Gas Management Question

[O-ring]

Those of you doing wreck diving...are you using rule of thirds when penetrating? Is that to any depth of penetration? What if you were doing a wreck at 150' and only going in and out of cargo holds?

On another note...do you dive thirds because deco = overhead or are you using "rule of halves" or rock bottom only because the surface is accessible during the dive and you have already calculated your deco gas requirements and have those in a separate cylinder?

Example scenario (note - this dive is NOT real):
You are doing a dive in the ocean to 170' using Trimix. Based on your SAC, you have calculated that you need 113 cf of bottom gas and 20 cf of deco gas. You have decided that you are going to deco on 50/50 and sling an AL40. You don't plan on penetrating the wreck. What cylinders would you bring for bottom mix? Why?

Does this change if you decide you may do some mild (cargo holds, open hatches, open windows) penetration?

 

[Rec2Tek]

What's your buddies breathing rate? Does she require the same amount of gas for the dive?

 

[O-ring]

Rec2Tek once bubbled...
What's your buddies breathing rate? Does she require the same amount of gas for the dive?

...or assume identical SAC rates.

 

[Munin]

O-ring once bubbled...
Those of you doing wreck diving...are you using rule of thirds when penetrating? Is that to any depth of penetration? What if you were doing a wreck at 150' and only going in and out of cargo holds?

For a penetration dive from the start, 1/3s. For a dive where I see something inside I want to take a look at, calculate 1/3s and do it. For a swimthrough, unless you are approaching rock bottom you will have enough gas for 1/3s, so don't worry about it.

On all dives, thirds is calculated from the useable gas after rock bottom is subtracted, eg 3300 - 1200 RB = 2100 Useable = 700 1/3s

On another note...do you dive thirds because deco = overhead or are you using "rule of halves" or rock bottom only because the surface is accessible during the dive and you have already calculated your deco gas requirements and have those in a separate cylinder?

Deco does not necessitate thirds. Dive and surface conditions determine gas planning. A drift dive is all useable (why would you need thirds?), a dive with a live boat might be 1/2s or something like 15 out 5 back. In these cases an immediate ascent is not risky. A dive with an anchored boat, shore dive, or a similar situation where you must get back to your entry point means 1/3s. Remember, your rock bottom includes gas for your buddy.

Example scenario (note - this dive is NOT real):
You are doing a dive in the ocean to 170' using Trimix. Based on your SAC, you have calculated that you need 113 cf of bottom gas and 20 cf of deco gas. You have decided that you are going to deco on 50/50 and sling an AL40. You don't plan on penetrating the wreck. What cylinders would you bring for bottom mix? Why?

Your numbers are a bit off here, assuming a normal SAC. 20cf of deco gas gives about 20 minutes of deco, or a 10 minute bottom time at 170, or about 45cf of gas. Ocean means Al80s for strict DIR, 45cf means backgas only is adequate. 113cf of bottom gas means about 23 minutes at 170, 35 minutes of deco, or more than you can do on a Al40.

Does this change if you decide you may do some mild (cargo holds, open hatches, open windows) penetration?

 

[Rec2Tek]

For starters I'd use twin steel LP 108's because thats what I have Basically I plan on thirds(2/3's for me 1/3for my bud). Using the 108's filled to rated pressure I have about 216 cubic ft of gas. 2/3 would be aprox 144 which would meet the gas requiremnts for the dive(plus a little buffer). I wouldn't change the plan based on penetration. I have only x amount of gas and time to do the dive. So I plan the penetration and turns accordingly. Since we are assuming identical breathing rates my doubles would also give my buddy a third of my gas in the event of a CF(plus the buffer for both of us ). Now the alu 40 is cutting it very close if there was a loss of deco gas by your buddy you'd run out as you finished deco. There are other things you may wish to take into consideration in your senario like loss of deco gas, buddy losses both back and deco gas. But I hope this helps with your basic senario.

 

[O-ring]

For a penetration dive from the start, 1/3s. For a dive where I see something inside I want to take a look at, calculate 1/3s and do it. For a swimthrough, unless you are approaching rock bottom you will have enough gas for 1/3s, so don't worry about it.

On all dives, thirds is calculated from the useable gas after rock bottom is subtracted, eg 3300 - 1200 RB = 2100 Useable = 700 1/3s

So, if you are on a dive and you see something neat, you will calculate thirds on the fly and go in and check it out? Yeah, I know what rock bottom is...

Deco does not necessitate thirds.

Ok. The boats up here don't live boat, but you can always do it jersey style or drift deco if you have to...would you use thirds in that scenario or not?

Your numbers are a bit off here

They are supposed to be...this isn't a real dive . They aren't that far off though, but that starts a whole other argument...software, etc. which I don't want to get into. Let's just say that the deco time I was getting was less than yours, backgas was probably using less helium, and was probably using a lower SAC.

 

[O-ring]

Rec2Tek once bubbled...
For starters I'd use twin steel LP 108's because thats what I have Basically I plan on thirds(2/3's for me 1/3for my bud). Using the 108's filled to rated pressure I have about 216 cubic ft of gas. 2/3 would be aprox 144 which would meet the gas requiremnts for the dive(plus a little buffer). I wouldn't change the plan based on penetration. I have only x amount of gas and time to do the dive. So I plan the penetration and turns accordingly. Since we are assuming identical breathing rates my doubles would also give my buddy a third of my gas in the event of a CF(plus the buffer for both of us ). Now the alu 40 is cutting it very close if there was a loss of deco gas by your buddy you'd run out as you finished deco. There are other things you may wish to take into consideration in your senario like loss of deco gas, buddy losses both back and deco gas. But I hope this helps with your basic senario.

Ok, you say the 40 is cutting it close...is that because you carry double your deco gas (1/2 for you and 1/2 for your buddy)?? The dive calls for 20cf of 50/50, which is roughly half of an AL40 @ working pressure.

 

[Hallmac]

I use thirds on any planned dive that includes deco or penetration. and 1/2 + 200 on simple rec dives.

Hallmac

 

[Munin]

O-ring once bubbled...
Yeah, I know what rock bottom is...

Sorry, just wanted to it to be clear to everybody that were're not talking about straight 1/3s.

Ok. The boats up here don't live boat, but you can always do it jersey style or drift deco if you have to...would you use thirds in that scenario or not?

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.

They are supposed to be...this isn't a real dive . They aren't that far off though, but that starts a whole other argument...software, etc. which I don't want to get into. Let's just say that the deco time I was getting was less than yours, backgas was probably using less helium, and was probably using a lower SAC.

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?

 

[Rec2Tek]

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