Mixed gas diving and varying tank usage

[R.Chisholm]

Hi,

I own 2 carbon dive 6.8l/300 bar tanks with nautec 300 bar DIN valves which I bought on a whim.

I believe I have so far managed to keep them oxygen clean – and only use them with an atomic aquatics M1 regulator that I also strive to keep oxygen clean.

I have used both tanks individually as a bailout filled with Nitrox 32 while cold water solo diving (usually with a 2x12l twin set as back gas).

I have considered using these tanks for the following:

- Shallow (60ft/20m) sidemount if I ever get round to setting up the gear properly.
- Continue to use them as bailout holding Nitrox 32 for solo diving up to 40m/130 ft.
- Use one for single tank deco diving holding Nitrox 50 for light deco dives
– i.e. up to 15 mins mandatory decompression.
- Fill one with 100% oxygen and keep it for emergency oxygen supply on the surface – possibly with a different constant flow oxygen regulator.

For the sake of this discussion let’s assume I understand the procedures and dangers inherent with high pct high pressure oxygen rich mixtures.

Here is my question to divers that regularly dive using high pct oxygen rich decompression gas.

Is it actually practical to switch the contents of a cylinder every few dives from say Nitrox 32, to Nitrox 50 to 100% oxygen or is the need to relabel the cylinders with a new MOD sticker and ensure that the tanks and regulators really stay oxygen clean (shop banked Nitrox w/o personal filter anyone?) just too much hassle all round?

Regards,

Rory

 

[Diver-Drex]

There are some things in your possible choices for the tanks you have that don’t make sense but I’ll stick to your question...

O2 clean is O2 clean. Whether your mixing 50%, 80% or getting 100%, the standard for clean is the same. Pure O2 goes in if you’re partial pressure blending. But we were supposed assume you knew this...

And how are you keeping them clean? Other than filling only with EAN or with O2 compatible air what does this mean? Are you getting them cleaned annually?

To me an O2 cylinder is dedicated. So is my 50% deco bottle. For a pony bottle you want air or the same mix you’re using for the dive. I prefer air for a pony as it’s MOD is well beyond rec limits and it’s easy to get fills and top offs. And it’s one less cylinder I need to O2 clean every year.

Also, you may want to rethink using EAN32 in a pony on 130fsw dives. You’re banging on 1.6 for no reason. Just put air in it, there is no advantage to using EAN.

 

[happy-diver]

I fill my own and put everything in all of them and breathe from anything

Interestingly humerous but perhaps not to all

 

[beester]

I also would normally comment on some of the things you are saying (tank choices, gas choices) but will also keep on topic.

I follow drex. I keep my 7L (and 40cuft) O² tank dedicated. they only get 100% O² and they are always labeled like this ( a sticker stating OXYGEN + the MOD 6M). Same with the 80cuft 50% stage. All other stages I have (all Alu 80cuft tanks) are changed on requirement of dive(s) and get a new MOD sticker when they change purpose.

For me this personally adds safety. yes I always analyse my tanks, yes I always switch gas in a team way, but still having these dedicated tanks, makes sense for me.

PS (yes off topic) I don't get the bailout tank when you are diving a (manifolded) D12 double set? You can isolate gas on this set in 15 secs flat (or close one of the tank valves or the isolator valve), so I don't see the use of a bailout tank in this config (unless you can't reach the valves).
PPS: 32% at 40m is too rich for my taste... why take risks?

 

[kensuf]

J valve? Really?

 

[TrimixToo]

The MOD should be prominent, for at least two reasons. First, and most importantly, you should check it before you breathe the gas. You need to be able to locate it quickly, and in my view you should follow the LP hose from the attached reg's first stage to the 2nd stage in your hand before you put it in your mouth. (This simple procedure once saved my butt when I inexplicably clipped my tanks on the wrong sides one day.) Second, your buddy or team members, if you are not solo, should be able to see the MOD and stop you from doing something stupid if you start to switch to the wrong gas, or already have. My tanks are all dedicated with permanent numbers, but I have a lot of them. Wide tape and a marker work, are cheap, and don't require dedicated tanks.

 

[PfcAJ]

Is it actually practical to switch the contents of a cylinder every few dives from say Nitrox 32, to Nitrox 50 to 100% oxygen or is the need to relabel the cylinders with a new MOD sticker and ensure that the tanks and regulators really stay oxygen clean (shop banked Nitrox w/o personal filter anyone?) just too much hassle all round?

I would highly recommend against that. There's more than a few divers who have passed away thinking their "oxygen" tank had something else in it when it didn't and they had a seizure and drowned.

Dedicated tanks for dedicated gases is one of the easiest ways to reduce variables. Mark them prominently with MOD stickers, and only put that gas into that tank. Yes, you still need to analyze it of course to make sure the gas you wanted is in the tank, but switching gases.

 

[helodriver87]

Yup, just dedicate individual tanks to individual gasses. It's expensive, but that's technical diving in a nutshell. My 190 tanks never have anything but 18/45 in them. My 100 bottles are always 32%, and so on. Obviously, everything gets analyzed before diving, but it removes lots of variables. The consistency and peace of mind is worth it to me. I'd hate to die because I made a mistake as a result of not wanting to spend an extra $100-150 on an AL80.

 

[R.Chisholm]

Thanks all,

I guess by way of clarification I should have mentioned that I can fill banked 32 PCT Nitrox from at least two local dive shops.
Allegedly and according to their air fill certificates these Nitrox compressors are kept oxygen clean.

Clearly however if you dedicate a diving tank, valve and regulator to 100 pure oxygen use and only fill oxygen purchased from a reputably gas provider there are presumably fewer risks to contamination of the tanks.
By using banked Nitrox I am clearly trusting the provider and the manufacturer of my personal filter.

As to some of the other comments:
1. Why use a Nitrox 32 bailout at 130 feet with pp02 of 1.6 bar.
Because the banked Nitrox 32 I can fill is certified clean – the air from the air compressors from the two providers isn’t. If I filled the bailout tanks with air I’d have to rely on my personal filter to keep the tanks clean and having had several problems with the filter I’m not sure that is a good idea.

If I did have to use the bailout at 130 feet I wouldn’t stay therefore more than a few minutes while I ascend to shallower depths so I figure this isn’t a big issue.

2. Why take a bailout tank at all if you have a twin set.
Last time I actually had to shut down the twin set in anger after a free flow at 130 feet I lost a total of 80 bar – mainly through having tried to see if I could persuade the regulator to stop free flowing any other way and through a few other errors on my part.
Also when wearing the tick under suit in mid-winter I find it hard to shut down the left post efficiently.

Given the above I’m not sorry to have a totally independent bailout – especially when solo diving.

 

[seeker242]

Clearly however if you dedicate a diving tank, valve and regulator to 100 pure oxygen use and only fill oxygen purchased from a reputably gas provider there are presumably fewer risks to contamination of the tanks...

Because the banked Nitrox 32 I can fill is certified clean – the air from the air compressors from the two providers isn’t. If I filled the bailout tanks with air I’d have to rely on my personal filter to keep the tanks clean

Ok, but if you had dedicated tanks/reg, you would not need to keep the air tank O2 clean, because it's dedicated to air and only air to begin with.