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Some O-Ring Basics: O-rings are one of the modern marvels that make our diving possible. To live and work happily with them, and to replace them when needed, it helps to be familiar with four O-ring basics: material, hardness, cross-section and diameter. Material: Most Scuba O-rings are black or gray, made of nitrile rubber or Buna-N, an artificial rubber-like elastomer mixed with carbon black to provide wear resistance (just like automobile tires). To quote: “Nitrile / NBR: (Buna-N) is the most widely used elastomer due to its excellent resistance to petroleum products, operating temperature range (-40°F to +257°F) and one of the best performance-to-cost values. It's an ideal material for aerospace, automotive, propane and natural gas applications” and “is reasonably priced and features good resistance to petroleum oils, ozone, sunlight and oxygen aging, relatively low compression set, good resilience and outstanding physical toughness.” (1) It is for good reason that Duro 70 nitrile O-rings are the most common and considered the most reliable (2). Infrequently used in Scuba equipment are Yellow O-rings, made of nitrile rubber dyed yellow, and Blue O-rings made of fluoro-silicone which is of relatively low tear strength and limited abrasion resistance (1). While the equipment’s brochure may state one should only replace an expensive and hard to obtain O-ring with one of the same specifications, I have replaced both yellow and blue O-rings with standard black nitrile O-rings. This approach may appear to be expedient, but saves time and money, and has worked well for me. Slightly softer gray O-rings, such as used by Nauticam, I suspect are made of nitrile rubber with a lower content of carbon black. I have replaced them with standard Duro 70 rings as needed, no problem. 2. Hardness is expressed in Duro, or Shore-A, units. It “is measured based on the depth of indentation by a standard size and shape impacting gauge”(2). The hardness of most available nitrile black O-rings is Duro 70, these O-rings are considered the most reliable and are the most widely available. 3. Inner Diameter: The I.D. of an O-ring can conveniently be measured, vertically and horizontally, with a metric ruler. I then use a school compass with a thin black ink pen to draw a circle of that radius (half the I.D.) on white office paper. When overlaid on the circle, the O-ring’s inner edge should barely touch it (see photo). 4. Cross Section: A good way to measure the C.S. is with an electronic digital metric caliper, available from eBay or Amazon for a very reasonable price (see photo). -- The C.S. and I.D. can be used, for example, to replace a lost O-ring. First measure the width of its groove to estimate the cross section. The I.D. is estimated by the length of a string fit into the groove. The length in mm. is divided by Pi, 3.1416 to approximate its I.D. Confirm you have the correct size using test O-rings and check for leaks. The housing’s main O-ring for the back to front seal, for example, must be the same size as the groove it fits in. In contrast, for small fittings sometimes the factory size O-ring is a fraction smaller that the groove they fit onto. For example, my Subal port EXR-3 extension needed an O-ring. The groove width is a bit over 3 mm, with a diameter of 95 mm. I tested the following O-rings I.D.: 85, 88, 90, 93 and 95. Obviously, the 85 thru 93 O-rings were a bit small and had to be stretched. The 95 O ring was almost bit loose and it was difficult to mount the EXR extension in the housing, as the O-ring did not fit and got pinched. The best fit was a 90 mm I.D. O-ring. As Subal uses a 3 x 90 mm, they must have intended for the O ring to be a bit small for a good fit. It is useful to keep written notes, measurements and paper work, including drawn circles, for future use. I have used this approach to recondition housings by replacing the defective or old O-rings. Tools: The tools are basic, a school drawing compass with fine tip pen, metric ruler, digital micrometer caliper, paper and pencil. References: 1. https://www.applerubber.com/products/o-rings/?gclid=Cj0KCQiAj4biBRC-ARIsAA4WaFhcEIFvbNl1aGmLdJLxJJX5qBzfJ0sv64oVftPGT51RgDB1Apc2a3saAitFEALw_wcB 2. https://www.globaloring.com/durometer/ Some O-ring suppliers https://www.applerubber.com/material-selection-guide/ http://www.theoringstore.com/ O-ring background https://en.wikipedia.org/wiki/O-ring ==