Pressure, temperature measurement at extremely low ambient temperatures

Standard pressure gauges and thermometers generally cope with ambient temperatures down to -40 °C. But what must be the features of a mechanical measuring instrument that works perfectly well in polar climates with even lower temperatures? This can be explained using the WIKA POLARgauge® as an example. This product family has been designed specifically for measurement at extremely low ambient temperatures down to -70 °C. The instruments are mainly used in crude oil and natural gas processing; for example, in the local pressure and temperature monitoring of pumping stations and pipelines.

Minus 40 °C – this value is often exceeded faster than one thinks. This is demonstrated with a look to Russia. There, tables with standard temperature values even indicate more than -60 °C for some regions. For mechanical instruments which will be operated in such ambient conditions, two critical points must be observed:

For pressure measurement at extremely low ambient temperatures only available with filled case: The WIKA pressure gauge model PG23LT.

1. The case filling

A measurement at extremely low ambient temperatures requires filled-filled measuring instruments. The liquids used for this protect the instrument from condensation formation within the case resulting from temperature fluctuations. They thus prevent the window from frosting, so that the measured value can still be read safely.

However, the majority of silicone oils used for this purpose begin to solidify at temperatures below -40 °C. The movement then no longer works, or can even freeze. Application-aligned monitoring of the parameters (e.g. process pressure) can no longer be ensured.

Therefore, for the POLARgauge® series a special low-temperature silicone oil has been specified. This oil remains in a state that enables correct measurement and measured value display, even at -70 °C.

The case of the POLARgauge® model PG23LT (r.) in comparison to a standard version (l.): It has no elastomers.

2. The elastomers

Elastomers are found in all mechanical pressure gauges or thermometers. They provide a secure sealing between the individual components. In detail, these are the flat gasket at the window, and with pressure gauges additionally the blow-out device on the rear of the case and the vent valve for pressure compensation.

However, the sealing materials used in standard designs cannot durably withstand very low ambient temperatures. They become brittle over time and lose their sealing properties. This results in the case filling escaping. This, in turn, leads to the formation of condensation and the frosting of the window.

Therefore, in the development of the POLARgauge® series, elastomers have been eliminated in all critical points. The seal between the window and case is made of a special material. It is adapted to extremely low ambient temperatures. The vent valve has a stainless steel cap.

Overview of the WIKA POLARgauge® series

 

Note
You can get further information on the POLARgauge® series on the WIKA website via the following links. The product family currently comprises four instrument types: The Bourdon tube pressure gauge model PG23LT, the differential pressure gauge models 733.51, 733.31 (filled versions of the models 732.51, 732.31), the bimetal thermometer model 55 and the gas-actuated thermometer model 73. Thus, a suitable variant is available for almost any application. If you have any further questions, your contact will gladly help you.



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