Thermocouple calibration tables

At a basic level, it seems that one should be able to take any thermocouple, attach it to a voltmeter, and determine the amount of electricity generated from the heat applied to the dissimilar junction. Then the user could look in some predetermined thermocouple calibration table to determine the amount of heat that the amount of electricity from that specific type of joined wires produces. 

Thermocouple calibration tables have been compiled by the NIST and can be found in a variety of sources such as the Handbook of Chemistry and Physics by the Chemical Rubber Company, published yearly. 

If you look at a thermocouple calibration table, you will see that it has a reference junction at 0°C. This reference point is used because of an interesting complication that arises when a thermocouple is hooked up to a voltmeter. To explain this phenomenon, first look at one specific type of thermocouple, a type T (cop-per-constantan design). Also, assume that the wires in the voltmeter are all copper. Once the thermocouple is hooked to the voltmeter, we end up having a total of three junctions (see Fig. 2.31) ... 

The main source of uncertainty was the absolute temperature measurement, which was made with a platinum, platinum-rhodium thermocouple, calibrated by the National Bureau of Standards to i but thought to be accurate to 1 ° G. AZ/q was obtained l y substituting the experimentally determined equilibrium constants and the spectroscopically derived free energy functions into equation 3.1.6. The results obtained are shown in Table 3.2.1. 

Where accuracy of measurement is not vital, commercially available electrical compensators (which assume the outputs of the thermocouples used will match the standard function within the tolerances stated in Table 16.9) are sufficient for direct readout of temperatures. With proper selection, accuracy of at least 2.2°C (or 0.75 percent of readings) for standard wires and 1.1°C (or 0.4 percent of readings) for wires of special tolerance can be achieved without calibration (Table 16.9). However, the precision or reproducibility of a single thermocouple is usually much better than these limits. 

It should be emphasized that the calibration tables are merely guides or interpolation aids. For reliability each lot of thermocouple wire should be spot-calibrated and compared with the calibration table. A calibration for the particular wire can be obtained by revising slightly the published table. Linear interpolation of the deviations has been found to be sufficiently accurate for small adjustments. 

To avoid or minimize unwanted thermal EMFs, thermocouple junctions are used in pairs as shown in the bottom portion of Figure 12.10. One of the junctions is maintained at a known temperature - often an ice bath. The temperature of the other junction can then be inferred from the resulting total EMF using standard thermocouple tables or special calibration tables prepared for your specific thermocouple. 

The experiments were carried out in a Netzsch STA 409 C (Simultaneous Thermal Analysis - STA) in the TGA/DSC configuration. The STA has a vertical san le carrier with a reference and a sample crucible, and in order to account for buoyancy effects, a correction curve with empty crucibles was first conducted and then subtracted from the actual experiments. Platinum/Rhodium crucibles were used in order to get the best possible heat transfer. The thermocouple for each crucible was positioned Just below and in contact with the crucible. The ten rerature obtained from the measurement is the temperature in the reference side. This temperature is converted to the temperature in the sample side by using the DSC-signal in pV and a temperature-voltage table for the thermocouple. The product gases were swept away by lOO Nml/inin nitrogen which exited the top of the STA, The STA was calibrated for temperature and sensitivity (DSC) with metal standards at each heating rate. 

When accuracy greater than the tolerances in Table 16.9 is required, the wires must be calibrated. Normally, this requires comparison calibration of sample thermocouples taken from each spool to account for spool variability. Typically, two thermocouples—one fabricated from the beginning and the other from the end of the spool—are calibrated to determine an average calibration for the entire spool. If the deviation between the two calibrations is not within the required uncertainty, a third thermocouple fabricated from the center of the spool should be used. If the results are still unsatisfactory, then each thermocouple should be calibrated individually, or a different spool should be used. 

Some electric tube furnaces incorporate a p3n ometer, which is not required in any of the experiments described in this manual, though Experiments 13 and 14 can be run by using an electric tube furnace at controlled temperature instead of an oil bath if so desired. A pyrometer may easily be improvised with iron and constantan wires twisted together and spot-welded to provide the junctions. The hot junction is placed against the outside of the combustion tube in the center of the heated portion and bound in position by asbestos tape, which also serves as insulation. The cold junction is kept in ice and water, and the electromotive force is measured with a millivoltmeter. With the cold junction at 0°C and the hot junction at 200°C, the electromotive force of the iron-constantan couple is given in tables as 10.77 millivolts. For accurate work the couple used should be calibrated and to assure uniform temperature distribution and electrical shielding, the combustion-tube and thermocouple wires should be encased in a tubular metal shield that fits inside the furnace and is groimded. 

In addition to these defining and secondary temperature standards, a thermocouple wire (SRM 733, a silver-28 at.% gold alloy) has been certified, which serves to compare manufactured wire to standard reference thermocouple tables between 4 and 273 K. SRM 767, a superconductive thermometric fixed-point device, provides temperature calibration in the range 0.5 to 7.2 K This device incorporates five high-purity elements (lead, indium, aluminum, zinc, and cadmium) in long, thin cylinders whose superconductive transition temperatures are certified to be reproducible within 1 mK. 

Temperature Calibration. As mentioned, temperature control and calibration of TGA equipment is more difficult. Not only are the sample and the thermocouple a finite distance apart, so as not to interfere with the weight measurement, but also the heat transfer between sample and oven in TGA is usually across an air or an inert gas gap. In addition, sample masses are often larger than those in DTA/DSC. There may be a large temperature gradient inside the sample. There are also other factors noted in the right-hand side of Table 5 that would affect temperature control. 

Polymer samples were placed in a tungsten crucible in a Knudsen cell inlet system of a time-of-flight MS. The system was evacuated. Samples were then heated at a linear rate. Spectra were determined at 1 minute intervals. The thermocouple was welded to a support rod 3 mm below the crucible and had been calibrated against a thermocouple in the bottom of the crucible at each heating rate. Details are summarised in Tables 9.4 and 9.5 for three polymers. 

The calibration period for thermocouples. Table 17.5, showed a wide spread in practice. The frequency should be governed by the operating temperature and type of thermocouple therefore it is not surprising that there is a wide spread in the reported calibration periods. Best practice is, of course, to calibrate before each test but experience may be sufficient for laboratories to relax this procedure as data on thermal drift are recorded and analysed. It is somewhat worrying that several laboratories did not specify any calibration period for their thermocouples, all thermocouples require... 

A time to reach blister temperature for paint removal was used to determine the relative speed of paint removal. In each case, the paint consisted of an alkyd resin binder on painted metal and wood surfaces. A calibrated thermocouple attached to a unit with a digital temperature scale was placed directly on the surface, and the temperature was recorded when the paint blistered Table 11.12 lists the results. This test showed that a metal substrate requires more cleaning time (+5-72 sec) and a higher temperature (+77-82 °F (+22-28 °C)) than less thermally conductive substrate, such as wood. 

---