How Do You Choose an O-Ring?

When installing a pressure sensor, especially on units with parallel threads, such as BSP, UNF etc, it is necessary to use an O-ring on the external face of the hexagon mounting face to ensure complete sealing.

It is important to choose the right sealing gasket to give long-life performance. Several factors must be taken into consideration when you choose an O-ring, including compatibility with the fluid under test, the temperature range of the application, mounting torque, size, pressure ranges etc.

There are many types of seals on the market, from gaskets in copper, lead, Viton, nitrile, teflon etc. The following are some of the more commonly used seals.

Viton

Viton O-rings are oil resistant and are capable of operating in temperatures between -15 and +200°C.

Teflon

Teflon O-rings have similar properties to Viton, but are used for pressure ranges above 500bar (7000psi).

Dowty bonded

Dowty bonded O-rings are versatile but relatively expensive. They comprise a metal annulus to which a medium nitrile elastometric ring of trapezoidal section is bonded. Dowty bonded O-rings are commonly used with hydraulic fluids or mineral-based engine lubricating oils. Temperature range is -30 to +110°C, and maximum pressure is 1500bar.

Nitrile

Versatile and economical, nitrile O-rings are suitable for temperatures from -40 to +110°C, and are resistant to mineral hydraulic oils, greases and certain chemicals. Nitrile O-rings are available in imperial and metric sizes.

In addition there are now many different adapter ranges available that have built-in seals. Alternatively there is the option of choosing a thread that self-seals or has a cone sealing arrangement, such as Swagelock, -3 fitting etc.

Read more: Index to all of our Technical Notes on Pressure

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