When protecting your pressure instruments from harsh process conditions, isolators such as diaphragm seals or isolation rings are essential. They are designed to effectively protect pressure gauges, switches and transmitters from corrosive media, particulates, pulsations and extreme conditions. However, the effectiveness of these assemblies relies heavily on one important factor: proper filling.
That’s where the challenge comes in. At Ashcroft, we’re often asked whether it’s acceptable to add fill fluid to a diaphragm seal or isolation ring in the field. On the surface, it may sound like a quick fix for a leaking or underfilled assembly. But in reality, attempting to add fluid outside of a controlled process may be problematic.
Read this article to understand the science behind why isolator fills matter, why field filling may add risk and what you can do to keep your instruments reliable. By the time you finish reading, you’ll know why field filling is risky and the steps you can take to get accurate, safe readings that last for years.
What is the role of fill fluid in isolation devices?
Isolation assemblies, such as diaphragm seals and isolation rings, are effective devices for protecting your pressure instruments from clogging, corrosion and in some cases, to accommodate extreme temperatures. A critical part of these systems is the fill fluid, which transfers process pressure from the isolator to the sensing element.
Fill fluids are carefully chosen depending on the application. For example,
- Silicone has a wide temperature range, is responsive and is commonly used in general industrial applications or processes where temperature swings are frequent.
- Glycerin is thick and stable, providing excellent dampening in applications with heavy pulsation, such as pumps or compressors, but is limited in high- or low-temperature environments, as well as in low-pressure and vacuum applications.
- Halocarbon is inert and essential for oxidizing media, making it a go-to choice in chemical and chlorine service where other fluids could react dangerously.
- Application-specific fluids for specialty applications are also in the mix. These include fluids such as food-grade silicone or glycerin for sanitary processes in food and beverage or pharmaceutical production, high-temp options like Syltherm (up to 750 °F) for extreme heat, and low-temp formulations (down to -150 °F) for cryogenic processes.
If any air is trapped in the assembly, it compresses under pressure, which throws off your calibration. Over time, that error only gets worse as the process temperature changes, pressure spikes occur, or pulsations hit the system.
Why does controlled filling matter?
There’s a reason Ashcroft has used a precise filling process for decades—it reduces risk. Here’s what happens behind the scenes when your isolator assembly is filled properly:
- Assembly: Components are put together with the right thread sealant to prevent leaks. If more than one instrument is used, they’re matched for range to avoid overpressure during calibration. Throttle plugs, typically used for pulsation control, are removed to avoid restricting a proper fill. If a capillary is used, a lower viscosity fill fluid (10 cSt or less) is recommended.
- Vacuum Evacuation: Once assembled, a fill port is connected to a vacuum pump, which runs for an extended period of time to pull every trace of air out of the system. The timing largely depends on the strength of the vacuum pump.
- Fluid Preparation: Air removal is also critical with the fill fluid prior to filling. This is evident during the removal process as you can literally see bubbles foaming out as the air escapes.
- Filling: Once a full vacuum is executed, the fill fluid is introduced and pulled into every nook and cranny of the assembly. Occasionally, seal assemblers (even the most experienced technicians) need to repeat the process for good measure.
- Calibration: Once filled, the instruments are checked using NIST traceable calibration equipment. Gauges are tested at multiple points across their range. Switches are exercised several times and setpoints verified. Transmitters are checked across all outputs. ASME B40.100 advises that accuracy degradation on a seal assembly is typical. At Ashcroft, we typically allow for an extra 0.5% tolerance on instruments when installed on an isolator.
This process may not sound simple, but it is a necessary practice to ensure reliable instrumentation. Instrumentation that is designed to protect operators from harmful situations. Instrumentation that is designed to protect expensive equipment, such as pump systems and chemical feed skids. Instrumentation that provides critical information about the process to make sure that ratios are acceptable, tanks are full, and operations are running as expected.
What are the risks of field filling?
When you try to fill in the field without a vacuum, you’re taking some risks, including:
- Reduced Accuracy: Even a small bubble can cause drift, slow response, and poor or inconsistent calibration results.
- False Fills: If the fill fluid to be used has not been fully evacuated of air, tiny (and somewhat benign) bubbles will enter your assembly. Calibration might look fine at first, but these bubbles can eventually merge into a larger (and now destructive) bubble. This in turn can cause calibration problems hours, days or weeks later.
- Affect on Sensitive Instruments: Low-span gauges or low-setpoint switches are especially vulnerable to hazardous conditions such as temperature fluctuation, the effects of which are amplified with entrapped air.
- Safety Hazards: Bad data can compromise not just equipment but also operator safety and process quality.
Why would you be tempted to field fill?
Instruments installed in the field that require fill-fluid adjustment are more than likely presenting bigger underlying issues. For instance:
- The assembly could have been improperly filled by the supplier during the initial assembly.
- It may be an example of “false fill”, in which case field-filling would at best only mask the problem.
- There could be a leak through the sensing element. If this is the case, field filling is temporary and does not address why the sensing element is leaking in the first place.
Assemblies are built to last years, even decades, without losing fill. If they don’t, the fix isn’t to add more—it’s to figure out what went wrong.
How can you solve the real fluid fill issue?
Instead of topping off fluid, you’re better off proactively addressing the actual problem, which could be any of the following:
- Material issues: Upgrade to a more resistant material if corrosion or abrasion has compromised the seal.
- Temperature issues: Add dissipation devices or switch to welded assemblies for protection and durability.
- Operator issues: Welded connections can also prevent overtightening or loosening, which often leads to leaks.
Ready to learn more?
The accuracy of your pressure instruments depends on the integrity of the fill. The controlled vacuum process is critical. If you try to field fill, you’re risking instrument accuracy, reliability and safety.
For more information about isolation devices and liquid fill challenges, here are a few articles that may interest you.
- When to use a diaphragm seal vs. an isolation ring
- Alternatives to liquid-filled pressure gauges for vibration protection
- What factors affect a diaphragm seal's performance?
- How do I safely select diaphragm seals for high-temperature applications?
- Why use a flushing connection on a diaphragm seal?
Feel free to contact us with any questions or concerns. In the meantime, download our guide to learn more about selecting an instrument assembly solution for your specific needs.
