Isotech 108462 copper point SPRT model

The ITS-90 specifies the Isotech 108462 copper point SPRT to the freezing point of silver, 961.78°C. Thermocouples can be used outside of this temperature, but it is difficult to see small changes in temperature. For a type R thermocouple, the voltage sensitivity at the copper point is 14µV/°C; a change of 1µV equals a voltage change of 71mK. When using thermocouples the electrical noise limits the ability to follow small changes in the copper freeze plate and therefore attention was focused on the development of a new resistance thermometer to allow better measurements.

Isotech has a long history of making SPRTs up to the Silver Point (Model 96178) and this experience was combined with new research to produce the new copper point SPRT (Model 108462).

Ro is nominally 0.25Ω the same as the Silver Point SPRT but the platinum winding is held in place on a new type of synthetic sapphire mandrel. The platinum ‘likes’ oxidizing but ‘hates’ reducing atmospheres. The jacket of the thermometer is made of aluminium oxide. It is filled with air and thus surrounded by 20% oxygen. Unique is that the jacket is connected to a small air pump to pressurize the 108462 with air. This allows any leakage to come out, while maintaining an oxygen-rich atmosphere around the winding. This is what gives the thermometer its stability.

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