Rock bottom, 500 PSI, or something else?

[boulderjohn]

It wasn't made clear to me in my GUE training whether they really believe it's better from a decompression standpoint or whether it's more along the lines of: for recreational dives to less than a maximum depth of 100 feet, regardless of whether it's better, it's probably not worse to any significant degree, and besides, it's a good skill to develop.

So you would not prepare for it in your calculations for an out of air emergency?

 

[tbone1004]

@boulderjohn can you clarify what PADI actually requires for SAC calculations at the recreational levels?

 

[boulderjohn]

@boulderjohn can you clarify what PADI actually requires for SAC calculations at the recreational levels?

In this thread it was referenced as being in the new AOW course. I teach primarily technical diving these days. I have not taught the new AOW course, and I do not have the textbook for it. I do have the instructor manual, knowledge reviews, etc.

SAC rate is not mentioned as a standard in the instructor manual. It is not in the knowledge reviews.

The dive requirements for the deep dive include the following:

1. With a buddy, plan and manage gas use, including
determining turn pressure, ascent pressure and
reserve pressure. Establish no stop and dive
time limits.​

I would have trouble teaching that requirement without teaching SAC rate. I imagine most instructors would include it, but I obviously have no way of being sure of that.

I always taught SAC rate as a part of the deep dive in AOW, even under the old standards.

 

[Lobzilla]

@boulderjohn can you clarify what PADI actually requires for SAC calculations at the recreational levels?

Yes, let's get this thread back to what new OW divers know these days and how they can get to a better awareness of how their gas resources relate to a safe completion of a dive.

This is obviously a safety issue but good awareness of our critical resources also leads to more comfort and enjoyment. And who would not want that?

 

[Lorenzoid]

So you would not prepare for it in your calculations for an out of air emergency?

I'm not sure I follow you. "Minimum gas" (aka rock bottom) for OOA emergencies is calculated based on 10 fpm. The "minimum deco" ascent strategy, where you take 30 seconds to reach the next 30-second stop at 10-ft intervals is, as I said, taught for reasons that I believe GUE doesn't make clear.

 

[PfcAJ]

It wasn't made clear to me in my GUE training whether they really believe it's better from a decompression standpoint or whether it's more along the lines of: for recreational dives to less than a maximum depth of 100 feet, regardless of whether it's better, it's probably not worse to any significant degree, and besides, it's a good skill to develop.

Minimum deco is just the ascent plan that decoplanner (or whatever your software of choice) spits out when it’s 1min stops to the surface. If you encounter a 2min stop somewhere then you’re past “minimum deco”.

 

[Kevrumbo]

Recreational -Minimum Gas Reserve (MGR) for a Gas Share Contingency/Emergency,
"CAT" vs old "Rock Bottom":

So as I understand it, the GUE "CAT" protocol using 100'/30m for example:
["CAT" is GUE's acronym-mnemonic for Consumption X Average ATA X Time]

100'/30m, take 1min at depth to donate long hose & problem solve;
(Ascent rate 10fpm, same as 3 meters per min, to surface takes -->10min);
Total Time to Ascend is 10 plus 1 equals 11min.

Average ATA is 4ATA plus 1ATA divided-by 2 equals 2.5 ATA.

Therefore given the new figure Surface Consumption Rate (SCR) of 0.75 cubic feet per minute, same as 20 liters per min,

CAT = 0.75 x 2 divers x 2.5 ATA x 11min = 41.25 cuft or 1200 liters in metric.

So the new easier to calculate GUE recreational min deco ascent strategy in a gas sharing contingency (after 1 minute at operational depth to problem solve) is a 30 second hold/30 seconds to ascend for every delta 10ft or 3m from operational depth -for an ascent rate of 10 fpm (3 mpm).

The above is a somewhat simpler uniform ascent rate, tactical wise, than UTD's Rock Bottom as outlined below:

------

By comparison for the 100'/30m example,
UTD recreational Rock Bottom procedure still retains the ascent strategy 30 fpm/9mpm rate from operational depth to 50% max operational depth, then the slower 10 fpm/3mpm from there to the surface, and using the higher SCR of 1 cfm (30 liters per min):

Rock Bottom = 1 x 2 divers x 2.5 ATA x 8min = 40 cf or 1200 liters in metric.

(See 14:30 mark in video below):

 

[Lobzilla]

Recreational -Minimum Gas Reserve (MGR) for a Gas Share Contingency/Emergency,
"CAT" vs old "Rock Bottom":

So as I understand it, the GUE "CAT" protocol using 100'/30m for example:
["CAT" is GUE's acronym-mnemonic for Consumption X Average ATA X Time]

100'/30m, take 1min at depth to donate long hose & problem solve;
(Ascent rate 10fpm, same as 3 meters per min, to surface takes -->10min);
Total Time to Ascend is 10 plus 1 equals 11min.

Average ATA is 4ATA plus 1ATA divided-by 2 equals 2.5 ATA.

Therefore given the new figure Surface Consumption Rate (SCR) of 0.75 cubic feet per minute, same as 20 liters per min,

CAT = 0.75 x 2 divers x 2.5 ATA x 11min = 41.25 cuft or 1200 liters in metric.

So the new easier to calculate GUE recreational min deco ascent strategy in a gas sharing contingency (after 1 minute at operational depth to problem solve) is a 30 second hold/30 seconds to ascend for every delta 10ft or 3m from operational depth -for an ascent rate of 10 fpm (3 mpm).

The above is a somewhat simpler uniform ascent rate, tactical wise, than UTD's Rock Bottom as outlined below:

------

By comparison for the 100'/30m example,
UTD recreational Rock Bottom procedure still retains the ascent strategy 30 fpm/9mpm rate from operational depth to 50% max operational depth, then the slower 10 fpm/3mpm from there to the surface, and using the higher SCR of 1 cfm (30 liters per min):

Rock Bottom = 1 x 2 divers x 2.5 ATA x 8min = 40 cf or 1200 liters in metric.

(See 14:30 mark in video below):

Once we have figured out how much gas volume we need, we can convert this to PSI by using the Tank Factor.

The tank factor tells us how many cubic feet our tank will hold for every 100 PSI.

For example, an AL80 holds 77 cubic feet at 3000 PSI. We divide 77 (rated full volume) by 3000 (rated pressure) and multiply by 100. That yields 2.57 cubic feet for every 100 PSI. Therefore:

• An AL80 has a tank factor of ~2.5
• A 3500 PSI 100 cubic feet tank has a tank factor of ~2.9
• Your ... tank has a tank factor of ??

We only need to calculate this once for every tank we have and memorize the number or write it on the tank.

Now if we plan 40 cubic feet for an air-sharing ascent, we divide those 40 cubic feet by the tank factor (e.g. 2.9 for a HP100) and get ~14 hundred PSI. That is the minimum pressure where we have to leave the bottom to be prepared for the worst case scenario.

As pointed out earlier, we can ascend to a shallower depth, recalculate our needed reserve for an air sharing ascent from there and continue diving until we reach that turn pressure. And so on.

However, if we continue at maximum depth past the turn pressure we know that our planned air-sharing ascent will become less and less of a viable option.

Every team has to decide where to draw the line (turn pressure) but we first need to be able to calculate how that pressure relates to the time we have for solving an unexpected problem.

I hope that the last two post allow every novice divers to do this. If not, ask questions here or in your class.

 

[boulderjohn]

I'm not sure I follow you. "Minimum gas" (aka rock bottom) for OOA emergencies is calculated based on 10 fpm.

I guess I must have misunderstood. I got the idea that for calculating rock bottom, the assumption was that in an emergency, the divers would go through the entire min deco profile, which you also said was only done as a training strategy for future decompression diving. I would not think that an OOA emergency would be the best time to work on skill acquisition for future decompression diving.

I also don't understand why 10 FPM is used for an OOA emergency.

 

[PfcAJ]

I guess I must have misunderstood. I got the idea that for calculating rock bottom, the assumption was that in an emergency, the divers would go through the entire min deco profile, which you also said was only done as a training strategy for future decompression diving. I would not think that an OOA emergency would be the best time to work on skill acquisition for future decompression diving.

I also don't understand why 10 FPM is used for an OOA emergency.

It shouldn’t be. It should be 30fpm to your first stop.

GUEs calculation method uses 10fpm and a .75 sac rate. It just so happens to give (close too, at least) the same value at the end of the day as a 1.0 sac rate with a 30fpm rate to the first stop and 10fpm thereafter.

It might be easier to calculate but it’s intellectually lazy af.