O-Rings, Lubricant & Tools

All oxygen compatible for SCUBA use.

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Sizing and Selecting O-Rings for SCUBA

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 or a PDF version that could be reliably reproduced in a printer. Sizing an existing O-Ring that has been in service can be error prone because O-Rings can take a compression set that changes their dimensions enough to be misleading. Also, depending on their material and exposure to specific lubricants or gases, some O-rings will swell in size over time. Luckily, the sizes and materials used in 'user servicable' SCUBA applications are fairly regular, so it's probably best to choose based upon the application.

Sizing -- The Society of Automotive Engineers (SAE) publishes the AS568 (Revision "D" issued 2014) aerospace standard that defines imperial sizes for O-Rings used in sealing applications. An AS568 number has a three-digit suffix that identifies the O-Ring's physical size. Technically, an O-Ring size is listed as AS568D-014, AS568D-112, AS568D-214 and so on, but most people use just the three-digit number. Sizes are specified by the inside diameter and the cross section diameter (thickness). The first digit denotes the O-Ring cross section width: 0xx = 1/16-inch, 1xx = 3/32-inch, 2xx = 1/8-inch. British Standards Institution (BSI) publishes the BS ISO 3601-1:2012 standard that defines equivalent imperial sizes, so a BS3601-112 O-ring and AS568-112 O-ring are the same physical size. (The obsolete British imperial standard BS 1806 is still widely referenced, but was superseded by the BS ISO 3601 standard.) BS ISO 3601 also defines metric sized standard O-rings, for example M2.4X11.3 has an inside diameter of 11.3 mm with a 2.4 mm cross section.

Materials -- O-rings can be made of a very long list of exotic materials; most are inappropriate for SCUBA applications. ASTM International (ASTM), originally known as the American Society for Testing and Materials, publishes technical standards for O-ring materials. The materials commonly used in SCUBA applications appear in the following table.

Material Description
Acrylonitrile-Butadine Copolymers
(NBR)
Most O-rings used in SCUBA are made from NBR, the ASTM standard designation for a class of hydrocarbon based synthetic nitrile rubbers. Nitrile is also known as Buna-N, one of the first such materials that was patented in 1934 and often now used as a generic name. NBR is a widely used and low cost elastomer that offers good physical properties with resistance to many chemicals. However, NBR is not considered suitable for use in SCUBA diving Nitrox applications.
Fluorocarbon Elastomers
(FKM or FPM)
O-rings made of FKM have been the preferred choice for use in SCUBA diving Nitrox applications. FKM is the designation for a class of fluorocarbon based synthetic rubbers as defined by ASTM standards, equivalent to FPM by ISO standards. Originally developed by DuPont under their trademark Viton®, FKM is a high performance elastomer with excellent oxygen resistance. FKM also offers excellent compression set resistance along with excellent resistance to rapid gas decompression. Even for use with ordinary air, most experts agree that FKM O-rings outperform NBR O-rings.
Polyurethane
(AU or EU)
The translucent O-ring occasionally seen in the face of K-valves and DIN connectors, sometimes called a 10,000 psi O-ring, is made of polyurethane. Polyurethane has excellent oxygen resistance along with excellent abrasion resistance but it is sensitive to UV light (sunlight). The ASTM standard designation for most polyurethane in O-ring elastomers is AU for polyester urethane, or rarely O-rings are made from EU for polyether urethane; in general polyurethane is sometimes abbreviated PU.
Ethylene Propylene Diene Monomer
(EPDM)
EPDM class O-rings are becoming more common in SCUBA because some people feel it's a less toxic material for cost effective use in breathing air systems. EPDM is an elastomer with excellent weatherability, heat resistance, and odor-free characteristics. However, EPDM is less resilient and is not recommended for use with petroleum derivatives.
Silicone
(VMQ)
The orange O-rings sometimes seen in rebreather loop seals are made of silicone. The term silicone describes a class of materials in which vinyl-methyl-silicone (VMQ) is the primary ingredient. VMQ is especially non-toxic, thus widely used in food service and medical applications. VMQ is not recommended with silicone based lubricants commonly used in SCUBA.

Rapid Gas Decompression - When a pressurized gas is released very quickly, rapid gas decompression can cause an O-ring or other elastomer seat to fail as a SCUBA regulator is purged prior to removal from the tank. This damage typically goes unnoticed until the regulator subsequently exhibits a leak or free flow, and the cause is not always visible upon inspection because small bubbles form on the inside of the material. The most commonly used O-ring material that offers excellent resistance to rapid gas decompression damage is FKM.

Hardness -- O-rings are commonly available with a hardness rating, as measured by an ASTM type-A Shore durometer, of about 70 (the "soft" kind) or more rarely a rating of around 90 (the "stiff" kind). The harder O-rings are generally better suited to most of the SCUBA applications we describe. The soft O-rings are appropriate for dynamic applications where the O-ring is providing a seal with a moving part, such as inside swivels. The stiff O-rings are more durable in static applications where the O-ring is providing a seal with stationary parts, such as valve fittings.

Color and Appearance -- O-rings are available in nearly any color, although most commonly seen in black regardless of material. There are no O-ring color coding standards and thus color is not an indication of the material or quality. Even though most FKM O-rings are black, the FKM O-rings sold by Dive Gear Express are usually brown or green; simply to make them easily distinguishable from NBR O-rings which are usually black. O-ring materials also can not be distinguished from each other by their surface appearance, more a function of the manufacturer than the material. NBR is incorrectly said to have a shiny surface and FKM a matte surface. This is not a reliable indicator, and certainly not true if the O-ring has been lubricated. Just to be clear, the COLOR and SURFACE APPEARANCE of the O-ring is MEANINGLESS.

Caution: O-rings made from Hydrogenated Nitrile (HNBR) class materials are commonly available due to their resistance to many automotive fluids as well as refrigerant gases and their color is often green. However, HNBR resistance to oxygen is not significantly different than ordinary NBR. Do not assume an O-ring is suitable for use with Nitrox just because the color is green or brown.

Shelf-Life -- The Society of Automotive Engineers (SAE) publishes ARP5316 (Revision "D" issued 2014) as the recommended practice for the storage of O-rings. It states the shelf life prior to use of PU material O-rings is 5 years, NBR material O-rings is 15 years and the shelf life of FKM material O-rings is "unlimited".

Counterfeits -- The O-ring industry sees a lot of products that do not meet their material specifications, substituting cheap or low quality materials for more expensive fluorocarbons and coloring them brown or green. Many people also refer to any and all fluorocarbon material O-rings as viton. However, Viton® is DuPont Performance Elastomer's brand name for their fluorocarbon based elastomers (FKM). DuPont is not the only company that manufactures FKM products and there are different types intended for various different applications. Counterfeit and mislabeled FKM O-rings are so widespread that Dive Gear Express makes up our own kits using O-rings supplied through trustworthy channels.

Dive Gear Express stocks a variety of O-Rings for SCUBA diving Nitrox applications along with lubricant and tools.

Our first SCUBA TekTip, this text was originally ©Copyright 2004 and substantially revised ©Copyright 2018.

Tek Tip Warning Image WARNING

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.



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