首页|Trials of Pt-20%Rh versus Pt thermocouples between 157 degrees C and 962 degrees C
Trials of Pt-20%Rh versus Pt thermocouples between 157 degrees C and 962 degrees C
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NSTL
Iop Publishing Ltd
For over a century the Type S (Pt-10%Rh) and R (Pt-13%Rh) thermocouples have been used as primary or secondary reference thermometers. However, the measurement uncertainties of both types at the silver point (962 degrees C) are typically limited to about 0.5 degrees C, due to drift caused by crystallographic ordering between 200 degrees C and 500 degrees C and rhodium oxidation between 500 degrees C and 900 degrees C. Although both processes can be reversed using an 1100 degrees C anneal, regular annealing can be inconvenient or impracticable. This paper follows up a prior study indicating that an alternative noble-metal thermocouple comprised of Pt-20%Rh and Pt is relatively insensitive to both drift mechanisms. Four thermocouples, assembled using wire from four different manufacturers, were evaluated using a gradient-furnace and homogeneity scanner. Measurements were also made using a salt-bath (200 degrees C to 500 degrees C) and fixed points between the indium and silver points. The experiments indicate that individual thermocouples are stable at the silver point to within 0.18 degrees C for periods of up to 100 h and all four thermocouples have emf versus temperature characteristics within 0.3 degrees C of each other. This intrinsic stability and similarity, coupled with cost, assembly and use conditions identical to a Type R or S thermocouple make the Pt-20%Rh versus Pt thermocouple an attractive alternative. Recommended annealing procedures enabling the greatest stability are also given.
NSTL
Iop Publishing Ltd
