Calculating Sac Rate Without Getting Wet.

[noj3333]

Being an Upstate NY diver I’d like to calculate my SAC rate so I can get a pony tank that’s an appropriate size. I know the formula, but I’m curious if I run the numbers at surface pressure and not at depth.

Thanks, jon

Formula I'd use:

[ {(PSI Start - PSI End) x 33} ÷ (Depth + 33)] ÷ Time in Minutes = SAC Rate in PSI/min

 

[The Chairman]

Sure, why not? It won't be accurate at all. The only way to do it accurately is during a dive.

As for determining the size of a pony? If you're under 130, a thirty should be plenty. If you're over that, then probably a 40 is more appropriate. Anything less might be easier, but if you're using it to replace your buddy (solo diving) then you're cutting it too slim.

Caveat. I don't believe that ponies should ever be a part of gas planning except as a bailout. It's not a redundant supply if you're relying on it to finish your dive.

 

[clownfishsydney]

No way you can work out SAC on the surface, people breath totally differently underwater, some are more relaxed, others more stressed.

 

[The Chairman]

No way you can work out SAC on the surface

As long as you do both, I think it might be rather beneficial. First, they get practice in calculating their SAC. Then they get to see just how stress and stuff starts to affect your SAC.

 

[noj3333]

I'm not looking to get into tech diving, and I don't see going deeper then 100ft (currently I'm only OW, and not AOW certified). Mainly I'm looking for some extra security with an insta-buddy, or possibly for shallow solo dives, should I go that route and get the training I would get a 40.

I'm leaning towards a 13cu/ft or maybe a 19 for now...

 

[tbone1004]

too small imho. Unfortunately your body goes under many physiological changes when it is fully immersed, especially in cold water, especially in a wetsuit and there is no way to draw a line between dry SAC and wet SAC.

Regarding pony bottles, the "rule" is to use 1cfm as a conservative number and plan accordingly. You need to learn and read up about rock bottom, why it is important, and how to use it appropriately
DiveNerd - About the Rock Bottom Calculator for Scuba Gas Management

They also have a snazzy calculator. 1cfm is not abnormal in an "Excited" state for most divers, and is only about 50% higher than your normal sac rate. I like to reserve a 500psi buffer because your reg needs at least 200psi to function properly, and you want to have an extra little bit of wiggle room. Based on this, you need 20cf for the actual ascent, based on 1 minute to figure out what is going on, 2 minutes to ascent to half your depth which I don't really agree with, but it doesn't really matter, and then 5 minutes to the surface. I don't think 1 minute is enough assessment time, especially for new divers, so I like to double that, so add in another 4 cubic feet to that. This is mainly due to time needed to get back to the anchor line, or just the general air sharing process. Then you need a PSI buffer which is independent of tank size. Based on this you have 24cf that you need, and 500psi in an AL30=5cf, so you need an AL30 to safely have a pony bottle at 100ft/30m without having to expedite your ascent. If you go deeper than 100ft, you need a 40.
Others will argue until the cows come home about how this is unnecessary and that you can make an ascent from 100ft with a 13 or 19, and yes, you absolutely can, but as soon as you are on that bottle you have to start going up. Remember, at 100ft, on a conservative number, every minute you will suck between 700-1000psi out of that AL13, and 400-600psi out of the 19. That doesn't give you hardly any time at depth, and you have to basically book it to the surface. The other factor is an al13 and an al19 are useless for anything other than a pony bottle. They are too big for an inflation bottle, and too small for a deco bottle if you get into any sort of accelerated deco, which I highly recommend every diver taking a basic decompression class.

 

[redacted]

With a good sac a 13 will do the job as long as you remain calm. Why panic when you have enough gas? But you can't waste time so assessments and mooring line are out. The nice thing about a 13 is it fits in your baggage for air travel.

A 19 give you more time for assessment and travel but does not pack in baggage as well.

A 30 or 40 should give you more than enough gas for any OW diving situation but will probably cost extra if you want to take it by air travel. There are some USA ops that restrict bailout bottle size to 30 or less.

 

[The Chairman]

There are some USA ops that restrict bailout bottle size to 30 or less.

Really? I've never run into this at all. But then, I rarely dive with a pony.

 

[redacted]

Really? I've never run into this at all. But then, I rarely dive with a pony.

The Spree and the Fling had that restriction when they operated at the FGB. While it looks like the Spree dropped that restriction, the Fling still has it.

 

[The Chairman]

Yeah, I dove with a 40cf deco bottle off of the Spree just last year. I've never been on the Fling.

I have over 500 dives using an AL19. Had a special bracket to hold it onto my HP120s. Being a fairly cautious diver, I never ever needed it. However, it was a constant hassle switching it from tank to tank. Also, it got snagged a couple of times on mono and so forth. I wasn't diving solo, so I decided to sell it off. It had actually gone out of hydro on just one fill. Kind of funny.

For many divers, a pony is a solution to a skills/attitude problem. Too often, ponies are used to extend dives rather than as a bailout. If you need more gas, just get a bigger tank. If you're prone to running out of gas, change your attitude. Learn to check your gas a bit more often. It certainly beats increasing the complexity of your kit to compensate for Scuba ADD.