Nitrox Question

[Diver Rick]

I've been using Nitrox on live aboard vacations for about five years now. My understanding was that there are two methods of filling a tank with, say, 32%. One was using a "membrane" method, and another method, the name for which I've forgotten.

I was recently on a live aboard that seemed to be creating the mix "by hand". They filled empty tanks with about 500 psi of O2 (from large on-deck cylinders marked "Aviator's Oxygen", then filled the remainder of the 80's with air from a compressor. Using this method gave us mixes ranging from 30% to as high as 36%. We had to drain our tanks empty after most dives to facilitate this procedure.

I thought this was a little unusual to mix this way, and had always been given the impression that creating 32% nitrox for us "recreational users" was somewhat more complicated than this, and required a specialized pumping system.

Can someone who knows more about these matters than I do, care to comment?

Thanks!

 

[Doc Intrepid]

The process you describe is known as partial pressure blending. It's as simple as you describe it. Fill an empty tank with a prescribed psi of O2. Top with compressed air to final psi. Analyze and mark the tank. No worries.

Most dive shops that don't move the volume to merit 'pre-mix', or larger volumes of a specific nitrox already blended, fill tanks using partial pressure blending. It's real common.

(When the T-bottles of O2 get low it helps if you have a booster pump available, such as a Haskel (or other). This lets you use most of the gas in the T, and may be the source of some of the additional complexity you referred to. Boosters are also necessary for filling O2 deco bottles, as Ts of O2 don't generally come in at 3000 psi. You need the booster to compress the O2 to higher psi levels.)

 

[Dive-aholic]

Doc already answered your question. My question for you is were the 30-36% mixes what divers were requesting or just what the PPB was resulting in. That's not a very good range for PPB. If they were trying to get 32%, then the range should have been no more than 31-33%. Whose analyzer were you using? If their blending is that bad, then they probably aren't changing the O2 sensor in the analyzer on a regular basis either, so there's no telling what your mix really was.

 

[DA Aquamaster]

When you partial pressure blend properly, it is common to hit the desired mix within 1%and you can normally get it within .5%. Being consistent with fill rates and compensating for any rise in temperature are critical and that part is almost as much art as science in practice.

I have known many divers who will automatically drain a tank to simplify the math. But if you have either the math skills or a blending program to figure out how much O2 you need to add to the top of the remaining mix and have enough pressure to transfill the extra O2 into the tank, there is no need to drain the tank. Doing it this way uses less 02 and can mean an extra fill or two overall.

Similarly, for a mix with excessive 02, you can bleed some pressure out of the tank and then top it off with air to dilute the 02 to the desired percentage. For mixes with inadequate 02, you are stuck with what you mixed.

There are also a couple of other nifty on the boat partial pressure mixing options. For example you can take your a 3000 psi tank with 886 psi of 36% left in it and top it off to 3000 psi with air to get 25% for tomorrow morning's deep dive.

Or, if there is no 02 (or helium if you are diving tri-mix) on the boat, you can mix tanks or manifolded doubles with mixes that will allow you to transfill the contents into empty tanks and then top with air to get the desired mix. In this case you fill the first tank with the mix needed for the dive and then keep transfilling into the empty tank and topping the tanks with air to produce the mixes for the next two dives. This gives you two dives from all but the first tank or set of doubles rather than just one.

In any case, if you dive nitrox, it's a good idea to have your own 02 analyzer.

 

[O2BBubbleFree]

I haven't taken a Nitrox class, but maybe you will entertain a question, just for educational purposes? I like to go into my classes as educated as I can be, so that I can tell if the instructor is wrong...

For mixes with inadequate 02, you are stuck with what you mixed.

Is that simply because of all the extra precautions necessary with HP O2*, or is there some other reason you shouldn't top off with O2?

Couldn't you put the right amount of O2 in a seperate tank, then transfil from the low-O2 tank?

*HP O2 is dangerous, need booster, tank/valve need to be 'O2' clean, not just 'Nitrox' clean...

 

[MikeFerrara]

I haven't taken a Nitrox class, but maybe you will entertain a question, just for educational purposes? I like to go into my classes as educated as I can be, so that I can tell if the instructor is wrong...



Is that simply because of all the extra precautions necessary with HP O2*, or is there some other reason you shouldn't top off with O2?

Couldn't you put the right amount of O2 in a seperate tank, then transfil from the low-O2 tank?

*HP O2 is dangerous, need booster, tank/valve need to be 'O2' clean, not just 'Nitrox' clean...

With a booster you can fix this too but most of us have to get used to life without one.

They probably won't teach much about blending or the math in a basic nitrox course. You don't need to be a blender to dive nitrox or even teach nitrox diving.

 

[Al Mialkovsky]

We just picked up a booster at our shop, now to hook the darn thing up.

 

[O2BBubbleFree]

http://www.scubaboard.com/showpost.php?p=1355397&postcount=1

 

[String]

Partial pressure blending is by far the most common method of blending nitrox on the planet. Its cheap and simple. Yes it has some drawbacks such as wanting empty tanks to start but its easy. You require O2 clean tanks. It can also go to high % O2 mixes whereas membrane cant.

IIRC there is only one membrane blending outfit in the entire UK and that may have closed down but it is still fairly new technology but does limit to < 50% or so mixes.

 

[dl348]

My LDS used to prepare their Nitrox with the Continuous Flow Method but switched to Partial Pressure Blending since they do business with several tech divers who need oxygen concentrations over 40%.