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Platinum Metals Rev., 1971, 15, (2), 59

Pressure and Thermal E.M.F.s

The Effect on Rhodium-Platinum Thermocouples

  • P. H. W.
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A thermal e.m.f. is not only a function of temperature, it is also affected by pressure, a fact that is not as widely known. To the geophysicist studying chemical and physical behaviour at pressures of several thousand atmospheres correction for the effect of pressure is essential.

In a recently published paper I. C. Getting and G. C. Kennedy (1) describe a technique for determining corrections on single thermoelements where, under thermally symmetrical conditions, one-half of a homogeneous wire is pressurised while the other half is maintained at atmospheric pressure. This is a method previously used by Wagner (2), Bridgman (3) and Bundy (4) among others.

Getting and Kennedy’s apparatus consisted of a piston-cylinder device with talc as a solid pressure medium. The thermocouple wire was fed through a tube at atmospheric pressure to the thermal centre of the furnace where it entered the cell through a pressure seal. Inside the cell the wire was coated with binderless boron nitride to preserve electrical insulation yet ensure uniform pressure transmission. The temperature of both seals was recorded so that the temperature gradient along which the pressure was applied could be calculated. The cell was heated internally by a co-axial graphite heater.

A number of excursions were made into the pressure-temperature field ranging up to a maximum of 1000°C and 35 kbars. The thermocouples selected were Chromel: Alumel and platinum: 10 per cent rhodium-platinum since these are two of the most commonly used combinations. These thermocouples have previously been the subject of investigation by Bell et al. (5), and Hanneman and Strong (6) among others.

Extrapolation of the results obtained shows that Chromel: Alumel may read as much as 28°C high at 1200°C and 50 kbars while platinum: 10 per cent rhodium-platinum reads low by a similar amount at 2000°C and 50 kbars.

The authors acknowledge that pressure is only one of several factors affecting the e.m.f. generated by a thermocouple and observe that chemical contamination is a particularly difficult problem. Unfortunately their paper does not indicate whether any steps were taken to measure the degree of contamination and if so how their results were adjusted to allow for this factor. Curiously they also omit an explanation of the value of extrapolating to 2000°C when the platinum limb of a thermocouple normally melts at 1772°C. The reader is left to infer that pressure raises the melting point sufficiently to make this a meaningful exercise.

These criticisms apart the results should prove to be valuable to workers in this experimentally exacting field of research where reliable data on e.m.f. correction are still very scarce.

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References

  1. 1
    I. C. Getting and G. C. Kennedy, J. Appl. Phys., 1970, 41, ( 11 ), 4552 – 4562
  2. 2
    E. Wagner, Ann. Phys., 1908, 27, 955
  3. 3
    P. W. Bridgman, Proc. Amer. Acad. Arts Sci., 1918, 53, 269
  4. 4
    F. P. Bundy, J. Appl. Phys., 1961, 32, ( 3 ), 483 – 488
  5. 5
    P. M. Bell,, F. R. Boyd and J. L. England, Carnegie Inst. Wash. Yr. Bk., 1968, 66, 545 – 547
  6. 6
    R. E. Hanneman and H. M. Strong, J. Appl. Phys. 1966, 37, ( 2 ), 612–614; J. Appl. Phys., 1965, 36, ( 2 ), 523 – 528

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