Gas Planning

[pnw_diver]

The Split from A&I Yukon thread: Gas Rules in OW Solo Dives thread has got me thinking about my own gas planning for OW dives with a buddy.

Gas planning hasn't been part of the course material for any of the courses that I've taken, and so I've been left to figure things out on my own. General rules such as "return to the boat with 500psi" haven't been particularly useful.

So, I'd like to get opinions on a particular profile.

Assuming the following - non-penetration wreck dive, wreck upright on a 130ft hard bottom, deck at 95 feet. Plan is to dive to the deck so we'll assume a maximum depth of 100 feet to allow a bit of wiggle room. The site has little or no tidal current. For the curious, the wreck I am using in this example is the HMCS Saskatchewan.

Diver is using a 3500psi 100cf tank with a 19cf pony. The diver's buddy is equipped similarly.

What is your gas plan for this dive and why?

 

[DevonDiver]

Need to know the diver's SAC/RMV.

 

[pnw_diver]

Assume a SAC of 0.6.

 

[rongoodman]

Take a look at this article on Rock Bottom calculations.

 

[TSandM]

This is interesting, because there are components of the scenario as presented that are not part of my diving practice, so they change things.

When I plan this dive, I plan to carry enough gas to get two stressed divers to the surface; with the assumptions and estimates I make, that's 40 cf of gas from 100 fsw. But you are EACH carrying backup gas, which changes things. If someone freeflows, they go to their own pony; if that were to fail, they go to the other diver's bottle -- it looks as though no one has to carry reserves in back gas. 20 cf is an adequate amount for an ascent with appropriate stops for one diver (assuming NDL dive).

So your gas goal is to arrive at the surface with roughly 500 psi, which gives you 3000 psi in backgas to use. Now, you have to decide how critical it is to return to the upline. If that's desirable but not critical (you can swim to boat, or boat has a skiff to pickup blown off divers) then you can dive half of your usable gas going out, and half coming back. If you MUST return to the upline (bad surface conditions, current, or boat has no way to retrieve divers) then you can use one third of your gas going out. This is because your pony is only enough for ascent; it will not get you back to the upline, so here, each of you has to carry backgas reserves for the other.

In the first scenario, turn pressure would be 2000 psi. In the second, it would be 2500.

The only place that sac rate comes into the picture is in calculating the "rock bottom" requirements. The 20 cf figure I mentioned is based on a stressed sac rate of 1.0 cfm; if your resting sac is close to that, you would have to increase the size of the pony bottle or reserve some volume in backgas to make up the difference.

 

[redacted]

Even with a sac of .6, that dive is NDL limited on air or EAN32 rather than gas constrained.

 

[tkdgodess]

Nov 14th....97 feet for 33 mins including 3 min. safety stop. 80+ vis.
"cold, clear, lots of life on guns, decking, railings all covered, dropped down bow hole (no ceiling above me) covered in tube worms, ascended, swam thru top cabin (full daylight) had a good look around, swam to helicopter landing pad, saw stern, returned back up line, long safety stop.

done on an AL80 air, with regular dive buddy using oceanic datamax sport computer. Previous dive was 2+ hours prior to the Breton 100' for 31 mins. Editted to add iirc it was actually a steel 85. Used charter boat tanks....

Didn't record my pressures for either dives. We had planned our routes around the wreck, watched our computers for NDL, and had a normal operating plan to start ascents with 800psi. Side by Side buddy diving with regular buddy.

 

[TSandM]

BTW, I slightly misspoke in my calculations above, because I didn't set aside gas for the ascent. You'd need 12 cf for my ascent for this dive, which is 400 psi, so that should have been subtracted from the gas to use on the bottom.

At that point, your MDL and gas supply are just about going to coincide.

 

[pnw_diver]

BTW, I slightly misspoke in my calculations above, because I didn't set aside gas for the ascent. You'd need 12 cf for my ascent for this dive, which is 400 psi, so that should have been subtracted from the gas to use on the bottom.

At that point, your MDL and gas supply are just about going to coincide.

Wait a minute... Are you suggesting that in your second case, you'd be turning at 2900psi (after using only 600psi of your back gas)?

 

[TSandM]

No -- you want to get to the surface with 500 psi; you need 400 psi to get there, so you need to leave the bottom with 900 psi. So you have 2600 usable, and if you're diving thirds, you have 800 psi (we always round down) to use going out. So turn in that case would be 2700, and you'd have a lot of gas left over when the dive was done.

The thing about reserves is you will rarely, if ever, need them. So you finish your dives with tons of gas. The joke in Florida is that the dive is over when your tanks are full . . . of course, that's related to cave fills. But on any uneventful cave dive, I will finish with AT LEAST a third of the gas I started with. You just face the fact that you're not going to get to dive all the gas you have, and that's the case with ANY diving situation where thirds is appropriate. As a result, small tanks will give you only very short bottom times in those circumstances.