Choosing Assemblies for Low-Pressure Applications

Monitoring low-pressure processes comes with unique challenges. In industries ranging from food and beverage to wastewater treatment to liquefied natural gas (LNG) transfer, even modest changes in pressure can affect efficiency, product quality, or safety. Directly exposing instruments to these processes often leads to media buildup, corrosion, or inaccurate readings. That’s where using assemblies that combine pressure gauges, transducers, or switches with diaphragm seals or isolation rings can help.

Ashcroft, a leading authority in pressure and temperature instrumentation, has decades of experience helping industries overcome these challenges. As the product lead for diaphragm seals and accessories, I help customers find the best assembly for their specific needs. Read on to learn:

• How different industries and instruments define “low pressure”
• Which assemblies work best with gauges, mechanical switches and sensor-based instruments
• How assemblies support applications across sanitary, industrial, utility and cryogenic environments — with LNG as a primary example

When you are finished, you will know how to ensure accurate measurement and keep your operations running efficiently with the right seal or isolation ring assembly.

What does “low pressure” actually mean?

The definition of “low pressure” depends on who you ask and which instrument or industry you’re working with. At Ashcroft, we define low pressure as anything below 15 psi. This threshold matters because diaphragm flexibility, seal fill fluids and instrument sensitivity all affect accuracy below this point.

However, in industry terms, HVAC or wastewater can also define “low pressure” as anything under 30–50 psi since those systems normally run higher. Finally, there are also instrument variations: 

• Pressure gauges: often anything under 15 psi.
• Switches: can go much lower, down to inches of water column (inH₂O) set points.
• Sensor-based instruments, such as transducers, generally require a minimum span of 15 psi when paired with a diaphragm seal.

These variations are why it’s important to choose the right assembly for your exact application. What qualifies as “low” in one industry may be considered normal in another, and the wrong pairing of seal and instrument can compromise performance.

How do assemblies support different industries?

Every industry faces different challenges when monitoring low pressure. Assemblies provide tailored solutions across a wide range of applications. When selecting assemblies, material compatibility is just as important as pressure range. Using diaphragm seals made from corrosion-resistant alloys such as 316 stainless steel, Monel® or Hastelloy® helps prevent instrument damage from aggressive or abrasive process media. 

Here’s how assemblies ensure accuracy, protection and compliance across critical applications.

Figure 1: Industry assembly challenges

In cryogenic environments, selecting wetted materials resistant to embrittlement — such as 316 stainless steel or Monel® — helps maintain long-term performance. To see detailed compatibility data to help with material selection for these specific conditions, refer to the Ashcroft® Material Selection and Corrosion Guide.

What are the instrument considerations for low-pressure assemblies? 

Selecting the right assembly isn’t just about matching a diaphragm seal to an instrument. It’s also about knowing the capabilities and limitations of each type of device. Low-pressure gauges, switches and sensor-based instruments all behave differently when paired with seals and isolation rings, and each has specific requirements. 

In this section, we outline some recommendations and restrictions for 4 instrument types. The Ashcroft product examples are used to illustrate how these rules apply in practice, helping you avoid misapplications and ensure your assemblies perform accurately and reliably. For a full reference, read the Ashcroft Diaphragm Seal Pressure & Temperature Min/Max Guide.

1. Pressure Gauges

Pressure gauges are widely used for local indication,  giving operators a direct, real-time reading of system pressure right at the process line. They’re simple, reliable, and require no external power, making them ideal for monitoring lines, vessels, or systems where visual feedback is critical.

Recommendations for use in low-pressure applications

• Bellows gauges are a good choice for applications where accuracy at extremely low ranges is critical, such as tank level monitoring or low-pressure gas systems. 
• For example, a gauge like the Ashcroft® 1188 Low-Pressure Bellows Gauges requires a minimum span of 60 inches of water (~2.2 psi), making it ideal when small changes in pressure must be detected.
• These gauges can be paired with the Ashcroft® 200/300 Series Viton™ or Kalrez® diaphragm seals or the Ashcroft® 740/741 Series metallic seal, which provides the flexibility needed for such low spans. 

Restrictions for use in low-pressure applications

• Don't use vacuum or compound ranges at these spans with metallic diaphragms because metallic materials lack the flexibility that elastomers provide.
• Avoid using any general-purpose or commercial gauges that are not designed for seal assemblies.

2. Pressure Switches

Pressure switches are designed for control and safety functions, automatically triggering an alarm, pump, or shutdown when pressure crosses a set limit. For instance, you will see them used in low-pressure processes that depend on stable pressure conditions, such as filtration systems, pump control, or pressure safety interlocks.

Recommendations for use in low-pressure applications

• Choose switches that can operate at very low set points, even down to inches of water column (inH₂O), to ensure precise control and early detection in applications where minimal pressure changes can impact safety, efficiency or product quality. 
• For example, the Ashcroft® Pressure Switch Series A, B, G, L and P can function at these extremely low set points, ensuring your process will operate reliably in sensitive, low-pressure environments. (See Figure 2 below.)
• Pair the pressure switch with a high-displacement diaphragm seal like the Ashcroft® 740/741 to achieve the desired low set points. Or, you can pair switches with flexible diaphragms like those found in the Ashcroft® 200 Series, offered with Viton™ or Kalrez®. These materials are used when setpoints are between 6 - 20 Inches of Water

Note: Set points higher than 6 psi can use any seal or isolation ring material. 

Figure 2: Minimum set points for mechanical pressure switches

Restrictions for use in low-pressure applications

• Do not set switches below their published limits. Some ranges are not recommended for remote mounting with a capillary because of signal dampening. 
• Avoid using incompatible fill fluids, like glycerin, which cannot be used in spans under 15 psi or in vacuum service. 

3. Sensor-Based Instruments (Digital Gauges, Transducers, Electronic Switches)

Sensor-based instruments are typically used in automated systems that require continuous monitoring, remote control, or data integration. They provide electrical outputs for control systems, making them ideal for modern process facilities, sanitary applications and critical environments like LNG or pharmaceutical production.

Recommendations for use in low-pressure applications

• Sensor-based instruments are the best choice when you need precise data and integration with control systems, such as in automated LNG or sanitary processes.
• For example, the Ashcroft® E2 Sanitary Pressure Transducer, Industrial Digital Gauges and NPI Electronic Pressure Switch deliver high outputs for advanced system integration.
• These instruments should only be used in applications with a minimum span of 15 psi when paired with diaphragm seals.

Restrictions for use in low-pressure applications

• Do not specify sensor-based instruments for spans below 15 psi, as accuracy cannot be maintained.
• Avoid overlooking temperature effects — using the wrong fill fluid can lead to calibration drift, especially at the low end of the pressure range.

 Figure 2: Low-pressure instrument assemblies comparison

Selecting the proper assembly isn’t just about protecting the instrument — it’s about ensuring your readings remain accurate and reliable over time. The wrong pairing of seal, instrument and fill fluid can lead to drift, clogging or even premature failure. 

In addition to proper sizing, material and fill fluid selection are critical factors in low-pressure assemblies. The Ashcroft® Corrosion Guide recommends matching seal materials and elastomers to the process fluid to prevent swelling, hardening or corrosion, which can lead to inaccurate readings or premature failure.

By considering a few key factors, you can narrow down the best assembly for your low-pressure environment.

• Match the pressure span of your application to the diaphragm seal’s displacement.
  
  
• Select the instrument type (gauge, switch or sensor) based on your specific needs.
  
  
• Consider media compatibility, temperature ranges and cleaning needs.
  
  
• Use Ashcroft’s expertise and selection tools to avoid trial-and-error and get it right the first time.

For pressure gauges, following ASME B40.100 guidelines, operating pressure should fall within the middle 25–75% of the gauge scale, and the instrument’s full-scale pressure should be approximately twice the expected operating pressure. This ensures both accuracy and safety in low-pressure measurement

Ready to learn more?

Now you’ve seen how low-pressure assemblies protect instruments, improve accuracy and adapt to the unique challenges of industries from sanitary processing to wastewater treatment to LNG transfer. You’ve also learned how different instruments — gauges, switches and sensors — can be paired with diaphragm seals or isolation rings to create reliable assemblies that fit your application. For a deeper dive into related topics, you may also find these articles helpful:

• What’s the Right Pressure Instrument Mounting Assembly for Me?
• Accurate and Reliable Isolation Ring Assemblies for Water/Wastewater Applications
• When to Use a Welded Instrument Assembly
• Why Did My Pressure Equipment Fail? 6 Instrument Killers

Contact us today to talk to one of our industry experts and get all your questions answered. In the meantime, download our guide to pressure instrument assembly.