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Scuba Diving O-Rings

Black is the normal color to be used and in fact, offers the optimum performance. Viton Black is stronger, more stable, and more elastic, even after continuous exposure to extreme conditions associated with the scuba diving sport (i.e. salt water, UV light, high temperature etc.). We recommend that you use a black Viton for your diving gear.
Green is the color of choice in Europe for color-coding Viton, whereas brown is the color in the US.  

Characteristics of Viton® O-Rings, Seals & Gaskets

Temperature range: -20°F to 400°F (-29°C to 204°C)

Standard hardness: 70-75 Shore A

Hardness range: 50-95

Standard colors: black, brown

The hardness of o-rings is measured by the Shore A durometer the higher the durometer, the harder the compound. 70 Shore A is softer than 90. The soft O-Rings are appropriate for dynamic applications where the o-ring is providing a seal with a constantly moving part, such as inside regulators. O-Rings commonly used in scuba diving equipment, that come in different hardness ranging from 70 to 90. Usually, HP and tank necks use 90, and LP uses 70. 
The hardness of the o-rings must be consistent with the technical requirements of manufacturers of diving equipment.

The materials commonly used in SCUBA applications

Trade names
NBR, Nitrile
Acrylonitrile-butadiene rubber
Most O-rings used in SCUBA are made from Nitrile, a hydrocarbon-based synthetic rubber. Nitrile offers excellent resistance to many oils and acids and has good physical properties. However, Nitrile is not a very oxygen resistant material and is not considered oxygen compatible. Nitrile is also often referred to as Buna-N.
Fluoro rubber
O-rings made of FKM Viton® have been the preferred choice for oxygen and nitrox compatibility in SCUBA diving applications. FKM Viton® is an elastomer with excellent oil and oxygen resistance at high and low temperatures, very good chemical resistance. Even for use with ordinary air, most experts agree that FKM Viton® O-rings outperform common Nitrile O-rings.
Adiprene® Millathane®

O-ring is made of Polyurethane. Polyurethane is very abrasion-resistant, but it is sensitive to UV light (sunlight). Good hydraulic oil and gasoline resistance. Resistant to pure aliphatic hydrocarbons (propane, butane, fuel).Resistance to mineral and silicone oils and greases.Resistant to water, oxygen, ozone, and aging. Excellent tear and abrasion resistance

Ethylene propylene diene rubber
EPDM materials generally have a high resistance to hot water, steam, aging, and chemicals, and are suitable for a wide range of temperature applications. EPDM has good resistance to hot water and steam, detergents, caustic potash solutions, sodium hydroxide solutions, silicone oil and greases, many polar solvents, many diluted acids, and chemicals. Special qualities are recommended for glycol-based brake fluids. EPDM materials are totally unsuitable for use with all mineral oil products (lubricants, fuels). They can be used between –45°C and +130°C (peroxide-cured –50°C to +150°C).

The Difference between FKM and Viton®?

  • FKM is the short form for the fluoroelastomer category according to the American standard ASTM.
  • Viton® is the registered trademark of DuPont Performance Elastomers.

Viton o-rings are commonly available in 75 & 90 Shore Hardness. These compounds are normally stocked in Black although other colors can be manufactured in Brown / Green / White

Black FKM VITON ® better properties over brown or green material.

Viton and Buna are two of the most commonly used elastomers for o-rings in scuba diving applications.
Viton is far superior to Buna for high-temperature applications. Viton seals provide an indefinite seal for temperatures up to 400°F, and for temperatures up to 600°F they offer an excellent seal for more than 48 hours. Buna, on the other hand, is only effective up to 250°F. However, Buna seals provide a low-temperature sealing option with effective sealing down to temperatures of –22°F, while temperatures below 5°F render Viton seals ineffective as they become quite hard and inflexible.
Buna is much less universally resistant than its Viton counterpart which suffers degradation from weather and ozone exposure. 
Both of these sealing options offer an extensive list of dive applications, and they both serve well in scuba gear.

Five Common Reasons why Scuba O-Ring Fail

  Explosive Decompression

  O-Ring Failure Description: The seal exhibits blisters, pits, or pocks on its surface. 

  Absorption of gas at high pressure and the subsequent rapid decrease in pressure. 

  The absorbed gas blisters and ruptures the elastomer surface as the pressure is rapidly removed.

  Contributing Factors: Rapid pressure changes. Low-modulus/hardness elastomer.

   Installation Damage

  O-Ring Failure Description: The seal or parts of the seal may exhibit small cuts, nicks, or gashes.

  Contributing Factors: Sharp edges on glands or components. Improper sizing of elastomer. 

  Low-modulus/hardness elastomer.  Elastomer surface contamination.

  Spiral Twisting Failure

  O-Ring Failure Description: The seal exhibits cuts or marks which spiral around its circumference.

  Contributing Factors: Difficult or tight installation (static). Slow reciprocating speed.

  Low-modulus/hardness elastomer. Irregular O-ring surface finish (including excessive parting line). 

  Excessive gland width. Irregular or rough gland surface finish. Inadequate lubrication.

  Abrasion - Friction

  O-Ring Failure Description: The seal or parts of the seal exhibit a flat surface parallel to the direction

  or motion. Loose particles and scrapes may be found on the sealing surface.

  Contributing Factors: Rough sealing surfaces. Excessive temperature. Process environment containing 

  abrasive particles. Dynamic motion. Poor elastomer surface finish.

  Compression Set

  O-Ring Failure Description: The seal exhibits a flat-sided cross-section, 

  the flat sides corresponding to the mating seal surfaces.

  Contributing Factors: Excessive compression. Excessive temperature. Incompletely cured elastomer. 

  Elastomer with a high compression set. Excessive volume swells in chemicals.

The information has been gained from manufacturing partners.

Have no worries, fatal o-ring problems are very rare if you are well informed and know how to use them correctly. 
O-ring leaks, however, are common and can ruin a dive or an anticipated dive vacation. Luckily, knowing how to deal with o-rings is simple and will reduce inconveniences a lot. You can replace most of the o-rings safely on the spot and with minimum skills. Many of them are includes in most Save-A-Dive kits.