Why use In-line Filters for Flow Instruments?

July 20, 2021 James Walton

One of my favorite phrases is, ‘buy cheap, buy twice’. This is never more apparent than when purchasing new flow meters or looking at ways to protect existing ones if necessary. There are various accessories that you can add-on flow meters.

With mass flow instruments, the accessory of choice often is a communication cable, these are essential in allowing you to communicate with an instrument and monitor or access the very information that you purchased the instrument to make available. However, one accessory is often over-looked and can be far more essential to the long-term performance and life-time cost of running an instrument, especially in industrial applications; in-line filters. 

In this blog I would like to share my ideas about in-line filters, in particular the ones used for gas flow meters.

filters, flow controller

Why using filters for your mass flow meters?

This simple add-on to a new flow meter can protect against a multitude of issues like:

  • Debris from contaminated gas lines

  • Particles that exist in industrial gases

  • Small amounts of oil from compressors

In-line filters are especially useful in applications where you have to deal with ‘dirty’ gases, gases with particles. This can be the case in an industrial environment, but also in research applications.  
You may think that in research applications you work with clean gases, but tiny particles can also occur here. Not only the particles in gases can be a problem, the dirt stored in the piping can be harmful as well. 

When using in-line filters you can filter the gas before entering the flow meter to make sure that the gas at the inlet of the instrument is clean - without any particles. This way you avoid contamination which can lead to a number of avoidable costs. With avoidable costs I mean costs due to down-time, service costs, calibration costs and engineering time to remove and re-install the damaged instrument.

Inherent to its construction, a thermal mass flow meter or controller for gases is more or less sensitive to contamination. The thermal flow meters for gas can be divided into two sensor principles:

  • Gas flow meters using the bypass principle

  • Gas flow meters using the CTA principle (Constant Temperature Anemometry)


Thermal mass flow meters for gases – Bypass principle

If we look at flow meters using the ‘bypass’ principle, these instruments are more sensitive to contamination. In these instruments only part of the gas stream flows through the sensor (bypass), the rest will flow through the laminar flow element. This flow element - flow splitter - contains small discs with high-precisions flow channels.  You can imagine that these channels may be clogged by contamination.

About 'bypass' principle

Thermal mass flow meter for gases – CTA principle

Instead of the bypass principle, instruments can also be designed by the CTA principle, also called Constant Temperature Anemometry, inline principle or direct-through principle. This principle has no bypass sensor but has a ‘straight’ flow channel. This construction is less sensitive to humidity and contamination.

About 'CTA' Principle
CTA sensor principle
Thermal mass flow meter for gases – CTA principle

In-line filter to prevent contamination of your instrument

Preventing contamination of your gas instrument, why is this important? To increase the MTBF (Mean Time Between Failure) it is important to make sure that the gas or liquid entering the instrument is dry and clean, in particular when using flow instruments with a bypass sensor. Depending on your fluid you can select different types of flow filters. 

Our flow instruments are designed for low flows and therefore have delicate and finely machined parts. This is needed to enable us to quantify the flow rates of gases that we can achieve with a good level of accuracy and repeatability. 

When you consider the potential damage that is possible from the different sources of contamination, and the delicate nature of the internal working of a mass flow instrument, it would appear to be a very straightforward decision to include a filter in your next purchase of a flow instrument.

Filters for gas flow meters and controllers

In-line gas filters

For our gas flow meters and flow controllers we have filters available which are placed in line with the instrument (our IN-LINE models). These filters are suitable for our gas flow instruments.

In-line gas filters are easy to use, you just screw them into the inlet of the flow instrument, and it guarantees a clean gas inlet.  It contains a 316L sintered metal filter cartridge that is suitable for general purpose filtration and can be cleaned with either a suitable solvent or by replacing the cartridge if heavily soiled.

If the gas contains large particles, we advise the use of a pre-filter. This pre-filter is recommended because it will remove a high percentage of the heavy particles, as it has a cartridge with a larger porosity than the actual filter, before they reach the main filter and reduce pressure drop from clogging and excessive maintenance/cleaning requirements.

In a previous article we have already mentioned the importance of in-line filters when installing a mass flow meter. 

“Ensure that the piping of the system is clean (before installing the instrument). For absolute cleanliness always install filters to ensure a moisture and oil-free gas stream. It is recommended to install an in-line filter upstream of the flow instrument, and if back flow can occur, a downstream filter or check valve is recommended too”. Read more tips before installation in our blog: 'Top 10 tips for installation'.

Would you like more information about in-line filters?

Check out our in-line filters Ask for advice

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USA - Bethlehem, PA 18017 
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