Don't breathe tanks to zero?

[chrpai]

Do people actually do that?

Fill Express does.

 

[Abdullah]

Topped with nitrogen? Kind of, not really though.

Curious, have you taken a nitrox class? It's called enriched air for a reason. Also I don't trust other people to analyze my gas for me. I'm breathing it, not them.

He's actually correct for the original binary Nitrox gas (nitrogen & oxygen) developed in the commercial dive industry. Of course recreational divers call anything EANx by the 'Nitrox" slang.

 

[boulderjohn]

And, unless the shop has a Haskel pump, the O2 delivery pressure is limited to whatever pressure remains in the oxygen supply tank. So PP blending on top of 500 psi or more of residual gas pressure limits the amount of oxygen the supply tank can donate to your tank (more hassles for the blender).

This is an issue a lot of people don't realize because they live in an area where this is not necessary. If you live somewhere with a lot of divers coming in for EANx fills, then banking makes sense. It also makes sense for the shop to have a Haskell so that they can fill at whatever pressures they want. There are lots of places, though, where the numbers of people diving nitrox do not make either of those a sound financial policy. Those shops end up filling tanks by partial pressure with no booster.

Since this is Basic Scuba, I will explain the process. For nitrox, you first put in your O2 to the calculated level. Then you top it off with air from the compressor (not simply nitrogen). A Haskell is a pump that allows you to boost the pressure of the gas you are adding. If you don't have one, then you can only add the gas to your tank up to the pressure of the tank from which you are getting it. A large O2 tank usually starts with 2,400 PSI. As you fill the scuba tanks, that pressure gets lower and lower. Eventually you reach the point that there is not enough pressure in the fill tank to put enough in the scuba tank. For that reason, as the fill tank pressure drops, you generally want the tanks you are filling to start empty so you can get the right amount of O2 in them. (It's a little more complicated than that, but not much.)

Calculating the amount you need to add to an existing tank is not hard; in fact, there's an App for that. It is usually for the reasons described above that people want you staring with empty tanks.

 

[nimoh]

The manual is telling the blender in that paragraph that he needs to know the residual mixture - analyze the residual gas, or start with an empty cylinder. Anyone blending trimix will have both oxygen and helium analyzers, so testing the residual mix is standard procedure.

I agree, but that's not what tplyons wrote

 

[tracydr]

I both dive trimix and fill it, and I would never do either without an analyzer.

As for draining the tank, bleeding out $100 worth of helium for no reason is just plain ludicrous. Perhaps the course material was written before analyzers were generally available.

I was wondering about this statement, too. How the heck would anybody be mixing trimix without an analyzer?

 

[chrpai]

It is usually for the reasons described above that people want you staring with empty tanks.

The problem with that is it's contributing to the problem. If you arbitrarily use draining as the solution to the problem then you accelerate your consumption of O2. If you know your O2 bottle pressure and the contents in the tank, you can make the best choice about what to do.

I think that for many they just do the "easiest" thing possible. Drain and fill....

I do agree there are business concerns and unfortunately that's a negative feedback loop. Increased cost decreases demand, decreased demand increases cost... Before you know it no one wants to bother O2 cleaning their tanks anymore to accomodate the PP blending and an EANx rental costs $15 while an air fill costs $4.

Apart from a few rare people O2 cleaning and inspecting their own tanks, filling with O2 at home and going in for an air top off, that's the current state of affairs in Austin, TX.

 

[Tortuga68]

I was wondering about this statement, too. How the heck would anybody be mixing trimix without an analyzer?

Not sure if you mean an O2 analyser or a He analyser...

Anyway, you can blend trimix just with just an O2 analyser

 

[Abdullah]

The problem with that is it's contributing to the problem. If you arbitrarily use draining as the solution to the problem then you accelerate your consumption of O2. If you know your O2 bottle pressure and the contents in the tank, you can make the best choice about what to do.

I think that for many they just do the "easiest" thing possible. Drain and fill....

In most places, the cost of the oxygen blown off will be less than the labor to custom blend every tank. For shops doing PP blending, the blender would have to take the time to analyze both pressure and mixture in every tank. The value of the residual oxygen in the tank probably isn't worth the labor used to define the variables.

Yeah, it's easier to blow the tank down. Especially if you're filling 20 of them and have other things to do to keep your business running.

 

[The Chairman]

Do people actually do that?

I know of at least six shops that do this. I like it!

 

[boulderjohn]

If they are charging for nitrox by volume, then I would have a problem with them draining my tank first.

Do people actually do that?

Fill Express does.

Fill Express (GoDive Florida) is where I got my advanced blender certification, and I use it frequently. It is the opposite of the kind of operation I described earlier.

They are a huge shop that does tons of business, including air fills, nitrox fills, and trimix fills. They have a huge bank of EANx 32, a huge back of EANx 36, and a huge bank of EANx 40. If you rent single tanks from them they come full, and you can have any of those mixes for the same price as air. If you want your tanks filled, then they charge by volume. That is an obviously correct choice, because there is a big difference between coming in for a fill with an Al 80 and a twinset of HP 120s.

They have a Haskell booster.

If you ask for a fill of banked gas on top of what you already have, no problem. They are not going to ask you to drain anything. For a lot of people, the procedure is to know what you have in your tank, ask for a banked fill on top of it, having a reasonable estimate of what you are going to end up with. You analyze and are done. If you want to change what you have in your tank to something different and quite specific, that is a custom blend and will take some work, so it will be more expensive. A shop like that has tons of options.

In contrast, if you walk into most shops in Colorado and ask for a nitrox fill, you will leave empty handed. Nobody dives nitrox in Colorado--the lakes are too shallow to make it worthwhile--so most shops don't have the capacity to make it. The shops that do will have a few O2 industrial bottles in the back, and they will transfill a custom blend for you with no booster. That is a completely different situation, and it calls for completely different procedures and policies.