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. #expertinstrumentation #diaphragmseals #PartofYourBusiness

    6 Responses to
    1. Dan Pearce

      How is the capillary fill fluid internal pressure determined? I’m trying to determine if a diaphragm bursts what will happen to the transmitter out let pressure?



      • Chi-Che Ma

        Hello Dan,

        Thank you so much for reaching out to us about your question regarding diaphragm seals.
        The internal pressure of the capillary is the same as a process pressure pushing against the diaphragm. The fill fluid acts as a safe barrier to directly translate the process pressure onto the measuring instrument. If the diaphragm bursts, the process would then enter the seal cavity and capillary to directly apply pressure to the instrumentation. The system will still take readings at this point. However, depending on the process, the instrument might get damaged by the process through heat, corrosion, abrasion, etc, in which case the instrument would fail. If the breach is in the capillary line and not on the diaphragm, the system will lose all backpressure and the diaphragm will be pushed flushed against the cavity bed. This means there is no fill fluid to transfer the pressure back up to the instrument, so there will be no pressure reading in the system.

        I hope this answers your questions. If you have any other questions, please feel free to reach out to our customer service experts on wika.us website.

        Best Regards,

        Chi-Che Ma

    2. Jose John


      Can you please explain the selection criteria of fill fluid in diaphraghm seal gauge in vaccum applications like

      750 mm @ 70 Deg C



      • EVG

        Hello, Jose. Someone from the diaphragm seal division should be getting in touch with you soon, if they haven’t done so already. We appreciate your interest!

    3. Dario Xavier Pazmiño Moreno

      Estimados que fluido de llenado debería utilizar para un manómetro compuesto, es decir solo me comentan que no debo usar glicerina sin embargo no me especifican cual seria el mas adecuado.

      Estimates what filling fluid should I use for a compound pressure gauge, that is, they only tell me that I should not use glycerin, however they do not specify which would be the most appropriate.

      • Faith Bergeron

        Hi Dario. Thank you for your comment. A product manager should be reaching out to you shortly regarding your inquiry.

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