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.

Citations

Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Dynisco. (2021, September 07). Pressure Transducer: Mercury Fill Sensors. AZoSensors. Retrieved on December 12, 2024 from https://www.azosensors.com/article.aspx?ArticleID=2204.

  • MLA

    Dynisco. "Pressure Transducer: Mercury Fill Sensors". AZoSensors. 12 December 2024. <https://www.azosensors.com/article.aspx?ArticleID=2204>.

  • Chicago

    Dynisco. "Pressure Transducer: Mercury Fill Sensors". AZoSensors. https://www.azosensors.com/article.aspx?ArticleID=2204. (accessed December 12, 2024).

  • Harvard

    Dynisco. 2021. Pressure Transducer: Mercury Fill Sensors. AZoSensors, viewed 12 December 2024, https://www.azosensors.com/article.aspx?ArticleID=2204.

Comments

  1. Marc-André Gingras Marc-André Gingras Canada says:

    Hi does anyone know what happens to mercury if compressed beyond its capacity? Does it become solid state?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoSensors.com.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Your comment type
Submit

While we only use edited and approved content for Azthena answers, it may on occasions provide incorrect responses. Please confirm any data provided with the related suppliers or authors. We do not provide medical advice, if you search for medical information you must always consult a medical professional before acting on any information provided.

Your questions, but not your email details will be shared with OpenAI and retained for 30 days in accordance with their privacy principles.

Please do not ask questions that use sensitive or confidential information.

Read the full Terms & Conditions.