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When to Use a Welded Instrument Assembly

measurement instrument accessories | instrument assembly

Instrument assemblies can help protect your equipment and ensure reliable and accurate pressure readings of your process. Assemblies that include accessories such as diaphragm seals, capillary, pulsation dampeners and Ashcroft type 2198 and 1198 siphons can help mitigate extreme effects, including high temperature, pulsation and vibration.

Along with dissipating these effects, diaphragm seals/instrument isolators can prevent clogging of the instrument from a dirty process. A seal is also available in many wetted materials for compatibility with the process, making them a popular choice to mount to instruments.

But what is a welded instrument assembly and when is it appropriate to use one?

This article will explain welded instrument assemblies and under what circumstances it’s best to use them.

What is a Welded Instrument Assembly?

Simply put, a welded instrument assembly consists of a mechanical measurement instrument and instrument accessories welded together. An example of this would be a pressure gauge welded to an Ashcroft type 2198 siphon and a diaphragm seal/instrument isolator. Throughout this article, I will use the words diaphragm seal and instrument isolator interchangeably.

Instruments attached directly to seals can be welded, and assemblies with some accessories can be welded at each connection. Snubbers and valves cannot be welded since they contain elastomers that will be compromised with the elevated weld temperature.

Materials to be welded must be compatible. For example, 316 stainless steel cannot be welded to Monel. If the materials to be welded are not compatible, the tamperproof feature can be compromised and become ineffective.

Generally, an accessory attached to a gauge does not change the accuracy of the gauge. However, attaching a diaphragm seal or instrument isolator to a gauge can change gauge accuracy. It typically adds an additional 0.5% accuracy tolerance to the gauge.

When a gauge welded to a diaphragm seal is required, Ashcroft confirms appropriate assembly accuracy tolerance only after the assembly is welded, filled and calibrated. For example, an Ashcroft 1279 gauge with an accuracy of 0.5% when attached to a diaphragm seal will usually have an accuracy of ±1% of span.

If the same 1279 gauge with an Ashcroft 2198 siphon and diaphragm seal are welded, filled and calibrated, the typical gauge accuracy of the assembly will also be 1% of span. So, additional accuracy occurs only when the diaphragm seal accessory is added.

Why Use a Welded Instrument Assembly?

More and more customers are seeing the value of a welded instrument assembly. A welded instrument assembly can be used in any application and is good for all industries.

Welding an assembly together can help prevent accidental tampering or disassembly. The welded tamperproof designed assembly also prevents a diaphragm seal from being detached from the gauge. This is important to note since the removal of the gauge from the seal breaches assembly system integrity, rendering the gauge inoperable.

The tamperproof welded design also prevents an important accessory from being removed. For example, if the process temperature is 800°F/427°C, design engineers have specified that an Ashcroft 2198 siphon be attached to the gauge to dissipate process temperature and protect the gauge. See figure 1 below:

Figure 1: A welded instrument assembly

welded instrument assembly

If removed from an assembly that is not welded, the gauge will lose its integrity and not give an accurate process pressure reading.

Instrument Assembly Tips

A pressure gauge is a window to the process. A welded assembly provides peace of mind that the components of the assembly won’t be removed. This feature removes the concern of compromising gauge accuracy.

If at some point your instruments need to be replaced, you would need to replace every part of the assembly. You wouldn’t be able to replace one or two parts of a welded assembly because breaking the weld would result in damage to the instruments.

Here are some benefits of a welded assembly:

assembly benefits

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

Now that you better understand the benefits and uses of welded instrument assemblies, you can choose the right solutions for your process to protect your pressure measuring equipment.

If you want more information on mounting accessories and assemblies, view our webinar about pressure instrument mounting assemblies.

We also have other instrument assembly articles you can read:

Feel free to contact us with any questions or concerns. Or you can download our Complete Guide to Pressure Instrument Assembly:New call-to-action

About Lou Altieri, Product Marketing Leader

Lou Altieri is a Product Marketing Leader with Ashcroft Inc. with more than 41 years of experience. He is responsible for pressure gauges, diaphragm seals, isolation rings and accessories that serve for the oil and gas, chemical/petrochemical, and water/wastewater markets. Lou has a passion for understanding customer needs and providing solutions to their problems. Most recently, to safely protect the gauge from elevated pressure beyond its full-scale range, he released a gauge that can withstand pressure up to 4X the range of the gauge without damaging the instrument. He has authored numerous articles. In his spare time, Lou enjoys power walking, hiking and winemaking.