Mercury Fill Sensors
A transducer is an electronic device that converts energy from one form to another. In the case of a Dynisco pressure transducer, the process fluid exerts a pressure force on a thin flexible metal diaphragm at the tip of the sensor. This, in turn, stresses a small sensing device called a strain gage. This strain gage then converts the pressure stresses to a low level millivolt signal, which can be further conditioned to yet another signal type.
For several reasons, the best sensor design is to have the strain gage as close as possible to the process fluid. However, the high temperature of polymer melt in an extrusion application takes the sensitive strain gage beyond its measurement capabilities, compromising its performance. To get the most out of the strain gage without compromising it, the sensing element is located remotely, isolating it from the high temperatures. So the challenge then becomes transmitting the process pressure to the strain gage, without losing the integrity of the pressure force. This is accomplished by using a transmission fluid that will best replicate the process pressure over a distance. The fluid must have the physical properties to do the job.
A liquid-filled assembly consists of a diaphragm that is in contact with the high temperature process material, an upper sensing diaphragm that has the strain gage bonded to it, and a liquid-filled capillary tube connecting the two diaphragms. The fill material can have positive benefits and negative trade offs. When selecting the liquid fill media, three things must be considered:
- Compressibility of the fill material correlating to the maximum pressure range
- Thermal expansion coefficient correlating to the pressure drift with changes in temperature
- Operating temperature range correlating to the max temp before the fill degrades or boils
For high temperature plastics processing applications where the polymer is melted and formed, mercury (Hg) is the most widely used transmission medium. Mercury's density makes it highly resistant to being compressed, so it will accurately represent process pressures beyond 30,000 psi. Mercury is also remarkably insensitive to thermal expansion when exposed to heat. Further, mercury maintains its liquid properties at below freezing and beyond 350°C, resisting solidification, boiling or vaporizing at temperatures that affect other liquid metals and fill materials. To date there is not another material that provides the same performance as mercury.
A trade off for mercury is that it is classified as toxic. To address mercury emissions in coalfired power plants, the US established the Clean Air Act in the early nineties. Follow-up regulations in the US and other countries have created programs to control mercury in all applications to the greatest extent possible. While there are alternate fill materials available for pressure sensing, none can adequately replace mercury for polymer melt applications. Alternative fill medium included the following:
- NaK is classified as non-toxic, Generally Regarded As Safe (GRAS) by the USFDA and has high temperature limits, but it cannot be used in FM Explosion Proof areas because NaK rapidly oxidizes (ignites) when exposed to air. Also, NaK's compressibility is not as good as mercury, limiting 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 placed. Dynisco offers NaK as an alternate fill.
- Oil is also classified as non-toxic and Generally Regarded As Safe (GRAS) by the USFDA. However, as an organic compound, oil degrades at higher temperatures, resulting in limited life cycles and temperature limits. Dynisco offers oil as an alternative fill for food and medical applications.
- Other fill materials have been used, with limited success. For example, galistan (liquid metal gallium, indium, and tin) classified as non-toxic. However, unlike mercury, galistan adheres to and is corrosive to other metals - such as the capillary tube - even when those metals are coated. The problem is further compounded at higher temperatures. Dynisco investigated galistan but does not offer it as an alternate fill.
The volume of Hg in a 6” Rigid unit is ~ .0016 cubic inches. For Flex configurations, the additional volume is .00095 cubic inches/ft (i.e. the total volume of a 6/18 is ~.003 cubic inches). Mercury represents the best technical alternative for accurate, repeatable, high temperature, and wide range pressure sensing applications. Dynisco also supports a reclamation program, recycling mercury from returned sensors.