Gold-alloy thermocouples

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. 

Gold-Iron Alloy Thermocouples. The increasing use of liquid hydrogen and liquid helium has created a demand for specialized thermometry below 25 K. Ordinary thermocouple combinations are only marginally acceptable because of their low sensitivity in this range. Dilute alloys of noble metals and transition metals, however, have relatively high temperature sensitivity below 25 K Au-2.1 at.% Co is perhaps the best known of this type. Unfortunately, this alloy is a supersaturated solid solution. Powell found that the... 

Platinum has many uses in dentistry. Pure platinum foil serves as the matrix in the constmction of fused-porcelain restorations. Platinum foil may be laminated with gold foil for cold-welded foil restorations. Platinum wire has found use as retention posts and pins in crown and bridge restorations. Heating elements and thermocouples in high-fusing porcelain furnaces are usually made of platinum or its alloys (see Platinum-GROUP metals). 

Tables 3.1-180-3.1-183 [1.216] and Figs. 3.1-240-3.1-242 [1.216,218] list the thermoelectric properties of gold and its alloys. Au—Fe and Au—Co-alloys are used in thermocouples for measuring very low temperatures [1.258], Au—Pd and Au—Pd—Pt alloys in thermocouples working under highly corrosive conditions.

Thermocouple types E, K, T, and KP versus Au-0.02, 0.07 at. % Fe can be used very well in the liquid oxygen range. If a gold-iron alloy is to be used above about 20 K, the positive thermoelement should be type KP. Use of the KP material allows the gold-iron alloys to be used from below the normal boiling point of helium up to room temperature. These couples have an extremely linear sensitivity above 20 K. Type E is recommended for general use when temperatures are not below 20 K. 

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