Why "O-rings," seals, etc let the water in!

Most O rings, seals, mastics and enclosures are excellent at keeping liquid water out - but are poor at sealing against water as a vapour, as it can pass straight through the material of many seals

Almost all O rings, seals, mastics and enclosures let water in.

So how can you tell which one will be the best for your purpose? Versaperm, a world leader in water vapour permeability measurement, has introduced a quick and simple way to test this and overcome the difficulty.

Water vapour problems cost the UK over GBP100M in damage every year.

The new Versaperm MKVI WVTR (Water Vapour Transmission) meter has been purpose designed for this task.

It can cope with several samples, seals or enclosures at a time, often give a reading in as little as 30 minutes - whereas conventional gravimetric measurements take several days and are significantly less accurate.

Seals, mastics, adhesives, gaskets, O-rings and enclosures can be highly sensitive to changes in pressure, temperature and other conditions.

Testing is the only way to ensure that a seal is fit-for-purpose and that Quality Control is being maintained.

The WVTR meter is easy to use and gives a series of digital readouts that can be saved or simply read off the screen.

It needs no re-calibration and requires, at most, minimal training to give results that can be as accurate to the PPM (parts per million) or PPB (parts per billion) range depending on the gas used.

The instrument can be configured to measure not just water vapour but most other gases, including hydrogen, helium, hydrocarbons, oxygen, nitrogen and carbon oxides.

As well as manufacturing the instruments, Versaperm offers a water vapour (and other gases) permeability laboratory service for companies that need to test samples on an irregular basis.

For information, the following gives daya relating to the more common 'O' ring materials:.

*Aflas FEPM: Resists oils, lubricants and some fuels, it is suitable for use with H2S, amines or high temperature water/steam.

* Butyl IIR: Offers low gas permeability and is can be used for vacuum and high pressure applications - It should not be used with mineral oils.

* Epichlorohydrin ECO: Provides good resistance to mineral oils, fuels and ozone - has poor compression set resistance.

* Ethylene-propylene, EPM/EPDM: Offers excellent resistance to weathering, ozone, water and steam.

* Fluoroelastomers: Dyneon, Viton, Tecnoflon: Works well in severe chemical conditions and high temperatures.

* Fluorosilicone FVMQ: Is extensively used for static seals in aerospace as it is highly resistant to many hydrocarbon types.

* Hypalon (and other chlorosulphonated polyethylenes): Offers excellent resistance to weathering and works well with many media for static sealing duties.

* Hydrogenated nitrile, Elast-O-Lion: Offers good mechanical and chemical resistance properties.

* Kalrez FFKM: These erfluoroelastomer compounds have excellent chemical and temperature resistance (grade dependant).

* Natural rubber.

Strong with good abrasion resistance, it is suitable for use with ammonia, ethylene glycol, dilute acids and alkalis, though please note that synthetic rubbers provide greater resistance to heat, weathering and oils.

* Neoprene: A general purpose elastomer that stands up well to sunlight and atmospheric ageing.

* Nitrile: Widely used with mineral oils water and some solvents, some grades are suitable for food applications.

* Polyurethane: Offers good mechanical properties as well as resistance to weathering.

hydrocarbon fuels and mineral oils.

- Stress relaxation may occur above +50C.

* TFE, Fluolion: Offers excellent resistance to most media and can also be used for 'O' ring back-up rings, though flow characteristics limit its use.

* Silicone: Offers good heat and weathering resistance as well as having excellent electrical properties - Sealing ability is limited due to high gas permeability, low tensile strength and poor resistance to tear and abrasion.

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