What are Mercury Fill Sensors?

Transducers are electrical systems that are capable of converting one form of energy into another. For example, a pressure transducer converts pressure into an electrical signal, which can be used to determine the extent of the pressure that the system is experiencing.

In the case of a pressure transducerfrom Dynisco the process is as follows:

  1. The surrounding fluid exerts pressure on a thin and flexible metal diaphragm located in the tip of the sensor
  2. The motion of the diaphragm in response to the pressure stresses an associated strain gauge
  3. The stress on the strain gauge is converted into a small electrical (mV) signal, which can be further processed if needed

Challenges and Solutions

There are a number of different reasons why it is important to locate the strain gauge as close as possible to the strain fluid however, this is not always possible. In extrusion applications the high temperature of the polymer fluid compromises the strain gauges ability to take accurate measurements.

To solve this problem the gauge’s sensing element is remotely located, where it is unaffected by the high temperature. However, the challenge now becomes how to transmit the pressure signal to the sensor without a loss of information. This can be achieved using a transfer fluid that can transfer the pressure experienced at the diaphragm to the sensor.

The transfer fluid needs to have the correct physical properties to achieve this. Such a system has two diaphragms – one in contact with the high temperature process fluid (e.g. polymer) and another in contact with the transfer fluid, which has the strain gauge attached.

The transfer fluid can bring both advantages and disadvantages to the system. When choosing which transfer fluid to use the following points should be considered:

  • What is the thermal expansion coefficient and how does this impact the pressure transfer at different temperatures?
  • How compressible is the fluid and does this fit with the maximum pressure range?
  • What is the boiling point/breakdown point of the fluid and is this below the operating temperature range?

Mercury: The Most Popular Choice of Transmission Fluid

In applications such as extrusion where high temperature polymers are used mercury (Hg) is the most popular choice of transmission fluid.

Mercury does not display significant thermal expansion and is very dense, which makes it compression resistant; meaning it can accurately transfer high pressures, even beyond 30,000 psi. In addition it also stays as a liquid over a wide temperature range (from below freezing to above 350 °C) meaning it does not solidify, vaporize or boil at the processing temperatures used.

Currently there is no other transfer fluid that can perform as well as mercury, however; mercury has the huge disadvantage of being toxic. Due to the toxicity of mercury it is highly regulated by the Clean Air Act in the US and similar legislation in other nations.

Types of Transfer Fluids

Whilst there are no true replacements for mercury the following transfer fluids can also be used in high temperature polymer applications:

Oil is a non-toxic medium that is Generally Regarded As Safe (GRAS) by the USFDA. When used at higher temperatures oil can degrade due to its organic nature, this limits its use in high temperature applications and means it can have a short lifetime. Dynisco can provide oil as a transfer fluid for medical and food applications.

NaK is also non-toxic and Generally Regarded As Safe (GRAS) by the USFDA. Unlike oil NaK can be used at high temperatures however, it can explode when in contact with air meaning it can only be used in FM Explosion Proof areas. In addition, the compressibility of NaK does not match mercury meaning it can only be used up to pressures of 10,000 psi and with short inter-diaphragm distances. Dynisco can provide NaK as an alternative transfer fluid.

Other transfer fluids have also been used, but with mixed success. Galistan (a liquid metal mixture of indium, gallium and tin) is non-toxic, however it can stick to and corrode other materials, including the tube it is contained in, and this can occur even when the metal is coated. When the galistan is at a higher temperature this problem is even worse. Following assessment of galistan, Dynisco decided not to offer it as an alternative transfer fluid.

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 is the best known transfer fluid for use in accurate and reliable pressure sensing in high temperature applications.

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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