GUE/DIR Standard Deco Gas Sequence

[allenw1972]

I am a little afraid to ask this question, but here goes...

When diving deeper profiles on standard bottom mixes, can the GUE standard deco gasses be used outside of the standard gas listing order?

Let me explain. For example, in planning a 250' dive, I have assumed the 100%, and 50% bottles will be used for the shallower stops (at the correct depths and in the correct sequence of course), but what if going to a 21/35 deco bottle as the 1st deco gas has advantages in the plan vs. going to the 35/25 first. In this case, I would use either 21/35 or 35/25, but not both. The advantages may be balancing the bottle volumes throughout the deco, breathing a deco bottle deeper to insure min bottom gas, and reducing the liklyhood of isoberic counterdiffusion when coming off a high % of He.

I am just GUE Fundies certified, but full trimix certified with another agency, so please cut me some slack here.

 

[RTodd]

I am a little afraid to ask this question, but here goes...

When diving deeper profiles on standard bottom mixes, can the GUE standard deco gasses be used outside of the standard gas listing order?

Let me explain. For example, in planning a 250' dive, I have assumed the 100%, and 50% bottles will be used for the shallower stops (at the correct depths and in the correct sequence of course), but what if going to a 21/35 deco bottle as the 1st deco gas has advantages in the plan vs. going to the 35/25 first. In this case, I would use either 21/35 or 35/25, but not both. The advantages may be balancing the bottle volumes throughout the deco, breathing a deco bottle deeper to insure min bottom gas, and reducing the liklyhood of isoberic counterdiffusion when coming off a high % of He.

I am just GUE Fundies certified, but full trimix certified with another agency, so please cut me some slack here.

As far as gas volume considerations, I can't think of many real world examples where moving deco gases around like this would be appropriate. You always need the proper amount of back gas bailout. Keep in mind, on a 250' dive in open water the DIR answer is a relatively short bottom time. So, deeper stops will be short on back gas and you would generally be much btter off going to 35% at the 120 stop. For cave dives where bottom times are longer, bringing a 120 and a 190 bottle is relatively simple and there is no need to skip one or the other.

 

[jjones9527]

I'm only Tech 1, so I'll base my answer off that. At the T1 level we can use either 50% @70ft or O2 @20ft, but not both. Since the 70ft bottle allows us to get on the gas sooner, and not have to worry about surface conditions, it is the preferred gas.

I can see a similar argument made for the use of 190gas straight to 70gas, forgoing the 120 bottle, in that we can get on the deco gas sooner. The only real problem I see is if that 190 bottle will get you to the 70ft stop. Hopefully someone with T2 can chime in.

 

[allenw1972]

As far as gas volume considerations, I can't think of many real world examples where moving deco gases around like this would be appropriate. You always need the proper amount of back gas bailout. Keep in mind, on a 250' dive in open water the DIR answer is a relatively short bottom time. So, deeper stops will be short on back gas and you would generally be much btter off going to 35% at the 120 stop. For cave dives where bottom times are longer, bringing a 120 and a 190 bottle is relatively simple and there is no need to skip one or the other.

Again, I haven't taken T1 or T2, but is min back gas calculated based on getting two divers to the 1st deco deploy based on a 1cf/min SAC for both divers? If the switch was deeper, in the case of 21/35, less min gas would be needed...or is there some other method for calculating bailout? IANTD uses a 1.5 multiplier for total required gas volume, or 1/2 of total projected usage added for bailout.

 

[jjones9527]

You need one and a half times the gas to get 2 people to the next gas switch at a SAC of 1Cf per minute. This goes for deco gas as well, so you would need 1.5X of 190 gas.

 

[allenw1972]

You need one and a half times the gas to get 2 people to the next gas switch at a SAC of 1Cf per minute. This goes for deco gas as well, so you would need 1.5X of 190 gas.

Please help me understand your response. We use 1.5 times the calculated bottom mix gas to provide enough gas to get two dives to the 1st switch. We have since been calculating min gas based on the an actual calculation of consumption (at 1ft SAC X 2) which comes out to less than 1.5 in some cases. Would you need to calculate the deco gas at 1.5 or something less (IANTD uses 1.2) and I think that is because they assume you now have options and could go onto your or your buddies back gas, or other deco gas.

If, in fact, if you did need to use 1.5, you may consider 21/35 as your 80 cf bottle and use two 40's for your 50 and 100. Although that may leave you with less shallow options in the event of lost deco gas shallow. Just a thought.

 

[RTodd]

Again, I haven't taken T1 or T2, but is min back gas calculated based on getting two divers to the 1st deco deploy based on a 1cf/min SAC for both divers? If the switch was deeper, in the case of 21/35, less min gas would be needed...or is there some other method for calculating bailout? IANTD uses a 1.5 multiplier for total required gas volume, or 1/2 of total projected usage added for bailout.

Ignore the bailout gas. The amount of time spent between 190 and 120 on a 250 dive in open water is not that long. If the dive is long enough to require a 3rd deco gas at all, you would be better off using a 120 gas where you would be spending some time. The 120 bottle will reduce the deco time whereas the 190 won't really do much. So choosing this deco gas over 120 for gas planning purposes is a bad idea.

The 50% vs. O2 on the otherhand is a different animal. On some dives, the deco optimization gained on the deeper stops makes which gas you choose a wash for total run time purposes.

 

[Steve R]

Ignore the bailout gas. The amount of time spent between 190 and 120 on a 250 dive in open water is not that long. If the dive is long enough to require a 3rd deco gas at all, you would be better off using a 120 gas where you would be spending some time. The 120 bottle will reduce the deco time whereas the 190 won't really do much. So choosing this deco gas over 120 for gas planning purposes is a bad idea.

The 50% vs. O2 on the otherhand is a different animal. On some dives, the deco optimization gained on the deeper stops makes which gas you choose a wash for total run time purposes.

And to continue Rtodd's excellent illustration further, if you have had to say bye-bye to for whatever reason to your 50%, you can more effectively use the 120 bottle to deco with vs the now almost useless 190 gas. You would also be able to actually carry enough in a 120 bottle to cover that, whereas you simply won't be able to do so in the 190. The deco would be rediculous to boot.

Not very well thought out at all.

As he says, the time from 250 to 120 is minimal on back gas anyway.

Oh yah, I am not T1 I haven't been to scuby-board in a while so I am not sure where the DIR forum currently stands so I figured I'd point that out right away.

 

[allenw1972]

Ignore the bailout gas. The amount of time spent between 190 and 120 on a 250 dive in open water is not that long. If the dive is long enough to require a 3rd deco gas at all, you would be better off using a 120 gas where you would be spending some time. The 120 bottle will reduce the deco time whereas the 190 won't really do much. So choosing this deco gas over 120 for gas planning purposes is a bad idea.

The 50% vs. O2 on the otherhand is a different animal. On some dives, the deco optimization gained on the deeper stops makes which gas you choose a wash for total run time purposes.

Yes. I do agree that the 21/35 would be really bad shallow for deco. I could pick a better choice for the 80 cf bottle. The difference between the 21/35 and 35/25 is 20 cf at my gas consumption (which is pretty high), but I calculate my bottom consumption at a greater rate than my deco, and am doing 10' per minute from 170' to 100' in the case of a 20 minute bottom time. Not a whole lot with that said.

I am hearing you guys loud and clear. I think I may run the numbers again, but your arguments are well presented and well taken. Thanks for the help!

 

[lamont]

Ignore the bailout gas. The amount of time spent between 190 and 120 on a 250 dive in open water is not that long. If the dive is long enough to require a 3rd deco gas at all, you would be better off using a 120 gas where you would be spending some time. The 120 bottle will reduce the deco time whereas the 190 won't really do much. So choosing this deco gas over 120 for gas planning purposes is a bad idea.

Maybe what is needed is a stage to give greater backgas reserves?

Yes. I do agree that the 21/35 would be really bad shallow for deco. I could pick a better choice for the 80 cf bottle. The difference between the 21/35 and 35/25 is 20 cf at my gas consumption (which is pretty high), but I calculate my bottom consumption at a greater rate than my deco, and am doing 10' per minute from 170' to 100' in the case of a 20 minute bottom time. Not a whole lot with that said.

And doing 10 fpm up to 100' doesn't sound like it requires either a 120 or 190 bottle?