Understanding SAC & RMV

[jborg]

So I did a couple of qualification dives today and I'm playing around with the data from on of them. I have a fairly simple situation where the second dive started with 168 bar in the tank and ended with 75 bars, on a 10 L (300 bar) tank. The average depth was about six meters. We were down for 23 minutes and 45 seconds, excluding a surface interval in the middle. I can calculate the SAC from this:

- Gas used: 168 - 75 = 93 bars
- Time: 23.75 minutes
- Depth factor: (10 + 6) / 10 = 1.6

SAC = 93 / 23.75 / 1.6 = 2.45 bar/min at surface

Given a 10 liter tank this works out to an RMV of 24.5 L/min.

Now, this isn't meaningful in any real way because we did a lot of buoyancy exercises, buddy breathing ascent, a surface interval with full BCD, etc. But let's just pretend that was all my breathing just for fun, I'm not trying to draw any conclusions from the actual number itself.

What doesn't make sense to me is how the programs I tried calculate this same thing. Here's Subsurface:

- Dive time: 24 min
- Gas consumed: 881.6 L
- SAC: 22.9 l/min

22.9 is 881.6 / 24 / 1.6 so so far so good. But how do we get 881.6 liters from 93 bars? Is there some ideal-gas-law magic going on here or something else?

I also have MacDive which shows this:

- Air used: 93 bar
- Average depth: 6.29 m
- Duration: 23:45
- SAC: 2.97 bar/min
- RMV: 29.65 L/min

Huh? I don't get how we arrive at 2.97 bar/min here. I'd have hoped that these programs all followed roughly the same rules.

 

[atdotde]

Subsurface takes real gas corrections into account as described on When real gas corrections matter – The Theoretical Diver

 

[IDNeon357]

Can you tell me how this applies?

Are you just trying to estimate how much time you'll have with how much air?

 

[rsingler]

Can you tell me how this applies?

Are you just trying to estimate how much time you'll have with how much air?

For dive planning, yes - knowing your SAC will help you gauge how long your tank will last, but only if you know your true average depth for the dive ahead.

Knowing your SAC more commonly plays a role in numerous gas calculations, and can serve as a rough measure of progress as a diver.

For example, a new, anxious diver "wastes" some of his gas in anxious extra breaths, inefficient breathing patterns (anything other than deep and slow), and excessive inflation and deflation of his bcd. More important, the novice diver wastes gas in the excessive effort required when swimming out of trim, or not streamlined. For this diver, progress to more relaxed, efficient diving can be gauged by watching the decreasing SAC that he often can download from his computer for dives over the course of days, weeks or months. It can also be hand calculated, as you saw above. Knowing it serves as a source of satisfaction for many divers actively trying to improve their technique.

It is more commonly used in dive planning to determine "rock bottom gas".
If you know your SAC, you can determine rock bottom for any depth for each of two scenarios: with and without a need to share air. When you reach rock bottom tank pressure, you MUST be ascending, or you will not surface before going OOA. Calculating RBG (depending upon the system used) is a function of SAC, depth, time to ascend at a certain rate, whether or not you add time to solve a hypothetical problem while still at max depth, whether or not you are sharing air with a (? panicked, hyperventilating buddy), whether or not you make a safety stop, and what pressure you want left in your tank when you surface.
Many experienced divers always have a RBG tank pressure in mind for their planned max depth, and always start their ascent before reaching that pressure. The required RBG pressure decreases as your dive meanders along the reef into shallower and shallower water, until at 20 feet looking up at the boat, you may no longer care, knowing that you can make a CESA to the surface (obviously skipping your safety stop), in the event of a sudden, unexpected emergency.

Finally, in decompression diving, knowing your SAC allows you to determine how much gas you need to bring for decompression.

For the OP of this thread, wanting to make use of this data, he is concerned at the variations in what should be a fixed value, given what he thinks are identical initial variables.

 

[IDNeon357]

SAC plays a role in numerous gas calculations, and serves as a rough measure of progress as a diver.

For example, a new, anxious diver "wastes" some of his gas in anxious extra breaths, inefficient breathing patterns (anything other than deep and slow), and excessive inflation and deflation of his bcd. For this diver, progress to more relaxed, efficient diving can be gauged by watching the decreasing SAC that he often can download from his computer for dives over the course of days, weeks or months. It can also be hand calculated, as you saw above.

For dive planning, yes - knowing your SAC will help you gauge how long your tank will last, but only if you know your true average depth for the dive ahead.

Or is more commonly used in dive planning to determine "rock bottom gas".
If you know your SAC, you can determine rock bottom for each of two scenarios: with and without a need to share air. When you reach rock bottom, you MUST be ascending, or you will not surface before going OOA.

Finally, in decompression diving, knowing your SAC allows you to determine how much gas you need to bring for decompression.

For the OP of this thread, wanting to make use of this data, he is concerned at the variations in what should be a fixed value.

I enjoy the difference in methods of scientific versus artistic.

I fall far more into the artistic category, where I determine things by "feel" rather than by data. And I'm very effective at that, but if anything it's probably because of a good retention/memory of the data I'm observing while doing an activity.

Like I know how much PSI I breathe with each breath at certain depths, for instance. I retain that in my head, same with how much PSI is consumed by my BCD over the period of a dive. And I have a good mental clock.

 

[rsingler]

I fall far more into the artistic category, where I determine things by "feel" rather than by data. And I'm very effective at that, but if anything it's probably because of a good retention/memory of the data I'm observing while doing an activity.

Well, without trying to sound snarky, you might want to rethink that. At 80 feet, when you're feeling fine, and your buddy comes zooming up to you, grabbing for your octopus, breathing like there's no tomorrow, and it's your third dive of the day with a significant nitrogen load...

Do you know you have enough gas to get you both to the surface, and both make the five minute safety stop that is probably advisable after that depth and that many dives? It's not only your own gas consumption that needs to be considered.

Jes' sayin'.

 

[IDNeon357]

Well, without trying to sound snarky, you might want to rethink that. At 80 feet, when you're feeling fine, and your buddy comes zooming up to you, grabbing for your octopus, breathing like there's no tomorrow, and it's your third dive of the day with a significant nitrogen load...

Do you know you have enough gas to get you both to the surface, and both make the five minute safety stop that is probably advisable after that depth and that many dives? It's not only your own gas consumption that needs to be considered.

Jes' sayin'.

I have to go unfortunately, and can't word something write at this time, but would like to continue this conversation later.

To run through your hypothetical a brief bit, I know that I have to sacrifice 300psi for BCD, and I know that at an average depth of 60fsw it'll be a solid 10psi per breath.

I know by watching the clock that each breath is approximately every 10 seconds so 3000psi starting fill, minus 300 = 2700, divided by 10psi a breath = 270, divided by 6 breaths a minute gives me a guesstimated time of 45 minutes.

Ignore finishing with 500psi (cuz I like this well rounded number for now) we'll follow the rule of thirds and rely upon a stop time of 5 minutes x 2 for two persons.

45 minus 10 minutes = 35 minutes and that gives about 11.5 minutes down, and 11.5mins back with the cushion needed for stop time and no consideration for "panic" which I don't consider is even calculable.

Maybe you could say a 2x-rate of breathing for panic, in which case let's say your friend's breathing is the same as yours, but panicking means he is breathing 12x a minute.

Obviously you'd want to figure an adjustment to your rule of thirds so that at any point along this square dive profile to 60fsw, you would still have a reasonable cushion.

That's about all I can say to that for now.

But I think it can be reasonably "figured' without the sciencey stuff...

Also sorry, but I'm rushing to get out of work, so I would love to continue this!

I don't think there's an exact answer in this scenario, but I'd love to continue the talk and think more about it!

 

[wetb4igetinthewater]

@IDNeon357

You might be interested in the gas planning/expectation portions of this doc.

 

[tursiops]

But I think it can be reasonably "figured' without the sciencey stuff...

It is not clear to me what is sciencey and what is artisitic, in your mind. You are using data, and making calculations. Is that not science?

 

[yle]

Well, without trying to sound snarky, you might want to rethink that. At 80 feet, when you're feeling fine, and your buddy comes zooming up to you, grabbing for your octopus, breathing like there's no tomorrow, and it's your third dive of the day with a significant nitrogen load...

Do you know you have enough gas to get you both to the surface, and both make the five minute safety stop that is probably advisable after that depth and that many dives? It's not only your own gas consumption that needs to be considered.

Jes' sayin'.

Also not trying to sound snarky... if he's at 80 feet and he has significant nitrogen loading from two previous dives that day, and he's advised to make a five minute safety stop because of that depth and that many dives... gas planning probably won't be much of an issue because that third dive will be limited by his NDL and not by his gas supply. In other words, he'll have plenty of gas for whatever happens.

If I was in the scenario you described, I know I'd still have more than half a tank when it was time to end the dive.