How is oxygen/nitrogen ratio controlled?

[Lee Taylor]

I almost exclusively dive Nitrox. After all, there is no good reason not to. Or at least that is what they say. Frankly, I enjoy it and I prefer it.

Prior to a day of diving, I check my tanks for percent oxygen. Doesn't everyone?

I often wonder about the procedure(s) and mechanics involved in producing a tank of Nitrox.

How controlled is this procedure? What is the likelyhood of getting a tank of Nitrox that has been accidentally filled with 80% oxygen?

Most dive cons and boat captains are very helpful and safety concerned in this regard.

I have, on rare occasion, run accross the dive con that behaves as if checking percent oxygen is:

silly
a waste of time
ridiculous

Why is that? Is an incorrect fill impossible? How unlikely is it?

 

[Thalassamania]

Checking is never a bad idea, but is a situation where say the cylinders live on the boat and are filled there with a compressor that only delivers air ... well ... the chances of having an oxygen fluctuation is rather slim, better to be looking for carbon monoxide.

 

[Bubbletrubble]

@Lee Taylor: The likelihood of getting a tank filled with 80% nitrox rather than, say, the requested 32% mix probably depends on how careful the tank fill guy is and whether the shop uses the partial pressure blending method.

At present, my local shop purchases 32% nitrox from a gas supplier and simply fills our tanks with it. Since the shop doesn't do any mixing at all, the chance of getting a rich fill is very, very, very low. I still analyze every tank fill.

A couple years ago, I was getting nitrox fills from a different shop with a membrane system. Since my tank never saw 100% oxygen, the likelihood of getting a rich fill was lower. Nevertheless, I was very careful about analyzing each and every tank fill (even the air ones) from this particular shop since it catered to a tech diving crowd, which uses tri-mix and other rich nitrox mixes.

If I got nitrox fills from a shop that used partial pressure blending, I'd analyze every tank fill for oxygen content (nitrox and regular air fills).

I would never think that analyzing a tank is "silly," "a waste of time," or "ridiculous." Being careful, especially when it comes to breathing gas, is a good thing in diving. But then again, I'm probably more risk-averse than the average diver.

 

[Mr Carcharodon]

I

How controlled is this procedure? What is the likelyhood of getting a tank of Nitrox that has been accidentally filled with 80% oxygen?


I have, on rare occasion, run accross the dive con that behaves as if checking percent oxygen is:

silly
a waste of time
ridiculous

Why is that? Is an incorrect fill impossible? How unlikely is it?

Well there are two main ways of doing the mixing. Partial pressure blending and continuous blending.

In partial pressure blending about 14% (assuming EAN32) of the desired fill pressure will be oxygen. So the oxygen is added first after which enough air is added to bring the cylinder to the fill pressure. The bugaboo with partial pressure blending is the temperature rise caused by filling the tank can throw the pressure readings off. Also if there is already nitrox in the tank it is possible there will be a math error on the mix. The worst case would be to get distracted and filling the tank with nearly 100% oxygen. Of course the oxygen supply is usually only about 2500 psi and that pressure will drop as gas is transferred. But still you could get well over 50% O2. But for a shop to fill a tank with close to 100% O2 and not know it would be a pretty spectacular mistake. I have never heard of it happening, but it could.

With continuous blending the oxygen is added prior to compressing the gas. Again it would be possible to get the mix wrong, or not consider what is already in the tank when doing the mix.

So the chances of getting the mix wrong and not realizing it are low, but the consequences are high, so we check.

 

[j yaeger]

exactly right!!!!!
never take anything for granted when it comes to mixes
period.
as you were taught,anylize your own mix
even if your dive con sucks-it aint his life
have fun anyways
yaeg

 

[Lee Taylor]

Thank you BT,

You told me enough that I can see that an error or mistake can happen. That is what I had suspected all along. The two things I have learned so far is the importance of measuring not only EAN, and AIR, but also carbon monoxide (I first heard about this a LONG time ago, and as Thal pointed out....it can still raise its ugly head today).

I do not think measuring dive gas is a waste of time in any respect. Most dive con's are excellent but occasionally we get a bad puppy. The reason for the post is that I recently had an experience with a dive con with a cavalier attitude concerning the importance of measuring ratios of gases.

I suspect in the isolated incident, the dive con just did not want to hunt and find the analyzer (too much like work) and on top of that he did not know how to use it. He had a superior attitude as if it were beneath him and it was really not important.

 

[rjack321]

But for a shop to fill a tank with close to 100% O2 and not know it would be a pretty spectacular mistake. I have never heard of it happening, but it could.

They do sometimes get filled with nearly 100% O2 but its a function of forgetting or not topping off with air. So there's a red flag because the tank isn't full, but the bigger issue is the remaining 100% having a MOD of 20ft. Problem is at its worst when the previous fill wasn't completely drained so 100% was added on top of 32% but then not topped off.

 

[TSandM]

If somebody is doing partial pressure blending, it's very possible to end up with more oxygen than you planned. But if I'm picking up a tank at a shop that blends, I'd be equally worried about ending up with air in the tank, when I thought it was 32%.

Getting filled off a bank makes it much less likely that you will get a fill that's really off (and a lot of other people will be complaining, too), and if a boat or shop is using a membrane system, they should be monitoring the mixture coming out of the compressor.

But honestly, the penalty for diving something that's really wrong is so high, whether it's DCS or toxing, and the cost of doing the analysis is so low (little time, almost no consumables) that I can't imagine why anyone would suggest you do otherwise.

 

[barc802]

I

I have, on rare occasion, run accross the dive con that behaves as if checking percent oxygen is:

silly
a waste of time
ridiculous

If you get a tank filled with Nitrox and they do not require you to test your own tank and fill out their Nitrox log. You should get your fills from some place else. It is a sign that they don't care and that leads to, do they change the filters on time? And whole host of other doubts.

If you were to dive that tank, and believe that there is 32% and it is actually higher. You exceed the MOD and get an oxygen hit. The shop is liable. That is why shops require that you check it an sign the log. If something happens they have a leg to stand on b/c you checked it and accepted it.

I personally do not, and tell my Nitrox students not to, accept a mix with greater than a 1% difference than what I have ordered. I have my tanks mixed for each dive to the best mix for that dive, so a difference of a few percentages can mess up your dive plan.

Remember you are the boss! If the does by have customers they are out of business.

 

[leabre]

If you get a tank filled with Nitrox and they do not require you to test your own tank and fill out their Nitrox log. You should get your fills from some place else. It is a sign that they don't care and that leads to, do they change the filters on time? And whole host of other doubts.

My LDS does not have a NITROX log but they will not allow you to pick up your NITROX fills until you first analyze and label your tank. Sometimes he'll double check but not often (could be that he knows me so is less concerned).

But, he takes cleanliness of the tank, valves, and manifolds seriously. If he suspects anything it's a new O2 VIP / Overhaul.

I don't get the impression he doesn't care. His procedures are just a little different. In fact, the only place around here I've filled out a NITROX log is on the boats, but not in the few shops I've gotten fills. They all just require you to analyze and lable before you can pick them up.