This article was originally published on August 16, 2021, and updated on February 12, 2025.
Pressure gauge accessories like snubbers, dampeners, capillaries, pressure-limiting valves, and siphons play an important role in keeping your processes running safely and efficiently. However, while these devices help ensure your pressure instruments maintain optimal performance in the most challenging applications, there is an underlying concern about their potential impact on pressure measurement accuracy.
As the product manager for pressure gauges and accessories at Ashcroft - a leader in pressure and temperature instrumentation, I am often asked for reassurance that these products don't interfere with gauge performance. In this article, you will learn about the factors that can affect pressure measurement accuracy, and the accessories that help, not hinder, instrument performance.
Factors that can impact pressure gauge accuracy?
As we explained in our article titled, 6 Reasons Pressure Gauges Fail, many factors can impact the accuracy of pressure instruments, including:
- Excessive pulsation/vibration: This occurs when there are rapid fluctuations in pressure, causing the gauge needle to flutter and potentially leading to inaccurate readings or damage to the instrument.
- Extreme low/high temperatures: Instruments exposed to temperatures outside their operating range can experience material expansion or contraction, affecting their accuracy and potentially leading to failure.
- Pressure spikes: Sudden pressure increases can push the gauge beyond its limits, risking permanent damage and inaccurate measurements.
- Instrument clogging: Accumulation of debris or particulates can block the instrument's pathways, hindering its ability to measure pressure accurately.
- Instrument corrosion: Exposure to corrosive substances can degrade the materials of the instrument, compromising its structural integrity and measurement accuracy.
The good news is the accessories we cover below will review the devices and instrument assemblies that are designed to help mitigate the negative effects of these issues on your pressure instrumentation. Knowing how these accessories work can help you understand how they may impact accuracy.
What you need to know about pressure gauge accessories
The ASME B 40.100 Standards Guide identifies several types of accessories that protect the performance and integrity of pressure gauges from the harsh conditions highlighted above. These accessories may also help with other pressure-measuring instruments such as pressure switches and transducers.
1. Flow restrictors protect against excessive pulsation and vibration
Pulsation can damage equipment or cause inaccurate readings. In processing equipment, surges in process pressure can lead to pulsation in gauges. These pulsations can cause poor gauge readability due to pointer flutter and decreased gauge integrity due to component damage. The following flow restrictors are designed to help with this challenge.
- Pressure snubbers and pulsation dampeners are used to lessen the effects of pulsation. These accessories restrict the flow of the process material to reduce pulsation before it reaches the instrument.
- Needle valves can change the size of the opening on a pressure instrument to counteract pulsation. Similar to a spigot on a garden hose, these devices can be opened wide or closed to control flow. Note that some accessories are more adjustable than others.
- Throttle screws can also change the size of an opening. It threads into the communication hole of a pressure gauge and shrinks it down from a standard hole to a pinhole, which effectively controls the impacts of pulsation.
2. Coil siphons and capillaries offer protection from steam and extreme temperatures
Instruments also need to be protected against extreme temperatures. Here are two examples of accessories that can help.
- Coil siphons are designed for steam applications. They help protect gauges from high-temperature steam by creating a condensate barrier and isolating the instrument from the steam.
- Capillaries, on the other hand, are designed to keep instruments safe from damage in extreme temperature applications. Because one end of the capillary connects to the process and the other connects to the gauge, you can mount the gauge a safe distance away from the extreme temperatures of the process. They also help reduce the temperature by reducing the volume of the media passing through and allowing the ambient temperature to cool (or heat) the media.
Because these devices have a small communication hole, they can also act as effective flow restrictors to help with pulsation. Plus, they offer flexibility in mounting, allowing instruments to be moved to more stable areas if there is heavy vibration. However, they do have the potential to get clogged easily. In applications that would be prone to clogging, you may need to protect the capillary with an isolation device such as a diaphragm seal or isolation ring.
3. Pressure-limiting valves help prevent pressure spikes
A pressure surge can push a gauge beyond an instrument's maximum limit, potentially causing irreparable damage. Pressure limiting valves will automatically close at a set point to protect the gauge and then reopen when it's safe. For instance, if a system typically operates at 10 psi, a gauge with a 0-30 psi range is ideal. If pressures could exceed 38 psi during cleaning or testing, a valve set to close at 30 psi would safeguard the gauge until the pressure drops to a safe level.
Pressure switches, which have setpoint limitations, can also benefit from pressure-limiting valves. For example, a 0-100 psi switch might have setpoints between 15 and 90 psi. If your system runs at 80 psi, a switch with a 0-100 psi span (or higher) is needed. For a 5 psi decreasing setpoint, your switch may not be able to accommodate because you’re limited to 15-90 psi, so a pressure limiting valve can help.
One strategy is to pair a 0-30 psi switch with a valve set at 30 psi allows safe operation at 80 psi, with the valve closing to protect the switch. When the pressure falls below 30 psi, the valve opens, enabling the switch to function, achieving the 5 psi setpoint.
Isolators protect instruments from clogging and corrosion
Diaphragm seals and isolation rings are other examples of accessories designed to protect pressure measurement instruments. In this case, isolators are designed to protect against clogging and or corrosion. They are installed between the instrument (as well as associated accessories) and the process.
Using the displacement of a transfer fluid (which occurs with the deflection of a thin diaphragm or liner) isolators transmit process pressure to the instrument. These devices can be made from materials that are suitable for applications involving harsh chemicals, or they can feature larger openings and more durable sensing elements to protect against particulates in the process.
Can pressure gauge accessories impact accuracy?
Now that you have a better understanding of the different factors that can impact the effectiveness of your pressure instruments, and the accessories designed to protect those instruments from those challenging conditions, let's get back to the original question. Do any of the accessories impact the accuracy of your measurements? The short answer is that most of them do not.
While the accessories mentioned above may influence the response time for registering a pressure reading, they do not affect the instrument's accuracy. More flow restrictions can lead to longer response times, but this is intentional when using a flow restrictor, as it helps reduce the impact of pressure pulsations.
An exception to this statement is diaphragm seals. Since they require additional pressure to displace their fluid, they can potentially affect measurement accuracy. Here's why:
- The diaphragm's displacement and spring rate must be sufficient to activate the instrument's sensing mechanism.
- Since the diaphragm has a finite spring rate, some pressure is needed to move the fluid necessary for the pressure element's operation.
- This impact on accuracy will be greater in instruments needing more fluid displacement, with diaphragms having higher spring rates, and in lower pressure range instruments.
Because of this, it is advisable to recalibrate the pressure instrument with the seal attached. Liquid-filled diaphragm seals may also create a detectable error in your pressure measurement due to temperature fluctuations causing expansion or contraction of the fill fluid. You would need to calibrate the seal and instrument at the process service and ambient temperature to ensure accuracy, as well as insulate any components exposed to outside temperature/elements.
The bottom line is that high-quality pressure instruments built to ASME code specifications will help ensure that you’re getting the most accurate readings possible from your pressure measurements.
Ready to learn more?
Now that you have a better understanding of the factors that can affect your pressure instrument and the accessories you can use to protect your instruments from damage, you may still have questions.
Here are a few more articles that address related topics:
- Six Reasons Pressure Gauges Fail
- What’s the Right Pressure Instrument Mounting Assembly for Me?
- When to Use a Welded Instrument Assembly
- Why Do I Need a Steam Siphon and How Does It Work?
- Selecting the Right Manifold Valves for Your Pressure Gauge
In the meantime, download our guide to pressure instrument accessories and assemblies: 