Heres a question

[rushrhees]

I am just a regular open water diver, never seen a rebreather in person, but just wondering how long does the air last in those things, lets say at 60 feet, I have heard really really long

 

[caveseeker7]

... but just wondering how long does the air last in those things, lets say at 60 feet, I have heard really really long

Depends on the kind of rebreather, not on the depth. The amount of O2 molecules the body needs stays the same no matter how deep you are.

The least efficient of them are constant mass flow semi-closed units like the Dräger Dolphin. They constantly inject more than is normally metabolized and eject the remainder in form of periodic, small bubbles. The injection rate depnds on the mix, which in turn depnds on the depth you want to dive. A 27 cu.ft. cylinder with a 32% Nitrox mix for example last about 45 minutes. Unlike open circuit, this is independent of depth (so if you leave the valve pen your tank will be empty 45 minutes later, too).

A RMV-keyed semi-closed rebreather like the Halcyon RB80 has about an 8:1 gas ratio when compared to open circuit. With this rig gas is added per breath, and a set (or in some military units adjustable) amount is expelled on inhalation. So twin 40 cu.ft. tanks, pretty much the same as a normal 80 cu.ft. recreational tank, lasts as long as if you were using 640 cu.ft. of gas.

The most efficient rebreather are closed-circuit rebreather, were no gas is expelled except on accent (like on a BC or drysuit). Two gases are used, pure O2 which replaces the metabolized O2, and diluent (air, heliox or trimix depending on depth) to make up for ambient pressure, dilute the O2 to a breathable mix, and bailout. The amount of O2 used is again depth independent, the deeper you go the bigger the savings. A 20 cu.ft. O2 tank should last for 6 or more hours.

Both the Halcyon SCR and CCRs have been used for dives exceeding 8 hours.
That is really, really long, isn't it.

Gas usage with the rebreather is not the limiting factor for dive duration, scrubber duration (how long/much CO2 can be removed from the loop by the absorbant) is, as well as decompression and bailout/emergency gas management.

 

[landfish]

the dreager scr i dive and dont understand here?
i dive the unit with and have done dives to 150 feet with a bottom time of 1 1/2 hours switched to 80 percent which is tied into the rebreather at 30 feet the main
tank is a 65 steal which had over 2000 pds left in it and the 19 with the 80 percent has 2200 left in it and the dive was just about 3 hours long?
gas comsumption is not the problem but cns and scrubber life is?
45 mins????

 

[caveseeker7]

... with the standard 4 ltr. cylinder using the apropriate meter nozzle for 32% Nitrox.

A 27 cu.ft. cylinder with a 32% Nitrox mix for example last about 45 minutes.

Page 50/51 of your manual will fill you in, the flow rate is supposed to be between 14.2 ltr./min and 16.9 ltr./min. The 4 ltr. tank is rated at 200 bar, so when full has 800 ltr. of gas. Equals a runtime between 56 min. and 47 min. if the flow is correct. Since you need some gas to do a positive pressure check before your dive 45 min. seems like a pretty good number.

I'm aware that Dolphins are used past their rated depth and scrubber durations, modified for gas switches and trimix use or converted into CCRs.
You'll need additional skills and knowledge both to modify and dive the unit, neither of which I think is the scope of the question. But you're right, extended times and depth are possible.

rushrhees, with a stock Dolphin and standard training for a dive to 60 fsw you can use a 50% Nitrox mix. Duration would than be between 100 and and 118 minutes.

 

[landfish]

and you are correct
but why would you use the 32 orifice anyway that flows about the same as open circut!
use the 60 and up the precent in the tank then go to the manual and figure out the duration
that 32 would make it feel like the mouth piece would want to blow out of your mouth at least it would with me
i believe the duration in the manual for the scrubber is 3 hours isnt it?
the only reason i said anything was it seems that most people put down the scr and i dont understand
yes ccr is more effecient but...

 

[caveseeker7]

but why would you use the 32 orifice anyway that flows about the same as open circut!
use the 60 and up the precent in the tank then go to the manual and figure out the duration

I would think
- because it is the intended use of the design
- because that's how diving the unit is taught
- because you better know exactly what you're doing (your metabolism, your pO2) if you don't
- because the unit needs to be modified (pO2 monitor, manual add valve ... if you add that you might as well KISS the unit as Jetsam's valve is a cmf valve with manual addition - giving you less rather than more than you actually metabolize)

that 32 would make it feel like the mouth piece would want to blow out of your mouth at least it would with me
i believe the duration in the manual for the scrubber is 3 hours isnt it?
the only reason i said anything was it seems that most people put down the scr and i dont understand

I hope you're talking about other people or posts putting down SCRs, not what I posted above.

There are several drawbacks to the cmf-SCR, you mention one above. Since the CL are usuallt full which means lots of lead rather than diving with minimal loop volume as you do a CCR.

No standard pO2 monitor, which I find irresponsible. There are divers overbreathing their units even though they dive them as designed.

That and the price are the usual complaints I see about cmf-SCRs, then there are some Dräger/Dolphin specific ones that keep coming up.

The way I see it, your or any other unit fits your needs and wants or it doesn't. So dive it or don't. Plenty of people put down my choice, too. While I usually try to explain it, in the end I don't give a sh!t if they get it or not.