Deep dive gas mixer?

[Frosty]

Folks I will start this post by saying I'm sorry if its old news or I'm asking something that's odviously impossible.
Out at the Poor Knights dive site on a surface interval I watched a couple of deep divers prepping for a dive.
Given the technology available in this the 21st century why don't deep recreational divers have a rig with bottles of the gasses they use attached to it and a computer doing the gas mixing for the various depths and deco requirements?
again I apologise but I can't see the answer.

 

[tbone1004]

If that's a serious question the answer is that is what rebreathers do, it would be a waste in open circuit. They are now bringing recreational rebreathers to the market, best one currently is probably the Poseidon 7even.

 

[victorzamora]

Lol, yeah, you described a rebreather.

 

[Frosty]

If that's a serious question the answer is that is what rebreathers do, it would be a waste in open circuit. They are now bringing recreational rebreathers to the market, best one currently is probably the Poseidon 7even.

Yep serious question and Yep Im aware of recreational rebreathers.
Im happy to admit im in uncharted waters knowledge wise regarding trimix open circuit so please forgive the ignorance.
MY (perhaps) incorrect understanding was that deep recreational divers run three tanks with different mixtures of OXY,helium nitrogen and other trace gasses to be used at different depths to avoid,getting narked at depth and oxy poisoning.My thinking was to use tanks with a single gas in them that the computer would mix and deliver to the second stage depending on depth.

 

[tbone1004]

nope, the three tanks you were likely seeing were two backgas cylinders with a mix either optimized for the depth or close enough, and a single decompression tank, typically 100% oxygen. The backgas is either mixed with a "standard gas" or a custom trimix blend, though custom trimix blends will typically have 3 cylinders on top of the back gas. With OC you would have too large of tank volumes because the fractions are so different. You would use much more nitrogen than the other two gasses and it would be very cumbersome. For stuff like this the CCR's make more sense because you can carry so little gas comparatively.

Typical OC dive to 100ft for an hour takes about 120cf of gas, rebreather divers will use a tenth of that give or take, so the unit is much smaller. On CCR the nitrogen, helium, and some oxygen is premixed and optimized or using standard gasses for the depth being dove and that is called the "diluent" or "dil" gas and then oxygen is added according to depth to keep your PO2 up. Quite complex, but quite simple at the same time. To do that on open circuit would just be far too complex, as would mixing all three gasses independently. Helium sensors are finicky little things and super super expensive. Getting the oxygen readings accurate is difficult enough, so getting the helium added to that at the volumes moving in open circuit would actually probably end up costing more money than a fully electronic CCR.

 

[victorzamora]

Yep serious question and Yep Im aware of recreational rebreathers.
Im happy to admit im in uncharted waters knowledge wise regarding trimix open circuit so please forgive the ignorance.
MY (perhaps) incorrect understanding was that deep recreational divers run three tanks with different mixtures of OXY,helium nitrogen and other trace gasses to be used at different depths to avoid,getting narked at depth and oxy poisoning.My thinking was to use tanks with a single gas in them that the computer would mix and deliver to the second stage depending on depth.

So, there are two kinds of rebreathers. CCRs are "Closed Circuit Rebreathers" and are the ones I'll be focusing on. They contain two tanks. One of "diluent" and one of oxygen. The diluent is chosen depending on target depth and a few other factors. It's not pure helium because of the potential for switching to breathing purely the diluent bottle (in case of rebreather failure)....whereas breathing pure helium or pure oxygen would kill you. So, one bottle is a SUPER rich helium mix (like 10% oxygen and most of the rest of it helium) and the rebreather automatically adds more oxygen as necessary to keep the breathing mix at a happy/healthy level.

The reasons this isn't done exclusively are few but serious. First of all, rebreathers are EXPENSIVE. I've been told to expect to spend $10k-$15k US for the rebreather and some training (not even all of your training). Secondly, they have many more failure modes and points and pretty much spend their days plotting ways to kill you. They're a lot more complicated and involved to dive. It's just a lot more going on than typical open circuit SCUBA gear.

 

[Dr. Lecter]

But they *do* let you blend whatever mix you want right there on your back in real time...assuming your O2 cells are working right.

 

[victorzamora]

They do a LOT of good things, that's for sure. I just meant for the purposes of the OP practically asking, "Why isn't everybody doing this??" There are plenty of reasons to stay away.

Of course, if I had the chance to be doing the dives your doing with HALF the frequency you were doing them, Doc, I'd probably go the way of the 'breather. Tbone, I don't want to hear a word......I AM maintaining logical consistency.

 

[Dr. Lecter]

For frequent deep diving in a locale where wholesale gas costs are roughly $0.35/cf O2 and $2.5/cf HE...they make a lot of sense below 200'. Above 200' in the same locale, not diving helium makes a lot more sense.

 

[ajduplessis]

But they *do* let you blend whatever mix you want right there on your back in real time...assuming your O2 cells are working right.

So if you have 10/60 trimix (diluent) the RB can change this on the fly to something like (30/30)??