What’s the Right Pressure Instrument Mounting Assembly for Me?

This article was originally published on June 4, 2021, and was updated on March 10, 2025.

Choosing the right assemblies for your pressure instruments can seem overwhelming. There are so many factors to consider and endless combinations of products and configurations that will influence what you need for your specific application.

So, how can you get the proper assemblies to suit your project’s needs? How do you start looking in the first place? These are questions that come up a lot in my discussions with customers. Luckily, my 20 years of industry experience, including the past five at Ashcroft, gives me the ability to share my expertise in this particular area. 

Read this article to learn the top five factors to consider when researching pressure instrument mounting assemblies, and how to choose the right assemblies that can protect your equipment and ensure successful processes for your specific application.

Why do you need a pressure instrument mounting assembly?

You may have started searching and found a variety of pressure instrument mounting assembly variations, so how can you find the right ones for you? You should start by asking what you’re hoping to accomplish and what your project needs to be efficient and accurate. These are just a few of the main considerations to keep in mind before you buy:

1. Minimize temperature errors with the right pressure instrument mounting assembly

If your process pressure is less than 100 psi, you may encounter temperature errors. To prevent or reduce these errors from occurring, there are a few solutions.

• Use low-volume street fittings to reduce the overall fill volume inside the assembly and minimize the impact of thermal expansion of the fill fluid on the accuracy of the assembly. Typical street fittings are not low volume, so look for a fitting that minimizes internal volume. Learn more about the benefits of low-volume fittings. 
• Minimize the number of fittings used and make sure all of your instrument connections are the same size (e.g., all ½ NPT or all ¼ NPT).
• Use compact instrument tees or cross fittings as another option that eliminates the need for “goalpost” configurations, which are bulky, expensive, and prone to damage. These accessories will reduce the internal volume of the assembly and reduce the temperature error of the instrument(s) attached.
• Dissipate process temperature with a minimal length capillary. A simple 6-inch length of capillary line is capable of reducing process temperature to the instrument(s) with elevated temperatures from 750 °F (300 °C) to less than 100 °F (38 °C).

Note: if you have an elevated process temperature, consider a siphon, which is an accessory that is specifically designed to reduce temperature. Some siphons can reduce the process temperature of the instrument(s) from 800 °F (427 °C) to less than 150 °F (66 °C). Learn more. 

2. Select proper ranges for your pressure instrument assembly

Multiple instrument assemblies, specifically gauge/switch combinations, work best when the nominal instrument ranges are similar. Ideally, the operating range of the gauge pointer should be mid-scale of the full-scale pressure range. In some scenarios, this might not be possible. For example, an assembly might need to actuate a switch at very low pressures relative to the pressure range on an indicating instrument.

In cases like this, ensure that the lower-pressure instrument has a proof pressure that exceeds the indicated range of the other instrument, or use a pressure limiting device or pressure limiting valve to protect it.

3. Protect your instrument from vibration and pulsation

If your mountings are installed close to a pump system, the pulsing surges of pressure can wear down the instrumentation and make it difficult to get a proper reading. Similarly, vibration can interfere with readability. To extend the life of a gauge, consider a liquid-filled gauge case. Liquid filling dampens the effects of process vibration and pulsation. Liquid-filled gauges typically come standard with a throttle device, which further dampens the effects of pulsation or process pressure surges.

An alternative to a liquid-filled gauge is a dampened movement gauge like Ashcroft® PLUS! Performance. This is a dry gauge case that works like a liquid-filled gauge with none of the liquid-fill headaches. The use of accessories like pressure snubbers, capillaries and pulsation dampeners is effective in dampening the effects of pressure pulsation.

4. Select the right fill fluid for your pressure instrument assembly

Glycerin is the most common fill fluid for single-instrument mountings. However, low-volume fittings and many types of accessories have narrow or constricted pathways. It’s best not to use a thick, viscous liquid like glycerin in these applications. Glycerin is typically 1,300 centistokes (cSt), while a more appropriate thinner liquid option such as silicone is typically 10 to 50 cSt.

If the process media is a strong oxidizer, the best practice is to use an inert fill fluid such as halocarbon for safety. Also, consider process temperature when selecting fill fluid. Fills like glycerin have temperature limits of 0 °F to 400 °F (-18 °C to 204 °C). Silicone temperature limits are -40 °F to 500 °F (-40 °C to 260 °C). Other fill fluids are available to meet process temperatures beyond these limits or special application services.

5. Ensure instrument and diaphragm seals or isolation rings are compatible

There are physical limitations to the pressure that isolators can sense and transmit to attached instruments based on the size and flexibility of the diaphragm. The range being measured must be above the minimum for that isolator. To find instrument compatibility with an isolator, view our min/max guide. When multiple instruments are attached to a single isolator, the minimum pressure ranges may differ.

For multiple instrument assemblies with ranges close to these minimums, it is best to confirm with the manufacturer that the assembly will work properly.

What pressure instrument mounting assembly options are available?

The labeling and identification of various mounting assembly options vary between manufacturers in the industry and there isn’t a standard identification system. However, Ashcroft created a Complete Assembly Guide as a comprehensive resource that describes each of our mounting variations. It includes depictions of assemblies, easy lookup by orientation and connection size, and instructions for correctly coding and ordering mounting assemblies.

 Once you've selected what you need, consider the following options:

• Instrument welded to isolators: Instrument welding creates a tamper and leak-proof assembly and is used when absolute leak integrity is critical for the safe operation of your process. Welded surfaces must be of like materials. Instruments attached directly to isolators can be welded, and assemblies with capillaries or siphons can be welded at each connection. Certain fittings (e.g. tees, nipples) cannot be welded, nor can snubbers, dampeners, or PLVs.

• Tamper evident sealant: Tampering with fittings can disrupt the fill fluid inside an assembly, causing instruments to read inaccurately or create unwanted shifts in setpoints. A brightly colored, tamper-evident sealant applied at each threaded joint in an assembly signals to an operator that they should not adjust these fittings and can serve as a quick diagnostic check if accuracy issues do arise.

• Elbow for vertical piping: When instruments are installed onto vertical pipe runs, consider using an elbow installed at an isolator for better readability of the instruments.

Installation constraints sometimes need custom configurations that aren’t covered by standard assembly offerings. You may require longer fittings, angled connectors or special instrument orientations to ensure that the instruments are readable, and the assembly doesn’t interfere with nearby equipment.

Ready to learn more? 

Now that you have an idea of what instrument assemblies are available and what they can be used for, you can find the right solution for your application. If you want more information, view our webinar to learn more about pressure instrument mounting assemblies.

Also, here are a few related articles that may interest you:

• Do Measurement Instrument Accessories Affect Accuracy?

• Alternatives to Liquid-Filled Pressure Gauges for Vibration Protection
• When to Use a Welded Instrument Assembly
• Isolation Ring Assemblies for the Water/Wastewater Industry that are Accurate and Reliable
• Best Practices for Installing Flanged Diaphragm Seals

Feel free to contact us with any questions or concerns. In the meantime, take a look at our guide to instrument assemblies to help choose the right one. For assemblies not included in our guide, reach out to Ashcroft for assistance setting up a custom pressure instrument mounting assembly to perfectly suit your needs.