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| Standard Thermometers | |
| High Temperature SPRT Model 962 | |
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High Temperature SPRT 0.25 Ohm, 0°C to 970°C |
 From Databook 2 |
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 Data Sheet |
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| The extension, in ITS 90, of the platinum range to the freezing point of silver (961.78°C) has required the development of a new platinum resistance thermometer; acronym HTSPRT. The first problem of extending the range is insulation. Mica can not tolerate the higher temperatures. For example, for a 25.5Ohm thermometer, suppose that the shunt resistance were 20MOhm. Then the network resistance is 25.499967Ohm. But we require measurement assurance of better than 1 part per million, so this will not do, even if the shunt were a constant (calibratible) value, which it is not. For a 0.25Ohm thermometer, a 20MOhm shunt gives a network resistance of 0.24999997Ohm, which is tolerable. The cost, and there is a cost, is increased difficulty on the electrical measurement side, particularly in the presence of noise, at high temperatures. This new design of HTSPRT offers solutions to the problems identified by previous researchers. 1. Allowing the sensor winding to expand and contract without friction. 2. Keeping the leads close to the sheath. 3. Preventing the lead wires from expanding toward the sensor winding. 4. Allowing expansion chambers for the platinum leads. 5. Keeping a high degree of electrical isolation between leads. 6. Preventing heat piping up the sheath or lead assemblies. 7. A patented technique is used to prevent unwanted gases penetrating the quartz sheath at high temperatures and is included with each thermometer. PERFORMANCE We have seen various ways of specifying the reproducibility and short and long term stability of the SPRT. A user of thermometers will know that this varies with temperature excursion and roughness of use. We have decided that our best way to describe our standards is to explain that they will meet the expected criteria of calibration, as shown in the specifications issued by N.P.L. The fact that over 200 of these thermometers are in use world-wide testifies to the popularity of this version of high temperature thermometer. Its particular features are reliability and simplicity. The model 962 is supplied with a calibration certificate giving RTPW and Wga. Alternatively we can provide a complete calibration certificate with our UKAS Schedule. |
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