Beyond blind rote air management

[kwinter]

I agree with you in general about thirds, but I think there may be situations in which it is advisable to think along those lines, even though it may technically not be true. For example, if you are visiting a wreck with an upline and a strong current, you might be advised to plan your dive to thirds because you would rather not do your ascent off the line in current. That might being you to the surface away from your boat and bring more complications than you would like.

I agree. There is also the time it might take to shoot a bag and tie it off to create your own upline in case you couldn't get back to the tie in. Simple reef swim on a drift dive where you can always go straight to the surface? No problem using half or even two-thirds or more, just so that you can surface with 500 psi (even accounting for increase RMV due to stress). Something more complicated like a wreck or reef in current near the shipping lanes or involving penetration and overhead? Then a more conservative plan and larger reserve are needed. The conditions of the dive dictate the plan.

 

[Steve_C]

. IF you must get back to a point certain, how can you be doing a "recreational dive" which has as one of its controlling parts that the surface is always an option?

The question is whether the surface is a good option.

On the dives I do a lot ( NC recreational dives) the surface is always an option. But it is often not a good option and often not a very desirable option. The best thing is to try and insure you are back at the upline with gas to spare. Otherwise you either have a swim if the current is not too bad or you create a hassle for the boat and other divers.

 

[DaleC]

I seem to recall the thirds rule being mentioned in recent solo diving discussions as a means of gas planning taught during such a course ??

 

[Steve_C]

Gas management was discussed a lot in my solo course with SDI. Thirds was mentioned as one option. But the idea that thirds was not always the best choice was also discussed.

 

[swimmer_spe]

Plan the dive, and dive the plan.

1/3 to go down, 1/3 to come up, 1/3 for when things go wrong.

It is easy for a newbie to look at their gauge and see that you start with about 3000psi, so, when you go down, you should have used 1000psi. Whatever you didn't, you can use to look around.

I was taught that, but I was also taught that you should never return with under 500psi.

As you get better, you can change how you do things.... untill it comes back to bite you.

 

[nimoh]

Plan the dive, and dive the plan.

1/3 to go down, 1/3 to come up, 1/3 for when things go wrong.

It is easy for a newbie to look at their gauge and see that you start with about 3000psi, so, when you go down, you should have used 1000psi. Whatever you didn't, you can use to look around.

I was taught that, but I was also taught that you should never return with under 500psi.

As you get better, you can change how you do things.... untill it comes back to bite you.

that's a reasonable plan if you go down and swim in one direction.

For many dives, you drop down an anchor line and swim around a wreck within 50' of the anchor line. Now you might swim 500' on your 1/3 of gas, but are still within 50' of the anchor line. Do you really need 1/3 of your gas to get back to the anchor line and back up to the boat...and do you really need 1/3 reserve for this?

On a 60' dive with an AL80, an average diver will dive for 20 minutes or so on 1000psi, but only need 5 or so minutes to make an ascent, getting them on the surface with half of their gas remaining unused.

 

[victorzamora]

that's a reasonable plan if you go down and swim in one direction.

For many dives, you drop down an anchor line and swim around a wreck within 50' of the anchor line. Now you might swim 500' on your 1/3 of gas, but are still within 50' of the anchor line. Do you really need 1/3 of your gas to get back to the anchor line and back up to the boat...and do you really need 1/3 reserve for this?

On a 60' dive with an AL80, an average diver will dive for 20 minutes or so on 1000psi, but only need 5 or so minutes to make an ascent, getting them on the surface with half of their gas remaining unused.

This is a great point, and this kind of dive is what motivated me to get a MinGas chart going for me. Just to use your sample dive as an example: Assuming 60fsw, good enough vis to see the line at all times, you and one buddy, AL80. I'd want to be back on the anchor line with 900psi. I should be surfacing with 590psi if all goes well. If something goes wrong at or before 900psi, that assumes my buddy and I have a combined SAC of 2 ft3/min (high) and take a long time to figure out what to do at the bottom....both of these are tools to add conservatism. If something goes wrong, we should still be surfacing with some gas in the tank, adjusted to account for a bad SPG (pretty common).

My plan is to laminate this, and/or get it permanently pasted on my slate, so that I can do this on the fly. My hope is to use this when a square profile is necessary, AND when it's not. My plan for when it's not is to ride the MinGas curve up, the way so many ride their NDLs. When I get close to my MinGas pressure, ascend some. Rinse and repeat as applicable.

 

[Kharon]

Perhaps there are some concepts in this article that you'll find helpful ... Understanding Gas Management

Couple of problems with the article.

Your SAC rate is defined as the amount of gas you breathe in one minute at the surface. It can be expressed as pressure (PSI) or volume (cubic feet).

Actually it can't be described in psi because that varies with the tank capacity and working pressure. Volume is constant.

For the purpose of this discussion, and to avoid confusion, we will refer to your SAC rate in terms of pressure. When expressing your air consumption rate as volume, we will refer to it as Respiratory Minute Volume (RMV).

Nope. SAC is volume of air breathed at surface at rest per minute. RMV is the actual volume of air breathed per minute on a single dive. It is specific to that dive and varies depending on a rather large number of factors.

You will often see someone describing their SAC rate as a number … this is not exactly correct, as it is only expressing your air consumption as calculated for a single dive. Your SAC rate is really a range, because it’s based on your breathing rate … which does not remain constant over a period of several dives, or even during a single dive.

Here again you are confusing SAC with RMV. SAC is a number and is unrelated to air breathed during a dive. SAC is volume of air breathed at surface at rest per minute. SAC usually remains nearly the same for a diver over time, again varying somewhat with fitness and a number of other factors. However it doesn't vary nearly as much as RMV over a number of dives. RMV is dive related. SRMV (raw, average, and deep) are values that are calculated from RMV and depth (for average and deep) describing what the air used during the dive would equate to had it been breathed at the surface.

Sorry to be pedantic - but there is way more than enough confusion relating to SAC, RMV, and SRMV (Surface Respiratory Mninute Volume).

 

[boulderjohn]

Couple of problems with the article.



Actually it can't be described in psi because that varies with the tank capacity and working pressure. Volume is constant.



Nope. SAC is volume of air breathed at surface at rest per minute. RMV is the actual volume of air breathed per minute on a single dive. It is specific to that dive and varies depending on a rather large number of factors.



Here again you are confusing SAC with RMV. SAC is a number and is unrelated to air breathed during a dive. SAC is volume of air breathed at surface at rest per minute. SAC usually remains nearly the same for a diver over time, again varying somewhat with fitness and a number of other factors. However it doesn't vary nearly as much as RMV over a number of dives. RMV is dive related. SRMV (raw, average, and deep) are values that are calculated from RMV and depth (for average and deep) describing what the air used during the dive would equate to had it been breathed at the surface.

Sorry to be pedantic - but there is way more than enough confusion relating to SAC, RMV, and SRMV (Surface Respiratory Mninute Volume).

Actually, the fact is that a lot of people use the terms in different ways. You may have been taught one use of the term; others have been taught another. In the Dive Planning course I wrote, I talk about this confusion. I point out that SAC rate has come to mean different things because of the way it is used. A lot of people use SAC to mean what other people mean by RMV.

If a diver is always using the same sized tank, it is a lot easier for that diver to think of SAC in terms of PSI. A large percentage of divers have never even seen anything but an AL 80. There is nothing wrong with them thinking of SAC in PSI, as long as they know it will be different should they ever try a different sized tank.

 

[mathauck0814]

Thirds, mingas, etc. are all meaningless unless you have a handle on what you breathe; not just your moderate SAC, but your wildly panicked "holy !@#*" breathing rate.

People plan for best case scenarios and enter the water with woefully little gas - especially at depth. I once bailed out and managed to suck down 120ft^3 on a ~100 foot linear swim to the anchor line and then up from ~80 feet in depth (with a normal OC RMV of 0.6ft^3).

It works most of the time, and that's testament to how reliable equipment is and how infrequently folks have cause for that momentary panic, but it doesn't demonstrate that folks are conducting good gas management because they just don't know how much they can breathe if properly motivated.

Once you know your potential gas requirements, you can pick the best equation to plug those needs into based on the dive ahead (whether it's thirds, rock bottom, etc)