Pressure Transducer: Mercury Fill Sensors

A transducer is an electronic device that transforms energy from one form to another. With regards to Dynisco pressure transducers, the process fluid applies a pressure force on a narrow flexible metal diaphragm at the tip of the sensor.

In turn, this stresses a strain gage, which is a small sensing device. This strain gage then transforms the pressure stresses to a low-level millivolt signal, which can be further conditioned to yet another type of signal.

For a number of reasons, the optimal sensor design places the strain gage and the process fluid as close together as possible. Yet, the performance of the strain gage is often compromised due to the high temperature of the polymer melt in an extrusion: this takes the sensitive gage beyond its measurement capabilities.

To get the best performance from the strain gage without compromise, the sensing element is positioned remotely, thus separating it from the high temperatures. So, the challenge then becomes relaying the process pressure to the strain gage, without diminishing the integrity of the pressure force.

This is achieved by utilizing a transmission fluid that will reproduce the process pressure over a distance. The fluid must possess the physical properties to perform the task at hand.

A liquid-filled assembly is made up of a diaphragm that is in contact with the high temperature process material, an upper sensing diaphragm with the strain gage affixed to it, and a liquid-filled capillary tube that bridges the two diaphragms together.

The fill material can have pros and cons. Therefore, when selecting the liquid fill media, three things must be taken into account: 

  • Compressibility of the fill material relative to the maximum pressure range 
  • Operating temperature range relative to the max temp before the fill degrades or boils
  • Thermal expansion coefficient relative to the pressure drift with changes in temperature 

Mercury (Hg) is the most extensively used transmission medium for high temperature plastics processing applications where the polymer is melted and formed. Mercury's density means that it is extremely resistant to compression, so it will represent process pressures beyond 30,000 psi accurately.

Mercury is also extraordinarily insensitive to thermal expansion when exposed to heat. Further, mercury keeps its liquid properties above 350°C and below freezing temperature, resisting boiling, solidification, or vaporizing at temperatures that usually impact other liquid metals and fill materials. 

To date, there is no other material that offers the same performance benefits as mercury.

One disadvantage of mercury is that it is categorized as toxic. The US established the Clean Air Act in the early 90s to tackle mercury emissions in coal-fired power plants.

Follow-up regulations in the US and around the world have established programs to manage the use of mercury in all applications to the greatest possible extent. While alternative fill materials are available for pressure sensing, mercury remains unparalleled for polymer melt applications. 

Substitute fill medium includes the following:  

  • NaK is ranked non-toxic – Generally Regarded As Safe (GRAS) – by the USFDA and has high temperature limits, but it is not suitable for FM Explosion Proof areas because when exposed to air, NaK rapidly oxidizes (ignites). Also, NaK's compressibility is not as good as mercury, reducing the high-end pressure ranges to 10,000 psi as well as limiting the distance from the process that the strain gage sensor can be positioned. Dynisco offers NaK as an alternate fill.
  • Oil is also classified as non-toxic and Generally Regarded As Safe (GRAS) by the USFDA. Yet, as an organic compound, oil deteriorates at higher temperatures, leading to reduced life cycles and temperature limits. Dynisco can provide oil as an alternative fill for food and medical applications.
  • Alternative fill materials have demonstrated limited success. For instance, galistan (liquid metal gallium, indium, and tin) categorized as non-toxic has been used. Yet, unlike mercury, galistan is corrosive to and sticks to other metals – including the capillary tube - even when such metals are coated. The problem is further compounded at higher temperatures. Dynisco explored the potential of galistan but does not offer it as an alternate fill.

The volume of Hg in a 6” Rigid unit is ~0.0016 cubic inches. For Flex configurations, the additional volume is 0.00095 cubic inches/ft (i.e., the total volume of a 6/18 is ~0.003 cubic inches). 

Mercury maintains its position as the best technical alternative for precise, repeatable, and wide range pressure sensing applications at high temperatures. Dynisco also supports a reclamation program, recycling mercury from returned sensors. 

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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