A diaphragm seal is designed to protect expensive instruments. It operates by transferring the pressure effect of a process media through a membrane-like seal to a fill fluid that then transfers the resulting pressure wave to a pressure gauge or sensor at the head of the assembly for measurement. This allows accurate pressure readings without directly exposing the delicate workings of the pressure instruments to potentially damaging media.
Selecting the Right Diaphragm Seal
Adding a diaphragm seal to your instruments is recommended if the instruments are to be used to measure corrosive process media or very high or very low temperature media. Selecting the right diaphragm seal for a specific application, however, requires knowledge about the location of the instrument, process connection size and type, process pressure and temperature limits and any nontechnical limitations on fill fluids (such as nontoxic fill fluids for sanitary applications).
Choosing the Right Diaphragm Seal Fill Fluid
The diaphragm seal experts at WIKA note that fill fluid characteristics largely determine the seal system performance, both in terms of response time and temperature dissipation. Various characteristics of fill fluids must be taken into account in selecting an appropriate fill fluid.
As mentioned above, an initial consideration in choosing a diaphragm seal fill fluid is process media compatibility. The process media temperature and pressure are also important factors because all fill fluids expand or contract with changes in temperature.
Keep in mind that the fill fluid viscosity and density have a direct relationship with the measurement response time. A more viscous fill fluid leads to a longer response time. Also, a fill fluid with a higher density may have an impact on mounting.
Types of Fill Fluids
Glycerin, silicone oils and halocarbons are the most commonly used diaphragm seal fill fluids, but WIKA actually offers around 50 different fill fluids for specialized applications. Note that extremely high and low temperature (refrigeration) applications often require the use of a special fill fluid.
The engineers at WIKA also point out that a fill fluid other than glycerin is required for vacuum and compound gauges. Note that glycerin fill fluid should not be used when there is a capillary line between the instrument and diaphragm seal.
Don’t hesitate to contact WIKA technical sales if you have any questions about diaphragm seals or to learn more about how to find the ideal fill fluid for your measurement assemblies.
WIKA: We’re Not Just Pressure Gauges.