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10 2020
When choosing the liquid filling medium
- Compressibility of the filling material according to the maximum pressure range
- Thermal expansion coefficient associated with temperature changes and pressure drift
- Operating temperature range corresponding to the maximum temperature before filling or boiling
Mercury (Hg) is the most widely used transmission medium for high temperature plastic processing applications where the polymer is melted and formed. Mercury density is highly resistant to compression, so it will accurately represent process pressures above 30,000 psi. Mercury is also highly insensitive to thermal expansion when exposed to heat. Also, mercury maintains its liquid properties below freezing point and above 350 ° C, resisting solidification, boiling or evaporation at temperatures affecting other liquid metals and filler materials. To date, no other material provides the same performance as mercury.
Mercury swap is classified as toxic. To address mercury emissions from coal-fired power plants, the US established the Clean Air Act in the early nineties. Tracking regulations in the US and other countries have established programs to control mercury to the greatest possible extent in all applications. While alternative filler materials are available for pressure sensing, none can replace mercury for polymer melt applications. Alternative fill media included:
NaK is classified by the USFDA as non-toxic, Generally Recognized as Safe (GRAS) and has high temperature limits, but FM cannot be used in Explosion Proof areas because NaK oxidizes (ignites) rapidly when exposed to air. Also, the compressibility of NaK is not as good as mercury, limiting the upper pressure ranges to 10,000 psi and also limiting the distance from the process where the strain gage sensor can be placed. The MPS Melt Pressure Sensor offers NaK as an alternative filling.
The oil is also classified as non-toxic and is Generally Recognized as Safe (GRAS) by the USFDA. However, as an organic compound, oil degrades at higher temperatures, resulting in limited life cycles and temperature limits. The MPS Melt Pressure Sensor offers oil as an alternative filling for food and medical applications.
The Hg volume in a 6 ”rigid unit is ~ .0016 cubic inches.
Additional volume for Flex configurations is .00095 cubic inches / ft (ie 6/18 total volume ~ .003 cubic inches).
Mercury represents the best technical alternative for accurate, repeatable, high temperature and wide range pressure sensing applications. The MPS Melt Pressure Sensor also supports a reclamation program that recycles mercury from recovered sensors.