A point about trimix diving

[Anthony Appleyard]

Closed-circuit automatic rebreather diving on nitrox is easily catered for, because ppO2 meters are readily available, and, knowing also the depth pressure, keeping the mixture correct is easy, as many of you know.

But trimix (oxygen and nitrogen and helium) diving is a different matter, and from pictures that I have seen, a trimix diver must trail about a big open-circuit cylinder of trimix nearly as long as he is tall. To make reasonably compact automatic trimix closed-circuit diving realistically possible, is there any chance that someone will invent a ppN2 and/or a ppHe meter small enough to fit it in a diving rebreather?

 

[broncobowsher]

Go read up on the sale materials on the Divesoft Liberty. Already on the market.

Although not needed. The ratio between N2 and He is constant. If that ratio is 1:2 (one nitrogen for two Helium) that ratio of inert gas stays the same (within reasonable tolerance anyway). So those 3 parts of inert gas (1 Nitrogen and 2 Helium) can be mixed with any amount of O2. Once you know the amount of O2, the seperate inert gasses are easy enough to figure out. If you measure O2 and figure the gas is 50%, 6 parts of gas will be 3 parts O2, 3 parts inert. 3 parts inert will be 1 part N2 and 2 parts He. Scale that all you like.

 

[Anthony Appleyard]

It came to me that if the rebreather had two diluent cylinders, one for nitrogen and the other for helium, and a ppO2 meter and ppN2 meter and a ppHe meter, then it could keep the ppO2 to the most that did not cause oxygen poisoning, and keep the ppN2 to the most that did not cause nitrogen narcosis, and the rest helium: that may save on expensive helium and on bulk of gas to carry about. And, if it had all three pp meters , their readings should add up to the depth pressure (allowing for CO2 temporarily in circuit, and suchlike), and if they do not, then one of the pp meters has failed.

 

[Wibble]

The point of CCR is the diluent inert gas isn’t consumed and is constantly recycled as you "rebreathe" it. I.e. you use very little diluent, say 150 litres / 5 cubic feet per dive (diluent is also used for filling the BCD)

You choose the diluent mix for the depth band you’re diving too. For me, down to 30m/100' it’s 21%, to 40m/132' It’s 21/15ish, to 50m/165' it’s 18/35ish, and to 70m/230' it’s 14/50.

I mix this from three twinsets with 100%, 21% (air) and 10/70. It’s not hard.

BTW you also need to take adequate bailout gas for the target depth.

 

[broncobowsher]

You are trying to make things WAY too complicated. No need to add complexity by changing the inert gas ratio during the dive. You start and end the dive on the same ratio of inert gasses, thus no need to add complexity by changing that ratio during the dive. The slight advantage is only there in extreme fringe dives, and it is so small that the error for getting it wrong is many times greater than any advantage you could possible gain from it.

If this is something you must have, go get a Divesoft Liberty and be happy. It is the only thing out there that will do this for you.

 

[wedivebc]

It came to me that if the rebreather had two diluent cylinders, one for nitrogen and the other for helium, and a ppO2 meter and ppN2 meter and a ppHe meter, then it could keep the ppO2 to the most that did not cause oxygen poisoning, and keep the ppN2 to the most that did not cause nitrogen narcosis, and the rest helium: that may save on expensive helium and on bulk of gas to carry about. And, if it had all three pp meters , their readings should add up to the depth pressure (allowing for CO2 temporarily in circuit, and suchlike), and if they do not, then one of the pp meters has failed.

Your plan would add way too much complexity for no return. Oxygen is a moving target because it is metabolized inert gases are in a fixed ratio and very predictable throughout the depth range

 

[Wibble]

To clarify, the ratio of oxygen to inert gas varies with depth. The absolute pressure of oxygen is fixed at the setpoint, typically at a pressure of 1.3 bar/atmospheres, the rest being the inert gas.

Thus at:

• 10m/33' (water pressure = 2bar/ata) oxygen will be 1.3bar/ata and inert gas will be 2-1.3 = 0.7bar/ata.
• 90m/330' (water pressure = 10bar/ata) oxygen will be 1.3bar/ata and inert gas will be 10-1.3 = 8.7bar/ata.

Hence the need to use the correct diluent mix.

 

[stuartv]

Closed-circuit automatic rebreather diving on nitrox is easily catered for, because ppO2 meters are readily available, and, knowing also the depth pressure, keeping the mixture correct is easy, as many of you know.

But trimix (oxygen and nitrogen and helium) diving is a different matter, and from pictures that I have seen, a trimix diver must trail about a big open-circuit cylinder of trimix nearly as long as he is tall. To make reasonably compact automatic trimix closed-circuit diving realistically possible, is there any chance that someone will invent a ppN2 and/or a ppHe meter small enough to fit it in a diving rebreather?

"Reasonably compact automatic trimix closed-circuit diving" is not just realistically possible. It's what we have now.

The big open-circuit cylinder of trimix that we carry around is bail out. We never use it except in emergencies. Fill it once and carry it around for a year (or more). The expensive of that is largely irrelevant to trimix CCR divers.

The cost of the helium that is used in a Diluent cylinder on a CCR is trivial. A 3L steel cylinder for dil is 23 cubic feet (in Freedom units). My old shop (where I was just a customer - not staff) sells Helium for $1 per foot. So, filling a new, empty 3L steel with TX10/50 costs about $12.... Topping it up after a day of diving is $6 - 8. A 3L steel filled with O2 (from empty) is another $10 or so.

The cost of gas in CCR diving is pretty much trivial, when you look at the entire diving budget for some diving a trimix CCR.

 

[beester]

Why, just why complicate matters. Your diluent ratio is fixed and doesnt need changing during a dive.

The diluent mix is selected on some factors..for example work of breath (gas density), end and finally making sure that there is sufficient gradient difference between diluent and max depth...so during a flush you can still see an effective ppo2 drop.. thats it. Easy.