O-Rings, Lubricant & Tools
All oxygen compatible for SCUBA use.
Sizing and Selecting O-Rings for SCUBA
Sizing --Society of Automotive Engineers AS568B is an Aerospace Standard that specifies sizes for O-Rings used in sealing applications. An AS568B number is a three-digit suffix that identifies the O-Ring's size. Technically, an O-Ring size is listed as AS568B-016, AS568B-112, etc., but most people use just the three-digit number. The first digit denotes the O-Ring cross section width: 0xx = 1/16-inch, 1xx = 3/32-inch, 2xx = 1/8-inch. While it might seem convenient to provide a picture sizing chart, we've never seen an online version that could be reproduced exactly in a Web browser. Also, sizing an existing O-Ring that has been in service can be error prone because O-Rings can take a set that changes their dimensions enough to be misleading. Luckily, the sizes used in 'user servicable' SCUBA applications are fairly regular, so it's probably best to choose the size based upon the application.
Materials -- O-rings can be made of a very long list of exotic materials; most are inappropriate for SCUBA applications. The materials commonly used in SCUBA applications appear in the following table.
(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.|
|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.|
|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 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 oxygen compatible O-rings sold on by Dive Gear Express 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.
Counterfeits --Many people refer to any and all generic "oxygen-compatible" O-rings as Viton. Viton® is DuPont Performance Elastomer's brand name for their fluorocarbon based elastomers (FKM). However, DuPont is not the only company that manufactures FKM products. Bear in mind that not all FKMs are equivalent, and there are different types intended for various different applications. The industry has also begun to see a lot of counterfeits, particularly with O-rings manufactured outside the US. As a result, you could end up with O-rings that are not the type of FKM you need and possibly not even an oxygen compatible O-ring at all. We've seen impostor O-rings that were actually a mixture of a little FKM and mostly cheap plain rubber, yet were also brown. We're going to be very frank: some of the 'asian import' o-ring kits in the little plastic compartmentalized boxes may not be 'as advertised'. Counterfeit FKM o-rings are so widespread that we make up our own kits using o-rings supplied through trustworthy channels.
Shelf-Life -- The Society of Automotive Engineers (SAE) issued ARP5316 as the recommended practice for the storage of o-rings. It states the shelf life prior to use of Polyurethane (PUR) orings is 5 years, Nitrile (NBR) o-rings is 15 years and the shelf life of Viton (FKM) o-rings is "unlimited".
Designers of modern SCUBA equipment often specify O-rings with special properties and materials for specific uses inside their equipment. The special o-rings are supplied as part of original equipment manufacturer (OEM) service parts kits. Substituting any other generic O-ring just because it's the right size can cause difficult-to-diagnose problems and degrade performance. Improper service could cause a failure which would have serious and life threatening consequences.