How many hours from a Prism with 19 cft tanks?

warjarrett

As a brand new Prism diver, with only pool time so far, I would like to know what people think is a typical expectation for dive hours available in one packed scrubber, one 19 cu-ft (3 L) fill of O2, and one fill of air diluent. If I need to specify some assumptions, lets say 65 to 70 degrees F (18-21 C), and 80 to 100 ft (25-30 m) diving.

kieranu

I'm not a Prism diver but one of my buddies, Ben who replied to your other post, is and we're both relatively new to the CCR game (~50 hours on our units) so I think its fair to say you can use our consumption as a reasonable guide. As far as gas usage is concerned the same rules should apply more or less regardless of unit. Essentially, my understanding is that in those conditions (similar to here in Perth) you'd be limited to about 6 hours of scrubber time and you're gas would last 4 to 8 hours (assuming you've got a booster, i.e. 25 to 50 bar per dive/hour, whichever is shorter) depending on dive profile and, assuming you use your dil for buoyancy control, how much gas you need to control your buoyancy.

I'm sure if I've got anything wrong, Ben'll be here to set me right.

warjarrett

I think I need to specify more assumptions. The 19s will be full to their maximum rated capacity (not short-filled), I will have no bail-out bottle (so I need to surface with an emergency contigent reserve), and my drysuit and buoyancy bladder have no separate supply (just use gas from the diluent tank). Then, the next question will be, well, how much more time will I get if I do bring a separate bailout bottle?

kieranu

You can use tools like V-Planner for gas-planning to calculate contingency for bailout, whether it be dil or off-board, for any given dive. If using on-board dil you can subtract this off and work out how long the gas will last given the estimates I provided from there.

IMHO, assuming all dives will be planned within no-deco limits I'd make sure I had at least 50 bar in reserve above 25m. Below 25m I'd say off-board bailout would be more or less essential. I always carry bailout and run my drysuit off that and my wing off my dil, but that's just me.

DeepBlueInnovation

Under those conditions I would get two dives a day above 25m, with the limiting factor being the diluent. In relation to the 02 question I believe 1litre per minute is the rough metabolism rate, so not taking into account sensor calibration or validation you would theoretically get 500 minutes on a single 19cf of oxygen. Of course that is well beyond the specified scrubber duration.

kieranu

Bear in mind this will depend on the profile of the dive, as you ascend more O2 will need to be injected into the loop in order to maintain set-point and if you then descend again dil will be injected in order to prevent an O2 spike. On top of that you'll be inflating/deflating wing and/or drysuit to maintain buoyancy and, at least in the beginning, blowing out gas to minimise counterlung volume. Particularly when diving with buddies on OC, I've found this can make the dive quite interesting

DaveC

What sorb are you using? 4-8, 6-12, or 8-12 This will also give you a indication of how much time you can do.

As some one as already mention, the profile will play a part, the up and downs will cause you to use more gas. The work load will play a factor in 02 uptake.

Typically, in the cold water we dive up here, we get two 2ish hours per dive on a scrubber of 8-12 sorb. The gas we have left can be anywhere from 1000 to 1500 PSI in each tank.

For not carrying a BO bottle, even its a little 30cuft, its still better then the on board 19. Plus it get you in the habit of carrying it.

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Mike

It becomes more about number of dives than duration - it's the descents that chew dil and the ascents that chew O2. I normally run out of dil before O2 - using dil for wing inflation if I'm doing a few dives.

Single dive - then 6+ hours would be completely feasible (If I ever decide I can tolerate a 6 hour dive that is) with a comfortable margin to spare.

Multi dives, then a couple of deep dives is fine for a couple of hours each. Shallower (air dil range) I'll do three dives in a day of an hour or so each. Perhaps a couple of days on the scrubber but normally fill tanks of an evening. Then again I'm a bit cautious and figure that gas in the tanks is a good thing to have. I've got in for a shallow pier dive with 50 bar O2 left, but I'd probably not jump off the boat with this little gas.

Mike

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viscya

This sounds like a good question for your instructor. He will know the specifications of the unit based on the scrubber material your using. However, with 19cf bottles, your scrubber should be the limiting factor, regardless of depth and temperature. Any dive greater than 30ft I would recommend bailout.

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dan@scuba-training.net

Carry a bail out cylinder!

If for no other reason than to fend off out-of-air-single-tank-snorkel-jockeys you may encounter at the depths you are talking about.