeter leads themselves create a new Triple Point of Hydrogen 13.81 -259.34 thermoelectric circuit. Liquid/Vapor Phase of Hydrogen 17.042 -256.108 at 25/76 Std. Atmosphere Let’s connect a voltmeter across a copper-constantan Boiling Point of Hydrogen 20.28 -252.87 (Type T) thermocouple and look at the voltage output: Boiling Point of Neon 27.102 -246.048 J3 Triple Point of Oxygen 54.361 -218.789 Boiling Point of Oxygen 90.188 -182.962 Cu Cu + + Triple Point of Water 273.16 0.01 v V1 J1 – C – Boiling Point of Water 373.15 100 Cu Freezing Point of Zinc 692.73 419.58 Freezing Point of Silver 1235.08 961.93 J2 Freezing Point of Gold 1337.58 1064.43 EQUIVALENT CIRCUITS Table 1 Cu + – Cu Cu V3 + J1 + J3 J1 V1 V THE THERMOCOUPLE – 1 When two wires composed of dissimilar metals are – + – + – joined at both ends and one of the ends is heated, there Cu V2 C Cu V2 C is a continuous current which flows in the J2 J thermoelectric circuit. Thomas Seebeck made this 2 discovery in 1821. MEASURINGV3 = 0 JUNCTION VOLTAGE WITH A DVM Figure 4 We would like the voltmeter to read only V1, but by Metal A Metal C connecting the voltmeter in an attempt to measure the output of Junction J1, we have created two more metallic junctions: J2 and J3. Since J3 is a copper-to-copper junction, it creates no thermal EMF Metal B (V3 = 0), but J2 is a copper-to-constantan junction which THE SEEBECK EFFECT will add an EMF (V2) in opposition to V1. The resultant Figure 2 voltmeter reading V will be proportional to the temperature difference between J1 and J2. This says If this circuit is broken at the center, the net open that we can’t find the temperature at J1 unless we first circuit voltage (the Seebeck voltage) is a function of the find the temperature of J2. junction temperature and the composition of the two metals. Z-21 The Reference Junction Cu Cu + +T + + V J1 v J v 1 V1 1 – – + – – V Cu Cu 2 C – + – V Voltmeter 2 J2 J2 T=0°C Ice Bath EXTERNAL REFERENCE JUNCTION Z Figure 5 The copper-constantan thermocouple shown in Figure 5 is a unique example because the copper wire One way to determine the temperature of J2 is to is the same metal as the voltmeter terminals. Let’s use physically put the junction into an ice bath, forcing its an iron-constantan (Type J) thermocouple instead of the temperature to be 0˚C and establishing J2 as the copper-constantan. The iron wire (Figure 6) increases Reference Junction. Since both voltmeter terminal the number of dissimilar metal junctions in the circuit, as junctions are now copper-copper, they create no both voltmeter terminals become Cu-Fe thermocouple thermal emf and the reading V on the voltmeter is junctions. proportional to the temperature difference between J1 and J2. V3 J Now the voltmeter reading is (see Figure 5): - + 3 ≅α + V = (V1 - V2) (tJ - tJ ) 1 2 v V1 If we specify TJ in degrees Celsius: – 1 + T (˚C) + 273.15 = t - J1 J1 V V1 = V Voltmeter 4 J4 if V3 = V4 then V becomes: i.e., if T = T V = V - V = α [(T + 273.15) - (T + 273.15)] J3 J4 1 2 J1 J2 = α (T - T ) = α (T - 0) J1 J2 J1 JUNCTION VOLTAGE CANCELLATION V = αT J1 Figure 7 We use this protracted derivation to emphasize that If both front panel terminals are not at the same the ice bath junction output, V2, is not zero volts. It is a temperature, there will be an error. For a more precise function of absolute temperature. measurement, the copper voltmeter leads should be By adding the voltage of the ice point reference extended so the copper-to-iron junctions are made on junction, we have now referenced the reading V to 0˚C. an isothermal (same temperature) block: This method is very accurate because the ice point Isothermal Block temperature can be precisely controlled. The ice point is J3 used by the National Bureau of Standards (NBS) as the Cu fundamental reference point for their thermocouple Cu Fe + tables, so we can now look at the NBS tables and v T1 directly convert from voltage V to Temperature T . – Fe C J1 Cu V2 Voltmeter Cu J J3 4 T Fe REF Cu + v J – 1 Ice Bath Cu C Fe REMOVING JUNCTIONS FROM DVM TERMINALS Figure 8 J4 J2 The isothermal block is an electrical insulator but a good heat conductor, and it serves to hold J3 and J4 at Ice Bath the same temperature. The absolute block temperature is unimportant because the two Cu-Fe junctions act in IRON-CONSTANTAN COUPLE opposition. We still have α Figure 6 V = (T1 - TREF) Z-22 Reference Circuit Let’s replace the ice bath with another isothermal This is a useful conclusion, as it completely eliminates block the need for the iron (Fe) wire in the LO lead: Isothermal Block Cu + Fe HI Cu Fe v J J J3 1 J3 1 Cu C – LO Cu Fe C Voltmeter J4 J REF J4 T REF Isothermal Block ELIMINATING THE ICE BATH T REF Figure 9a EQUIVALENT CIRCUIT Figure 11 The new block is at Reference Temperature TREF, and α α because J3 and J4 are still at the same temperature, we Again, V = (TJ1 - TREF), where is the Seebeck can again show that coefficient for an Fe-C thermocouple. α Junctions J3 and J4, take the place of the ice bath. V = (T1-TREF) These two junctions now become the Reference This is still a rather inconvenient circuit because we Junction. have to connect two thermocouples. Let’s eliminate the Now we can proceed to the next logical step: Directly extra Fe wire in the negative (LO) lead by combining measure the temperature of the isothermal block (the the Cu-Fe junction (J ) and the Fe-C junction (J ). 4 REF Reference Junction) and use that information to We can do this by first joining the two isothermal compute the unknown temperature, TJ . blocks (Figure 9b). 1 Block Temperature = T Cu REF HI Fe Cu J J1 3 J3 Fe LO + + Cu Fe C v V1 J – – 1 J4 J REF J4 C Voltmeter Cu Isothermal Block @ T REF R JOINING THE ISOTHERMAL BLOCKS T Figure 9b EXTERNAL REFERENCE JUNCTION-NO ICE BATH Figure 12 We haven’t changed the output voltage V. It is still α V = (TJ1 - TJREF ) A thermistor, whose resistance RT is a function of Now we call upon the law of intermediate metals (see temperature, provides us with a way to measure the Appendix A) to eliminate the extra junction. This absolute temperature of the reference junction. empirical “law” states that a third metal (in this case, Junctions J3 and J4 and the thermistor are all assumed iron) inserted between the two dissimilar metals of a to be at the same temperature, due to the design of the thermocouple junction will have no effect upon the isothermal block. Using a digital multimeter under output voltage as long as the two junctions formed by computer control, we simply: the additional metal are at the same temperature: 1) Measure RT to find TREF and convert TREF to its equivalent reference junction Metal A Metal B Metal C Metal A Metal C = voltage, VREF , then 2) Measure V and add VREF to find V1, and convert V to temperature T . 1 J1 Isothermal Connection This procedure is known as Software Compensation because it relies upon the software of a computer to compensate for the effect of the reference junction. The Thus the low lead in Fig. 9b: Becomes: isothermal terminal block temperature sensor can be any device which has a characteristic proportional to C Cu C Cu Fe = absolute temperature: an RTD, a thermistor, or an integrated circuit sensor. T REF It seems logical to ask: If we already have a device T REF that will measure absolute temperature (like an RTD or thermistor), why do we even bother with a LAW OF INTERMEDIATE METALS thermocouple that requires reference junction Figure 10 Z-23 compensation? The single most important answer to this question is that the thermistor, the RTD, and the integrated circuit transducer are only useful over a Fe certain temperature range.
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