How to Choose an In-line Filter – Physical Connections

Miniature in-line filter physical connection choices ISM puzzle graphicIn-line Filter Physical Connections
There are many considerations in carefully choosing an in-line filter. Common choices focus on the expected particle load and the chemical exposure. Sample tests will show that it works in the intended operating environment. However, critical options exists with the physical connections on in-line filters. This post will review connection choices for in-line filters: barbed, threaded, Luer and push-in connections.

Barbed Connections
A barbed connection has one or more ridges or barbs. This is the section of an in-line filter pushed into a tube during installation. Pushing a tube end over a barb causes it to expand and then relax to its original size on the far side of the barb. Relaxation of the tubing around the cylindrical surface behind the barb provides a connection seal. Gripping forces of the barb need to be greater than pull off forces on the tubing.

An exploded view of an ISM modular check valve. Click here to go to a landing page where you can get more information about these valves.

Modular Check Valves

We've taken spring-loaded check valves to a whole new level. Mix and match imperial and metric connections. Watch the video.

A play button for a video about ISM's modular check valves. Click here to go to a landing page where you can watch the video.

Barb features that are critical to gripping force:

  • Barb shape
  • Number of barbs
  • Spacing of multiple barbs
  • The outer diameter (OD) of the barb relative to the ID of the tubing
  • How the barbed fitting was made

Sharper barbs grip better but can cut into soft tubing. Steeper, deeper barbs also grip better but can be difficult to install and replace. Single barbs work better with soft, stretchy tubing. Multiple barbs (double and triple barbed) work over a wider range of hardness (durometer) but work best with harder tubing materials. How suppliers make barbed connection ports is important because minimizing or eliminating parting lines eliminates potential leak paths.

What is durometer?
Durometer is a reference measurement for material hardness based on its resistance to penetration. You will usually see durometer describing the hardness of polymers, elastomers and rubbers. The lower the tube durometer the softer it is and the more likely a barbed connection is a good choice.

Threaded Connections
Machinery ports use threaded connections most of the time. Because miniature fitting connections handle liquids and gases, they need to be leakproof. There are many standards for threads but most fluidic and pneumatic threaded ports use tapered threads. The most common tapered threads are NPT or BSPT. It is the combination of tapered thread and thread sealant that provides a leakproof seal. Installation without over tightening is necessary for a good seal. Please keep an eye out for my future blog on threads and all the standard thread types.

Luer Connectors
Luers use conical or tapered connectors. Connected male and female Luers create secure, detachable, leak-proof connections without an O-ring or gasket. Producers make Luer connectors in a wide variety of configurations. Luer connections are commonly used in medical applications.

Cutaway color model of a dual O-ring push-in fitting
Push-in Connectors

Push-in connectors are leak resistant because they use built-in O-ring seals. This type of connector has built-in metal teeth that grip the tubing. In order for a push-in connector to function properly, the tubing needs to be soft enough for the teeth to grip but not so soft that the tubing bends instead.

Putting It All Together
This is only a quick look at in-line filter connections. Barbs, threads, Luers and push-ins all have detailed technical specs but you can safely use these tips:

  • Barbed connectors should be matched to the tubing
  • Threaded connectors should have the right threads, sealant and correct installation
  • Luer connectors are very flexible but their size limits their uses
  • Push-in connectors provide quick, easy installation but should be matched to the tubing
  • Adapters provide options for in-line filter installation.

Look for our upcoming blog article about the adoption of standing desks in the ISM office:

There is More to Standing Desks than Just Standing

Have you had trouble choosing the right in-line filter connection for one of your projects? Did thread type cause problems? How about tubing durometer? Help us by telling others about what you learned.

Still have questions about how to select the right in-line filter for your application? If so, send me an email – You can also ask questions using the comments section below.

Previous posts in our How to Choose an In-line Filter series:


Steven C. Williams headshot March, 2018.

About the author
Steven C. Williams my LinkedIn profile link button.
Steven C. Williams, BS, is the technical writer and an inbound marketing specialist at Industrial Specialties Manufacturing (ISM), an ISO 9001-2015 supplier of miniature pneumatic, vacuum and fluid circuitry components to OEM's and distributors all over the world. He writes on technical topics related to miniature pneumatic and fluidic components as well as topics of general interest at ISM.       

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