I was reading an article about my Octo (Sherwood Shadow) there was a part in the article about it not being Nitrox compatable, I emailed Sherwood and they said this only was for using a mix of 40% or higher, for the standard mix, 32% and 36% it would be no problem. Does anyone have any thoughts on this?
There are a couple of things going on here with the manufacturers. First, they came up with this "nitrox compatible" thing a couple of years ago in an attempt to sell new equipment to divers advancing to nitrox. It was mostly hype. There is no special oxygen cleaning required for equipment used with gases that contain below 40% oxygen.
As a mechanical engineer who has specified oxygen cleaning to US Air Force specifications, let me debunk a couple of the manufacturers myths here. First, many (quote-unquote) scuba "experts" will insist that viton O-rings are an absolute must especially for regulators used with mixes above 40% O2. This is not necessarily so. Viton has a slightly lower rating as a fuel then Buna-N, but your fuel rating is not the only issue. You have to look at the “fire triangle”. You need a fuel, and oxidizer and an ignition source for combustion. For the most part we control this in scuba equipment a little by reducing the fuel source but mostly by eliminating the ignition source. It is notable here to see that the one plastic piece that sees the release of high pressure gas and so is most exposed to the possibility of friction heating is... THE HIGH PRESSURE SEAT and they don't make that in a different material. O-rings that see low pressure are not near as much of a risk as is the high pressure seat.
In my regulators that I rebuild FOR MYSELF I use Buna-N O-rings. When working on customer equipment I always use the industry specified components for liability reasons.
Now having said above I don't worry about the fuel rating of Buna-N I do worry about the fuel rating of petroleum products in tanks, either from dirty compressors or from cleaning solvents. I've had (quote-unquote) scuba “experts” tell my customers that there is no need to clean the tank in tanks we are going to be doing partial pressure mixing because the tanks only sees pure O2 at atmospheric pressure, that they just needed to clean the manifolds. My answer to this (besides yelling and swearing at the “expert”
is to remind the customer the agency that knows the most about handling oxygen is... NASA. Lets look at their track record on handling oxygen as it relates to accidents. They burned up an Apollo capsule on the launch pad with a 100% oxygen environment at atmospheric pressure (14.72 PSI), the blew up an oxygen tank on the way to the moon on Apollo 13 at below atmospheric pressure (10 PSI) and the they blew up the Challenger Space Shuttle by burning a hole in the oxygen tank at well below atmospheric pressure (60,000 ft is what, maybe 8 PSI). All three of these tragic accidents were from introducing an ignition source to complete the fire triangle when fuel was present. In partial pressure blending we put 100% oxygen into the scuba tank then top with air to make nitrox. If you open a valve too fast you can get friction heating past the valve seat in the tank valve. If I'm the one operating the valve, I don't want any fuel with a rating as high as petroleum products present if that accidentally happens.
So O2 cleaning of the regulator for nitrox percentages below 40% isn't necessary, but O2 cleaning a tank is.
Added 2/19/09: Thanks, guys. I agree pre-blended nitrox doesn't require tank cleaning. I didn't include that here but probably should have. Thanks.
