Air-only vs. Nitrox-only submersible pressure gauge

[Ricky B]

I recently purchased a Mares SPG, and the user manual has this warning:

The pressure gauge is designed for use with only one gas mixture. At the time of purchase, the purchaser must choose the mix to be used: breathable air or oxygen enriched breathable air (Nitrox).

I know that once a tank has been filled with nitrox, it shouldn't be filled with air and vice versa (unless cleaned), but I have never heard of not using a pressure gauge with a nitrox-filled tank once the pressure gauge had been used on an air-filled tank.

There's nothing about "we mean nitrox that is more than 40% oxygen."

The PADI manual doesn't say anything this, so far as I can find. There is a statement that "Some countries, including several in Europe, require a special dedicated air nitrox valve on enriched air cylinders." Otherwise I can find no mention of not using an SPG with nitrox-filled tanks if the SPG has been used with air. Is this simply a European approach?

In theory, trace hydrocarbons found in normal compressed air could lodge in the SPG, but since the SPG is not connected to the tank when being filled, I don't see how that could lead to an explosion.

What gives? Is this a real problem? Or do divers switch an SPG between air-filled and nitrox-filled tanks with reckless abandon?

 

[gcarter]

Air is EAN21. As long as you fill with good source gas, you should have no issues switching between EAN21 and EAN 32 or EAN36.

It is all about the lawyers and CYA. Keep you gear in good condition and you will be fine.

 

[jcaplins]

Sounds like crazy talk to me.

But this comment:

I know that once a tank has been filled with nitrox, it shouldn't be filled with air and vice versa (unless cleaned),

Is not necessarily true. You just need clean air. If a shop says they can't fill a nitrox tank with air, then I would start to question the quality of their air.

 

[Ricky B]

But this comment:

Is not necessarily true. You just need clean air.

Correct. There's a certain rating for air that can be used in a nitrox tank, but I don't recall what it is and so didn't go into it.

If the shop says they can't fill your nitrox tank with air, it might simply mean that the air is not hyper-filtered to remove all traces of hydrocarbon that are present in normal compressed air.

 

[Phil_C]

Pretty sure the issue is whether the air is double filtered air or not. Double filtered is fine for topping off or filling nitrox tanks, but most portable compressors are just single filtered, so should not be used with nitrox tanks unless you are going to get them cleaned before the next nitrox fill. My club has a double filtered air bank so that is fine for nitrox, topping off or air fills

I haven't come across anything in the UK about requiring a 'special' or dedicated spg for Nitrox as opposed to air, and nor have I when diving in Greece, Cyprus or Turkey.

Perhaps there is a 'theoretical' risk of the high pressure hose becoming contaminated with single filtered air then there being a fire/explosion risk when turning on the nitrox when it 'rushes' into the hose and spg and meets the hydrocarbons or contamination when turning the tank on .- Phil.

 

[Dr. Lecter]

Don't worry about it. At all.

First: If you're using pre-mix nitrox of 40% O2 or less, you need not clean the tank at all. Partial pressure blending of any nitrox mix (which requires adding 100% O2 as the first step) and/or mixes over 40%, is the only thing that requires O2 cleaning. Even then, using air is fine, so long as it's grade E, hyper filtered air. It's used for partial pressure nitrox blending.

Second: short of pouring motor oil into the SPG, I still wouldn't worry about it, even if you're putting it on an O2 reg. Sure, clean the threads and the HP port o-ring where it screws into the first stage body, and use O2 compatible lube on the HP spool that connects the SPG to the hose. But as a general rule, it really doesn't matter whether your SPG is O2 clean or not.