From Dive Gear Express site;
Acrylonitrile-Butadine Copolymers
(NBR or Nitrile) | 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. |
Fluorocarbon Elastomers
(FKM) |
O-rings made of FKM have been the preferred choice for oxygen and nitrox compatibility in SCUBA diving applications. FKM 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 O-rings outperform common Nitrile O-rings. |
Polyurethane
(PUR) |
The milky white O-ring occasionally seen in the face of K-valves, sometimes called a 10,000 psi O-ring, is made of Polyurethane. Polyurethane is very abrasion-resistant, but it is sensitive to UV light (sunlight). |
Ethylene Propylene Diene Monomer
(EPDM) |
EPDM O-rings are becoming more common in SCUBA because some people feel it's a safer material for use in breathing air systems. EPDM is a elastomer with excellent weatherability, heat resistance, dielectric qualities and odor-free characteristics. However, EPDM is less resilient and is not recommended for use with petroleum derivatives. |
Hardness -- O-rings are normally available with a hardness rating, as measured by an ASTM type-A durometer, of 70 (the "soft" kind) or more rarely a rating of 90 (the "stiff" kind). The O-rings sold on this page are 90-duro (except for the -003 & -006 O-rings), and well suited for the SCUBA applications we describe. 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. The stiff O-rings are more durable in
static applications where the O-ring is providing a seal with stationary or occasionally moving parts, such as a valve.
Color -- O-rings are available in nearly any color, although most commonly seen in black.
Most Viton O-rings are black. The Viton O-rings sold on this page are brown, simply to make them easily distinguishable from Nitrile O-rings which are usually black and EPDM O-rings which are often purple.
Just to be clear, the COLOR of the O-ring is MEANINGLESS. Some people believe there is a difference in quality based on color, but that is more likely attributed to unknowingly using soft dynamic O-rings in static applications. Our O-ring manufacturer states there is little difference in quality for O-rings based on color alone, and independent testing supports that statement. Some people also incorrectly believe a Viton O-ring can be distinguished from a Nitrile O-ring by the surface appearance. Nitrile O-rings are said to have a shiny surface, and Viton O-rings are said to have a matte surface. This is not a reliable indicator, and certainly not true if the O-ring has been lubricated.
---------- Post added May 2nd, 2015 at 02:08 PM ----------
Nitrile has a lower flash point thus a greater fire risk compared to O2 compatible O rings, and also if exposed continually to high levels of O2 go soft.
Often if Nitrile is used in tank valve neck seals with the tanks containing high levels of O2, they can be found to be deformed into the shape of the groove, rather than remaining a O ring shape.