O-Rings - Lubricant - Tools

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 serviceable' SCUBA applications are fairly regular, so it's probably best to choose based upon the application.

Sizing Nomenclature -- The Society of Automotive Engineers (SAE) publishes the AS568 (Revision "F" issued 2020) 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 AS568F-014, AS568F-112, AS568F-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+A1:2019 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 (The obsolete British imperial standard BS 1806 is still widely referenced, but was superseded by the BS ISO 3601 standard.)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 variety of exotic materials, but some 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.

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. In SCUBA applications, the most commonly used O-ring elastomers that offer resistance to rapid gas decompression damage are fluorocarbon and polyurethane.

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 hose 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 regarding their compatibility with oxygen.

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 compatibility with oxygen is typically considered poor. Do not assume an O-ring is suitable for use with Nitrox just because the color is green.

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. 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.

Lubrication -- Oxygen compatible lubricants such as Tribolube and Christolube are compatible with all elastomer materials commonly seen in SCUBA. Oxygen compatible lubricants are generally considered best practice within the SCUBA industry as they will typically outperform silicone grease and are also much less migratory.

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 and 2021.