Platinum Metals Rev., 1977, 21, (1), 15
Superior Thermocouple for Cryogenic Use
A New Positive Noble Metal Thermoelement
As the applications of low temperature technology increase so does the need for reliable means of measuring the temperatures involved. Thermocouples using limbs of material specially developed for the purpose can give sensitivity reproducible results, but only over a limited temperature range. Alloys of pure gold containing very small amounts of iron, of the order of 0.07 atomic per cent iron, have large thermoelectric powers and are successfully coupled with either copper or Chromel arms for cryogenic purposes but both have disadvantages.
Investigations carried out by V. M. Beilin, L. A. Medvedeva, I. L. Rogel’berg and T. F. Tarasova of the State Scientific Research and Project Planning Institute of Alloys and Treatment of Non-Ferrous Metals, Moscow, and reported in Cryogenics, 1976, 16, (9), 551–552, have resulted in the production of a positive limb made from palladium-0.8 atomic per cent chromium-0.6 atomic per cent ruthenium. The alloy was prepared by arc melting, using a non-consumable tungsten electrode and an argon atmosphere. The resulting ingot was mechanically worked to 0.2 mm diameter wire, then vacuum annealed for 1 hour at 800°C. When used with the gold-iron negative limb the thermocouple has a sensitivity better than 15 μV/K over the temperature range 2 to 100K and the calibration is practically linear over this range, an important consideration for temperature control systems. Electromotive force against temperature measurements, and calculated sensitivity vs. temperature data are presented graphically. In each case the corresponding curves for copper/gold-iron and Chromel/gold-iron are also given.
Both limbs of this thermocouple consist of noble metals having stable reproducible properties thus suggesting that, as well as the obvious industrial applications, there could well be a future use as a reference standard.