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Fill Fluid Options for Pressure Instruments

pressure gauge | liquid filled gauge

Some pressure instruments require a fill fluid to help with pulsation and vibration in certain applications. Filling the cavity of a pressure gauge with liquid lubricates and dampens the internal components to reduce or eliminate the effects of heavy vibration and/or pulsation.

A system fill is commonly used in assemblies when one or more instruments are installed above a diaphragm seal. There are many types of fill fluid options available today, so which one is best for your application? 

This article will discuss different types of fill fluids and under what circumstances they should be used with your equipment.

Using Fill Fluids

First, let’s discuss what fill fluids are available. Fill fluids are primarily used as case fills or system fills. Case fills refer to liquid filling (usually glycerin, silicone or halocarbon) that are used in mechanical pressure or temperature gauges to help with pulsation and/or vibration issues.

System fill is used to transfer the process pressure below a diaphragm seal to the sensing elements of the instrument.

A diaphragm seal is used to isolate the instruments from the process media mainly to protect against clogging or corrosion. The sensing elements of many instruments are limited in materials and are not always compatible with the process media.

Diaphragm seals, which can be made of many types of materials whether they are metallic, plastic or elastomeric, require some form of fill fluid to transfer the pressure fluctuations to the sensing element(s) of the instrument(s).

Case Fill Fluid Types

For a case fill, ASME recommends two primary options for filling a pressure gauge: glycerin and silicone. Glycerin may be the most effective as it is generally the most viscous, which makes it a better dampening agent.

The downside to glycerin is the temperature window. If glycerin is exposed to process or ambient temperatures lower than 20 °F it may impact the instrument’s ability to stay in tolerance. Above 150 °F, the glycerin may discolor making the instrument difficult to read. Always note the temperature ratings of a fill fluid to ensure it fits your application from both a process and ambient temperature standpoint.

Silicone, although significantly less viscous, still makes a significant impact on the longevity of a gauge exposed to pulsation and vibration. As a case fill, silicone’s temperature window is -40 °F and as high as 200 °F. Both glycerin and silicone are commonly used in equipment all over the world.

Halocarbon is not as common, primarily due to cost. However, applications with oxidizing media require an inert fill like Halocarbon, in which oils are low molecular weight polymers of chlorotrifluoroethylene (PCTFNew call-to-action

System Fill Fluid Options

For a system fill, there are many more options used for various applications. Glycerin and silicone are also very common for system fills. In this case, however, the fact that glycerin is so viscous (1300 cSt) is more of a disadvantage. Materials like silicone with a lower viscosity (10-50 cSt) allow the instruments to be more responsive. Picture trying to stir honey versus a glass of milk.

There are other factors to consider when specifying a particular system fill. Because the system fill is above a diaphragm seal (a very thin material in which the risk of a rupture is a fair concern), it may be necessary to ensure that in the event of a rupture the system fill will not compromise the process media. For example, food and beverage, as well as pharmaceutical applications, often use food-grade options such as glycerin, (food-grade) silicone, distilled water or Neobee.

The most significant factor, however, is the temperature parameters of the fill fluid. Applications that require extremely high or extremely low temperatures require special silicone-based fluids such as Syltherm, which have formulas that can see up to 750 °F and formulas that can see down to -150 °F. It doesn’t need to be an extreme temperature situation for temperatures to be a concern.

Often, applications that involve sanitary conditions in combination with oxidizing media will call for distilled water as a more responsive and economical material (compared to halocarbon). Water has a very tight temperature window, freezing at 32 °F and boiling at 212 °F. Glycol mixed into the water can help with the freezing but it also negates the food-grade compliance.

We don’t like to pressure you, but we have more information. 

In summation, when choosing a fill fluid to protect your gauge from vibration and pulsation or to use a transfer fluid for diaphragm seal assemblies, consider the viscosity, temperature requirements and other special requirements (especially if it’s a sanitary application).

Now that you better understand the various fill fluids recommended for your applications, you can help protect your instruments from pulsation or vibration and use the best type for your process.

If you want more information on mounting accessories and assemblies, view our webinar about pressure instrument mounting assemblies or download our Complete Guide to Pressure Instrument Assembly.

We also have a few related blog articles:

Feel free to contact us with any questions or concerns. Our industry experts at Ashcroft can help answer your questions and get you back on track!New call-to-action

About John Girard, Product Marketing Leader

John Girard is currently the Product Marketing Leader for Diaphragm seals at Ashcroft. Formerly, he was the Territory Sales Manager responsible for supporting the distribution network in the Northeast. He began with Ashcroft as a Product Specialist for mechanical pressure gauges and accessories. He transitioned to a Sales Engineer role working with engineering firms on specifications. John achieved an M.B.A from Johnson & Wales University and has 10+ years of experience working with Sales and Marketing.