Gas Management Question

[O-ring]

Rec2Tek once bubbled...
I would plan on using a oms 45 with a bit extra if you know what I mean. I would plan for my gas, then 1.5 x my gas to allow for my buddy and some extra( to compensate for increased breathing rate). In this case 50 cu ft should be enough. I know steel stages are not DIR. I will not carry more than 1 or anything larger than a 45. Next step would be an AL80, which would give you more than enough gas. FYI I have done almost this exact dive in Lake Erie. I also planned it with this amount of gas. I ended up buddy breathing thru the deco. I had enough gas to complete the dive for both of us.
DSAO

I was reading it as 1.5 * deco too (the TDI standard). That would mean I would need 20 cf3 * 1.5 = 30 cf3. Looks to me like a 40 would be more than ample. Am I looking at that wrong?

What SAC do you use for deco for planning? Do you crank it up to simulate a stressed deco?

 

[O-ring]

It's really very straightforward. If you absolutely have to be back to your starting point, use thirds. If you would like to be back but know that the boat will be able to pick you up if need be, ie you are not creating a more dangerous situation by drifting away from the boat, you can use something else.
For example, here in South Florida live boating works well because of generally fair seas. A dive on a wreck might involve an grappel and float ball. Gas planning in that situation might be 15 out to explore, 5 minutes to head straight back to the grappel, at which point deco would be done drifting under the float ball. If something comes up and you can't return it's not a safety issue.
If you were up in the NE where boats remain tied in to the wreck (AFAIK), you'd better dive thirds or you'll be in a dangerous situation if you are drifting.

We are technically in the mid-Atlantic (I think..), but it is basically the same as the NE...the boats hook the wreck, the mate descends and ties in with a shackle or something similar. Some boats have a zodiac, some don't, etc. Hopefully you could avoid drifting if you are out on the wreck, have hit your max BT, and need to ascend by shooting a bag from the wreck and ascending on it (kinda jersey upline style, but with a reel and not leaving the line behind). If not, and you were blown off the wreck, you would be drifting.

That isn't software deco. It's also using standard mixes and very reasonable SACs for planning purposes, 0.5 for deco and 0.75 for bottom. What sort of deco did you have for that dive, and what SACs are you using?

A-ha....that's the difference. This is software deco, but not Decoplanner and is using 18/30. Also the SACs in this case were .7 bottom and .4 deco. Deco was in the neighborhood of 30 mins.

 

[Rec2Tek]

O-ring once bubbled...

I was reading it as 1.5 * deco too (the TDI standard). That would mean I would need 20 cf3 * 1.5 = 30 cf3. Looks to me like a 40 would be more than ample. Am I looking at that wrong?

What SAC do you use for deco for planning? Do you crank it up to simulate a stressed deco?

You need 20 ft3 for you and 20 for your buddy plus the .5 (10ft3) for a buffer to compensate for possible increase breathing rates. which happen till you get a rythm going for your breathing. 50 ft3

My rate can be as low as .3 on a nice relaxed deco. I usually use a rate of .65 for bottom and .5 for deco. I usually don't crank it up for a stress deco as you will calm down after you get comfortable.

 

[O-ring]

Rec2Tek once bubbled...


You need 20 ft3 for you and 20 for your buddy plus the .5 (10ft3) for a buffer to compensate for possible increase breathing rates. which happen till you get a rythm going for your breathing. 50 ft3

My rate can be as low as .3 on a nice relaxed deco. I usually use a rate of .65 for bottom and .5 for deco. I usually don't crank it up for a stress deco as you will calm down after you get comfortable.

Ok, now I understand. Double the gas (half for my buddy) and then do the * 1.5. Man...that's a lotta gas!

 

[Munin]

I use 1.5 times deco gas needed for dive, on a single 40 that gives about 30cf useable and 30 minutes of deco. Deco gas is not buddy breathed. In that range you can deco on backgas until your team is done and then use theirs shallow without much risk.

 

[jeepster0000]

Okay guys, I will admit I am new to deco diving. I have taken the TDI Adv Nitrox + Deco Class and the DIRF class and know what rock bottom is. You guys really confuesed me. So what you are saying that O-ring should do is, if he starts with the working pressure of 2700 on double 108's, he should turn the dive at 1800+rock bottom?? Or are you saying he should turn the dive at 900 + rock bottom. The first option would be alot of gas wasted to me.

Since he is not doing that much penetration ie where he can still see daylight, would i be the rule of thirds or the rule of halves like Hallmac said.

Could someone give me the low down at exactly what pressures to turn the dive and why? Don't flame me, I am trying to learn, and if i asked a stupid question, then forgive me.

Thanks
ADAM

 

[O-ring]

jeepster0000 once bubbled...
Okay guys, I will admit I am new to deco diving. I have taken the TDI Adv Nitrox + Deco Class and the DIRF class and know what rock bottom is. You guys really confuesed me. So what you are saying that O-ring should do is, if he starts with the working pressure of 2700 on double 108's, he should turn the dive at 1800+rock bottom?? Or are you saying he should turn the dive at 900 + rock bottom. The first option would be alot of gas wasted to me.

Since he is not doing that much penetration ie where he can still see daylight, would i be the rule of thirds or the rule of halves like Hallmac said.

Could someone give me the low down at exactly what pressures to turn the dive and why? Don't flame me, I am trying to learn, and if i asked a stupid question, then forgive me.

Thanks
ADAM

I posted the original question because I was confused too..nothing wrong with being confused. I think that thirds is overkill for non-penetration wreck diving and that just diving RB is enough, even with deco. However, I think thirds makes sense in the penetration scenario. I got confused because I thought I remembered someone telling me to dive thirds with rock bottom backed out, which would make it REALLY conservative.

Let's do an easy example and see if we can avoid getting flamed (that is, if I can do an easy example correctly ). Let's say we have 104s with a working pressure of 2700 (KISS).

Diving thirds, we would turn at 1800 psi, using 900 for the trip out, 900 for the trip back, and 900 for our buddy.

Diving rock bottom, we would need more info...but let's say we were at 200' and were each carrying a 70' bottle. We need enough gas to get from 200' (at the deepest, farthest point of our dive, back to our first gas switch) to 70', which is 130'. Given that we want to maintain a nice 30 fpm ascent rate, it should take us 4:20 to get there. The rest comes down to SAC and depth (in ATA) averaging, but let's just say we need 22 cf for the ascent x 2 for our buddy, for a total of 44 cf (let's not argue mechanics here...I did this in Decoplanner and it doesn't really matter if it is right, since this isn't a real dive plan). Since, in a set of 104s, 8 cf3 is roughly 100 psi, rock bottom is around 550 psi. Therefore, 2700 - 550 = 2100 /2 = 1050. So, I guess we would do 1000 psi out and turn at 1700.

You can see how conservative this can be...if we take 2700 psi and back rock bottom out (2700 - 550 = 2150) and then figure thirds 717 psi = 1/3, so turn the dive at 2700 - 717 = roughly 2000 psi.

 

[ericfine50]

O-ring,

Thanks for the second example. You guys were confusing me a bit.

Eric

 

[O-ring]

ericfine50 once bubbled...
O-ring,

Thanks for the second example. You guys were confusing me a bit.

Eric

...but let's not stop...someone has to come in here and set us straight. My buddy and I are playing it safe and dive thirds in the aforementioned scenario. We each get back to the deco stop with a little over 900 psi (since 900 was our reserve and we aren't quite there yet...maybe we have 1000 or so in our backgas). We go to switch to our 70' bottles and find that I dropped mine (improperly attached bolt snaps...damn!). Why can't we finish the deco on the one bottle of EAN50 and a combination of the backgas in both of our doubles? Why do we need to carry so much deco gas? IMHO, 2(deco gas reqd) * 1.5 is an insane amount of deco gas to be lugging around...

 

[ericfine50]

Eric

I think you could deco on one cylinder (loss of attached cylinder = major cf) - I would share the 40, deco is not that huge and go to backgas.

Eric