Temperature Calibration Equipment: Application Note a Technician’S Guide
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Temperature calibration equipment: Application Note A technician’s guide Have you ever been brought a new thermometer to calibrate, and asked yourself, “What am I going to need to calibrate this?” This guide is intended to help you work out the kind of equipment you need for your particular calibration situation. Of course, there are many considerations—including accuracy, temperature range, automation requirements and budget. This guide covers the main points, but you’ll probably want to speak with an experienced Fluke application specialist before you make a final decision; as always, we stand ready to help keep you up and running. Getting started placed into a heat transfer medium in the temperature Most often, thermometer type source. The heat transfer determines the kind of equip- medium might be a stirred fluid, ment needed. Some of the most a metal block, or a fixed point common items that need to be cell. The heat transfer medium calibrated are listed in Table 1. maintains a constant and The equipment types shown in uniform temperature environ- the Needed equipment column ment that allows the reading should not be considered of the thermometer under test definitive. For example, the to be compared to a more same equipment used to cali- accurate known temperature. brate an RTD or an SPRT could also be used to calibrate a PRT, The known temperature but this is a good guide to value is going to come from what you would most likely one of two places: use. In addition, choice of • Naturally occurring equipment may depend on phenomena, such as where the calibration is the triple point of water performed. For example, you (0.010 °C) might use a calibration bath • A temperature measure- and temperature standard to ment, made by a calibrate an RTD in a labora- temperature standard tory, but a portable calibrator These two different methods would be more appropriate for of getting the more accurate on-site calibrations. More about known temperature lead to two the equipment needed will be distinct methods of calibration: explained below. comparison calibration and fixed point calibration. Calibration method Comparison calibration is the No matter what your tempera- most common type. ture calibration application, you’re going to need a temper- ature source to heat or cool your thermometers to a known temperature. During the cali- bration, the thermometers are Table 1. Common thermometers that need to be calibrated Workload Where calibrating? Needed equipment Dial thermometer On-site A portable temperature calibrator Liquid in glass Laboratory Calibration bath, fluid level adapter, magnifier, carousel, and temperature standard RTD On-site A portable temperature calibrator and (optional) temperature standard Laboratory Calibration bath and temperature standard PRT On-site A portable temperature calibrator and (optional) temperature standard Laboratory Calibration bath and temperature standard Thermocouple On-site A portable temperature calibrator and (optional) temperature standard Laboratory Calibration furnace and temperature standard Thermistor Laboratory Calibration bath and temperature standard SPRT Laboratory Fixed point cells, maintenance furnaces/baths, a standard resistor, and a resistance bridge Infrared On-site Radiometrically calibrated infrared calibrator thermometer Laboratory Radiometrically calibrated infrared calibrator, or an infrared calibrator (plate) and a reference radiometer, or an infrared blackbody (cavity) Comparison For non-contact thermometer calibration comparisons you will need: • A radiance source to gener- For contact thermometer ate the known radiance comparison calibrations, you observed by the infrared will need: thermometer • A temperature source to heat • A radiometric temperature or cool the thermometer(s) standard to provide the under test accurate known temperature • A temperature standard to that is compared with the provide the accurate known thermometer under test temperature that is compared The radiance source can be with the thermometer under either a painted surface or a test blackbody cavity. Good black- • (Optional) Measuring devices body cavities have a well to read the temperature stan- known emissivity value (i.e. dard and/or thermometer(s) 0.95 ±0.001). The key perfor- under test mance indicator of a radiance As the name implies, during source is its spectral emis- a comparison calibration, a sivity. The spectral emissivity thermometer under test is depends on wavelength, the compared to a more accu- geometry of the surface, the rately calibrated temperature finish of the surface, and the standard, while both are main- types of plate material and tained at the same constant paint used. The emissivity of a temperature in the temperature painted surface is different for source. Typically the standard each wavelength; therefore, its is four times more accurate radiance is only known if it is than the thermometer under measured over the same wave- test. Any thermometer can be lengths used by the infrared calibrated by comparison, and thermometers being calibrated. comparison calibrations can For example, measurements of take place either in a labora- surface by a radiometer over tory or on-site. the band of wavelengths from 2 Fluke Calibration Temperature calibration equipment: A technicians guide and a temperature standard. Fixed point cells are the most accurate type of temperature calibration equipment but they are also the most difficult to use and are found mainly in primary standards laboratories. For fixed point calibrations, you will need: • A fixed point cell (tempera- ture source) • A device to maintain the temperature surrounding the cell (i.e. bath or furnace) • A resistance bridge to measure the SPRT being calibrated Choosing a temperature source When choosing a tempera- 8 to 14 microns will be good Fixed point ture source, you often need to for calibrating thermometers of calibration choose the best compromise the same bandwidth (8 to 14 For the most accurate ther- between accuracy and some microns). mometers under test, the only other technical requirement. Surfaces used to calibrate sufficiently accurate tempera- Table 2 compares various infrared thermometers should ture standard is a primary types of temperature sources be calibrated radiometrically standard. Fixed point cells are against some common techni- over the correct bandwidth, or the primary standards used cal requirements. else a radiometric temperature in temperature calibration. In standard (radiometer) with the a primary standards labora- correct bandwidth needs to Choosing a calibrated tory, SPRTs are placed in fixed be compared with the ther- thermometer point cells and given ITS-90 mometers under test during (temperature standard) calibrations. The ITS-90 is calibration. For example, the the international temperature For comparison calibrations, Fluke, Hart Scientific 4181 scale used by the International you need to choose a cali- Precision Infrared Calibrator is System of Units (SI) to define brated thermometer for your calibrated radiometrically from Kelvin and Celsius temperature temperature standard. There 8 to 14 microns and does not values for the world. are several types to choose require a separate radiometric Fixed point cells rely on the from. Your choice depends temperature standard over that intrinsic properties of nature on your temperature range bandwidth. Alternatively, the to provide a very precisely and the required accuracy of Hart 9132 Infrared Calibrator is known temperature. Extremely your measurements. Table 3 not radiometrically calibrated pure substances (i.e. tin, zinc provides a guide for finding the and does require a sepa- or water) under the right right type of thermometer for rate radiometer for infrared conditions of temperature your application. Other condi- traceability. and pressure become very tions that should be considered precise and reliable tempera- are degree of ruggedness, ture standards. Thermometers and needed probe dimensions are calibrated by placing such as length and diameter. them inside the cell so that A general rule for resistance the thermometer and the thermometers such as PRTs, fixed point cell are resting at SPRTs, and HTSPRTs is that the the same temperature. This more rugged the instrument, means a fixed point cell is the less accurate it becomes. both a temperature source 3 Table 2. Temperature sources Technical needs Fixed points Baths Metrology wells Field metrology Micro-baths Dry-wells wells Accuracy ±0.001 °C to ±0.01 °C • Accuracy ±0.01 °C to ±0.1 °C • • Accuracy ±0.1 °C to ±1.0 °C • • • Calibrate on-site • • • • Eliminate fluids • • • • Get to temperature fast • • • • Calibrate multiple sizes and shapes • • Automate calibration • • • • • • Use fewer tools • Learn in less than 15 minutes • Use easily • • • • • Once you have chosen the type of thermometer probe you will use as the temperature standard, you need to choose a device to measure it. This device could be a special- ized thermometer readout, or it could be a general purpose instrument like a digital multi- meter. Thermometer readouts deliver the best results because their ranges, current settings and firmware are designed for temperature measurement. Selection criteria for thermome- ter readouts include the type of thermometer and level of accu- racy required and other factors like need for battery power, Caption to go here number of inputs, and whether data can be recorded for future Table 3. Thermometer types retrieval. See Table 4. Technical needs Thermistor PRT SPRT