Big Heavy tanks!

[Wibble]

I'm not asserting that either one is more "correct" as there appears not to be a consensus, but I observe that Wibble's definition is not the same as SDI's definition -- I was using the one from SDI. It seems that experienced divers disagree on the exact method of gas planning -- e.g. berndo suggests determining that the buddy has 40-50 min of time with a 12l tank at roughly the same depth, from conversation. The assumption, then, is that the buddy's reply is based on his own past dives, so this is similar to estimating RMV from past data, except that it estimates the total consumption at depth, rather than the rate of consumption. It seems that scubadada has analyzed data from a large set of past dives.

You state the SAC as a Surface Air Consumption, i.e. at 1 ATA.
Example 20 litres/min.

Then you multiply it by your depth in ATA:
e.g. 20m/66ft = 3ATA

Therefore you will consume 20 (litres/min) x 3 (ATA) = 60 litres per minute.

How much fuel in the tank? Simple, multiply your tank volume (litres) by the pressure (bar).
e.g. 11 litre (ali80) x 100 bar = 1100 litres.

How long will your gas last? Simple, divide your gas amount by your consumption at depth.
e.g. 1100 (litres) / 60 (litres/min) = 18 minutes before it's empty.

If you want to live, you might want to work out how much gas it takes to reach the surface WITH a safety stop from your depth. Worked example:

• 2 minutes to get your act together on the bottom (SMB, tell your buddy, get your A into G...), so that's 2 x 60 = 120 litres
• 1 minute to ascend from 20m/66ft to 15m/50ft (no, it's not fast, it's sufficient): use the bottom amount for contingency: 60 litres
• 1 minute from 15m/50ft to 10m/33ft. ALWAYS use the lowest amount 15m/50ft = 2.5 (ATA) x 20 (litres/min SAC) = 50 litres
• 1min from 10m/33ft to 5m/15ft = 2 ATA x 20 = 40 litres
• 3 minutes at 5m/15ft safety stop = 3 (mins) x 1.5 (ATA) x 20 (SAC) = 90 litres
• 2 minutes from 5m to the surface (contingency, slowly slowly, maybe waity for buddee) = 2 (mins) x 1.5ATA x 20 (SAC) = 60 litres
• Ascent total: 120 + 60 + 50 + 40 + 90 + 60 = 420 litres.
• THEN add your "must return contingency amount" which is 50 bar (or 500 PSI = 33 bar in the USofA) which equates to 50 (bar) x 11 litres (volume of your ali80) = 550 litres.
• Grand total of gas needed from the bottom at 20m/66ft = 420 + 550 = 970 litres.
  Call it 1000 litres.

Therefore your end the dive pressure in your ali80 tank is:
1000 (litres required) / 11 (litres for ali80) = 90 bar.

Note: Even though our working SAC will be less than 20 for an experienced diver, you NEVER use the lowest rate as you must have a contingency for real problems; like that gas hog grabbing your alternate air supply or rescuing that person who's panicking, or having a big surface swim... etc.


For your homework, redo those sums for:

• An emergency, say using 25 litres/min SAC.
• Then try 30 litres/min SAC.
• Now do that for two divers sharing gas at 20 litres/min SAC EACH (that's 40 litres/min SAC)
• Then redo the sums for 30m/100ft.
• Consider the implications for those consumptions if using a small 3 litre (20 cubit ft) PONY

 

[formernuke]

Note the pressure indicated by @Wibble is where you start your ascent. Not turn around pressure.

The problem with this thread in my opinion is it is basic scuba. A lot of us are very experienced divers and regard what is taught in OW as severely lacking in this area. However that is what is taught. And being basic section we are not supposed to getting into what we do and were taught in tec level classes.

 

[gordonscuba]

For your homework, redo those sums for:

• an emergency, say using 25 litres/min SAC.
• Then try 30 litres/min SAC.
• Now do that for two divers sharing gas at 20 litres/min SAC EACH (that's 40 litres/min SAC)
• Then redo the sums for 30m/100ft.
• Consider the implications for those consumptions if using a small 3 litre (20 cubit ft) PONY

Nice way of explaining it, I like that very much. Rather than calculating for 25, 30 and 40 separately, I would just note that in your example, the ascent total is 21 times the sac rate (which I call surface RMV). So for 25L/m it's 525L and for 30L/m it's 630L, and for 40L/m it's 840L.

Starting at 4 ATA the analogous calculation, always taking 1 min for each 5m until the safety stop, where s is the sac rate, gives ascent total
2 * 4 * s + 4 * s + 3.5 * s + 3 * s + 2.5 * s + 2 * s + 3 * 1.5 * s + 2 * 1.5 * s
= 30.5 * s
Plugging in s = 20, 25, 30 and 40 (all L/min) gives 610, 762.5, 915, 1220, in Liters.

Conclusions: a 3L pony at 200 bar is 600L so it's less than ascent total for most of these scenarios. (I use a 40cf as a pony.) These depths are all squarely recreational depth, so my personal opinion is that this kind of calculation should probably be taught sooner. Divers going from a boat with an instructor/DM might be okay trusting the DM to do this for them, but what if it's just buddies, both recently AOW, doing a shore dive together to 30m? That's allowed under padi standards...

 

[Wibble]

Note the pressure indicated by @Wibble is where you start your ascent. Not turn around pressure.

The problem with this thread in my opinion is it is basic scuba. A lot of us are very experienced divers and regard what is taught in OW as severely lacking in this area. However that is what is taught. And being basic section we are not supposed to getting into what we do and were taught in tec level classes.

It really bugs me that those calculations are just simple arithmetic yet people don't do them, relying on simple "end the dive at 100 bar" or whatever the DiveMaster says (have they done the arithmetic?).

This is especially the case for people who have a high SAC rate who MUST end their dive earlier than others, or mitigate their high consumption with larger tanks. Better still, most high gas consumption will improve with practice and coaching for skills, leading to better confidence in the water. Good trim (=flat), good buoyancy (=not overweighted), good finning (=efficient) leads to a far more enjoyable and stress free dive... and less gas consumed.

 

[steinbil]

Note the pressure indicated by @Wibble is where you start your ascent. Not turn around pressure.

The problem with this thread in my opinion is it is basic scuba. A lot of us are very experienced divers and regard what is taught in OW as severely lacking in this area. However that is what is taught. And being basic section we are not supposed to getting into what we do and were taught in tec level classes.

I disagree with this sentiment for several reasons:

1. I was taught basic gas planning in my OW
2. I'm not tech trained or do tech dives, but I find gas planning very useful
3. I think it should be part of OW training
4. There is no harm in exposing OW divers without this knowledge to the very simple concept of gas planning with the very simple math it requires
5. If you are a certified OW diver (not supervised diver) you should be able to plan and execute a dive safely with a buddy - gas planning should be part of this

Do casual OW divers that never dive without a DM need this knowledge? Probably not. Are there other ways of mitigating the risk of not having enough gas for a given dive? Probably. Should we scare or make fun of OW divers that didn't learn this? Absolutely not! Should we encourage them to learn about it, so they're better equipped to dive without a DM if they want to? Absolutely!

 

[Angelo Farina]

These calculations are truly simple in SI units. They are a bit more complex in Imperial units.
I suppose this is one of the reasons for which not-US based agencies, such as CMAS and BSAC, teach these calculations at the very basic level, whilst US-based agencies shift this training to more advanced levels.

 

[steinbil]

You state the SAC as a Surface Air Consumption, i.e. at 1 ATA.
Example 20 litres/min.

Then you multiply it by your depth in ATA:
e.g. 20m/66ft = 3ATA

Therefore you will consume 20 (litres/min) x 3 (ATA) = 60 litres per minute.

How much fuel in the tank? Simple, multiply your tank volume (litres) by the pressure (bar).
e.g. 11 litre (ali80) x 100 bar = 1100 litres.

How long will your gas last? Simple, divide your gas amount by your consumption at depth.
e.g. 1100 (litres) / 60 (litres/min) = 18 minutes before it's empty.

If you want to live, you might want to work out how much gas it takes to reach the surface WITH a safety stop from your depth. Worked example:

• 2 minutes to get your act together on the bottom (SMB, tell your buddy, get your A into G...), so that's 2 x 60 = 120 litres
• 1 minute to ascend from 20m/66ft to 15m/50ft (no, it's not fast, it's sufficient): use the bottom amount for contingency: 60 litres
• 1 minute from 15m/50ft to 10m/33ft. ALWAYS use the lowest amount 15m/50ft = 2.5 (ATA) x 20 (litres/min SAC) = 50 litres
• 1min from 10m/33ft to 5m/15ft = 2 ATA x 20 = 40 litres
• 3 minutes at 5m/15ft safety stop = 3 (mins) x 1.5 (ATA) x 20 (SAC) = 90 litres
• 2 minutes from 5m to the surface (contingency, slowly slowly, maybe waity for buddee) = 2 (mins) x 1.5ATA x 20 (SAC) = 60 litres
• Ascent total: 120 + 60 + 50 + 40 + 90 + 60 = 420 litres.
• THEN add your "must return contingency amount" which is 50 bar (or 500 PSI = 33 bar in the USofA) which equates to 50 (bar) x 11 litres (volume of your ali80) = 550 litres.
• Grand total of gas needed from the bottom at 20m/66ft = 420 + 550 = 970 litres.
  Call it 1000 litres.

Therefore your end the dive pressure in your ali80 tank is:
1000 (litres required) / 11 (litres for ali80) = 90 bar.

Note: Even though our working SAC will be less than 20 for an experienced diver, you NEVER use the lowest rate as you must have a contingency for real problems; like that gas hog grabbing your alternate air supply or rescuing that person who's panicking, or having a big surface swim... etc.


For your homework, redo those sums for:

• An emergency, say using 25 litres/min SAC.
• Then try 30 litres/min SAC.
• Now do that for two divers sharing gas at 20 litres/min SAC EACH (that's 40 litres/min SAC)
• Then redo the sums for 30m/100ft.
• Consider the implications for those consumptions if using a small 3 litre (20 cubit ft) PONY

Or an even simpler method:

• Assume 20L/min x2 divers = 40L/min
• Calculate a 3/min ascent rate*
• Use avg ATA
• 1 minute to solve problem

So your example of a dive to 20m:

• Max ATA = 3 / Avg ATA = 2
• Solve problem at depth: 1 * 40L * 3ATA = 120L
• Ascent: 20m @ 3m/min = 7min * 40L * 2ATA = 560L
• Total = 680L

If you want 2 minutes to solve problem, it would be an even 800L. The 3m/min ascent rate is conservative, to make up for possible delays, panicked SAC rate etc, but makes the calculation easier.

PS.
I see no point in planning to have any "must return contingency". These gas reserves are planned for an emergency. If you have an emergency you should use all the gas in the tank if you need it. If you don't have an emergency, you will still have the reserves when you exit the water, which would cover the must return amount.

 

[Wibble]

Or an even simpler method:

• Assume 20L/min x2 divers = 40L/min
• Calculate a 3/min ascent rate*
• Use avg ATA
• 1 minute to solve problem

So your example of a dive to 20m:

• Max ATA = 3 / Avg ATA = 2
• Solve problem at depth: 1 * 40L * 3ATA = 120L
• Ascent: 20m @ 3m/min = 7min * 40L * 2ATA = 560L
• Total = 680L

If you want 2 minutes to solve problem, it would be an even 800L. The 3m/min ascent rate is conservative, to make up for possible delays, panicked SAC rate etc, but makes the calculation easier.

PS.
I see no point in planning to have any "must return contingency". These gas reserves are planned for an emergency. If you have an emergency you should use all the gas in the tank if you need it. If you don't have an emergency, you will still have the reserves when you exit the water, which would cover the must return amount.

"Must return with" pressure.

This is Basic Scuba with many inexperienced novices; returning with less than 50 bar (or the lesser US practice of 500psi = 33 bar) should get a grilling on the boat for nearly running out of gas which is dangerous and an extremely bad practice.

If nothing else, a tank that is empty may need a visual inspection for water ingress. Hire operators will not be happy if there's less than the minimum upon return.

 

[steinbil]

"Must return with" pressure.

This is Basic Scuba with many inexperienced novices; returning with less than 50 bar (or the lesser US practice of 500psi = 33 bar) should get a grilling on the boat for nearly running out of gas which is dangerous and an extremely bad practice.

If nothing else, a tank that is empty may need a visual inspection for water ingress. Hire operators will not be happy if there's less than the minimum upon return.

I'm simply saying if you calculate an emergency reserve, you will return to the boat with enough unless you have an emergency - in which case it doesn't matter.

 

[berndo]

It really bugs me that those calculations are just simple arithmetic yet people don't do them, relying on simple "end the dive at 100 bar" or whatever the DiveMaster says (have they done the arithmetic?).

No, the dive does NOT end at 100 bar (more fake news). It's the TURING pressure at which you swim back. Nobody ever said in this thread or elsewhere that a dive ends with 100 bar.