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Platinum Metals Rev., 1967, 11, (1), 10

A Rhodium-Platinum Thermocouple for High Temperatures

A Reference Table for the “Six-Thirty” Couple

  • P. H. W.
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The thermocouple formed by two rhodium-platinum alloys containing 6 per cent and 30 per cent of rhodium respectively was first introduced in Germany some thirteen years ago by Degussa of Hanau for temperature measurements in molten steel. Since then it has been widely used in other high temperature applications; J. A. Stevenson (1), for example, has recently given an account of its use in the control of glass melting furnaces in Russia.

The “Six-Thirty” thermocouple, as it is commonly known, was developed because the standard platinum : 10 per cent rhodium-platinum and platinum: 13 per cent rhodium-platinum thermocouples were found to have limitations when used much above 1500°C. The pure platinum limb becomes extremely weak mechanically and is very susceptible to contamination by rhodium from the alloy limb. The “Six-Thirty” thermocouple overcomes these disadvantages by the use of two rhodium-platinum alloys that enable it to be used for continuous temperature measurements up to 1700°C and intermittently up to about 1800°C, the limiting temperature being the melting point of the 6 per cent rhodium-platinum limb at 1820°C.

The thermocouple is very stable under clean oxidising conditions. Walker, Ewing and Miller (2) have shown this couple to be more stable than any other rhodium-platinum alloy combination. The use of two rhodium-platinum alloys produces a lower e.m.f. output, but above 1000°C the sensitivity of the “Six-Thirty” closely approaches that of the platinum : 10 per cent rhodium-platinum thermocouple and may thus be used without any appreciable sacrifice in the accuracy of measurements. At room temperature the e.m.f. and sensitivity are so low that below 50°C cold junction correction may be neglected for all but the most precise measurements.

The growing importance of this thermocouple led to the redetermination of basic values, and new and more accurate reference tables compiled by Obrowski and Prinz of Degussa (3) were published in 1962. These tables are now regarded as standard values in Europe.

In the U.S.A. the “Six-Thirty” has become the preferred thermocouple for high temperature measurements and to facilitate its use and calibration in America reference tables have been derived by George W. Burns and John S. Gallaher (4) of the National Bureau of Standards. In a commendably detailed paper the authors describe how their test programme was conducted using thermocouples from three major American manufacturers and one from Degussa. The compositions chosen, 6.12 per cent and 29.6 per cent rhodium respectively, reflect the desire to avoid creating a new and conflicting set of standards, since these alloys had been previously found to generate e.m.f.s that conform closely to the values determined by Obrowski and Prinz.

The American reference table is therefore similar to the 1962 table and the differences between them, shown graphically in the diagram, are claimed to be less than the differences found between the couples being tested.

Burns and Gallaher give a full account of the techniques used to calibrate their thermocouples; these include comparison with a platinum resistance thermometer, comparison with a platinum: 10 per cent rhodium-platinum thermocouple and comparison with an optical pyrometer. A large number of e.m.f. determinations were also carried out on each limb of the thermocouples under test, the e.m.f.s being measured with respect to platinum reference wires. In addition a study was made of the effect of variations in the rhodium content on the temperature-e.m.f. relationship. The results of this and other parts of the test programme are shown concisely in graph form.

Reference Table for 6 per cent Rh-Pt : 30 per cent Rh-Pt Thermocouples

(Temperature in °C (IPTS 1948); electromotive force in absolute millivolts; reference junction 0°C

t(°C)100011001200130014001500160017001800
04.8445.7936.8007.8668.97910.12411.286.2.45313.616
0.0920.0980.1040.1100.1140.1160.1170.1170.115
104.9365.8916.9047.9769.09310.24011.40312.57013.731
0.0930.0990.1050.1100.1130.1160.1160.1160.116
205.0295.9907.0098.0869.20610.35611.51912.68613.847
0.0930.1000.1050.1100.1140.1150.1170.117 
305.1226.0907.1148.1969.32010.47111.63612.803 
0.0940.1000.1060.1110.1140.1160.1170.116 
405.2166.1907.2208.3079.43410.58711.75312.919 
0.0950.1000.1060.1110.1150.1170.1160.116 
505.3116.2907.3268.4189.54910.70411.86913.035 
0.0950.1010.1070.1120.1140.1160.1170.117 
605.4066.3917.4338.5309.66310.82011.98613.152 
0.0960.1010.1080.1110.1150.1160.1170.116 
705.5026.4927.5418.6419.77810.93612.10313.268 
0.0960.1020.1080.1130.1150.1170.1170.116 
805.5986.5947.6498.7549.89311.05312.22013.384 
0.0970.1030.1080.1120.1160.1160.1160.116 
905.6956.6977.7578.86610.00911.16912.33613.500 
0.0980.1030.1090.1130.1150.1170.1170.116 
1005.7936.8007.8668.97910.12411.28612.45313.616 

After averaging the results of the calibrations three quartic equations were derived with the aid of an IBM 7094 digital computer. In common with previous investigators, Burns and Gallaher found that no equation of simple form will adequately describe the e.m.f.-temperature relationship over the entire temperature range.

Difference between the NBS reference table and the 1962 Degussa reference curve for the “Six-Thirty” thermocouple

Difference between the NBS reference table and the 1962 Degussa reference curve for the “Six-Thirty” thermocouple

The authors emphasise the arbitrary nature of their reference table, but conclude that the thermocouples used by Obrowski and Prinz had essentially the same characteristics as those of American manufacture. Thus thermocouples made to comply with the earlier table of Obrowski and Prinz will comply with the American table to within similar tolerances.

The authors conclude their paper with a discussion of the results obtained and a consideration of the uncertainties in the measurements. Burns and Gallaher estimate that they do not exceed ±2°C at 1400°C and about ±3 or 4°C at 1750°C.

It is interesting to note that the American results are based almost entirely on comparison techniques, whereas Obrowski and Prinz used the fixed melting point technique. If criticism of this paper may be made it is that the value of the work would have been given more weight if more than one set of batches from each manufacturer had been tested. Apart from this one reservation the authors have presented an extremely competent and valuable paper and the publication of these tables underlines the international recognition of the “Six-Thirty” thermocouple as an accurate, sensitive and stable sensor for the measurement of high temperatures.

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References

  1. 1
    J. A. Stevenson, Platinum Metals Rev., 1966, 10, 128 – 131
  2. 2
    B. E. Walker,, C. T. Ewing and R. R. Miller, Rev. Sci. Instrum., 1962, 33, ( 10 ), 1029 – 1040
  3. 3
    W. Obrowski and W. Prinz, Archiv Eisenhüttenwesen, 1962, 33, ( 1 ), 1 – 4 ; Platinum Metals Rev., 1962, 6, 96 – 97
  4. 4
    George W. Burns and John S. Gallaher, J. Res. NBS, 1966, 70C, ( 2 ), 89 – 125

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