Do you avoid people with bad sac rates?

[miketsp]

The problem with correlation is that you have to look at what you're correlating.
I just had a quick look through the electronic log of my last 196 dives and what I found was that most of my deeper dives actually tend to be warm-water dives and when warm-water diving I get a lot colder than when cold-water diving.
Cold water diving I wear a 7mm semi-dry and warm water I wear a 3mm shorty or full.
So in fact it is true to say that I'm normally a lot colder on my deeper dives and I've definitely noticed that when I'm cold my SAC increases. On deep dives with my semi-dry there is no SAC impact compared to a shallow dive.
So when I bring body temperature into the equation I find that this clearly overrides any depth effect on SAC.

 

[ClayJar]

Stop and think a moment...if I have longer dive times with the same size tank....gee maybe my SAC will decrease?

If you always breathe down to your limit, sure. With lower SAC, dive time can increase, as you won't be OOA.

Does the correlation remain when you filter data to only use dives for which there was ample gas remaining (i.e. where gas consumption was not the reason the dive was turned)? None of the data I've seen can address that, but I'm open to considering ways of testing any hypotheses we may come up with.

Of course, if we step back and try not to lose the point, the point would be simply that SAC is not an appreciable function of depth. While a best linear fit of the data at hand may indeed show SAC decreasing slightly with depth (something like 0.02 cfm for a 40 foot change, given my data), the effect, if it indeed exists, is an order of magnitude less than the potential day to day variations I see with my SAC (based on things like currents, temperatures, etc). Just as there's little point to finding your perfect to-the-ounce buoyancy (it will vary by more than that based on what you eat), there's little point to SAC as a function of depth, given the normal range of variation even in a single analyzed diver.

After an hour at 66 feet, a 0.02 cfm SAC variance would mean 3.6 cubic feet, or about 140 psi in an AL80. On my SPG, that would be about the difference between the top of the 500 psi tick mark and the bottom of the 500 psi tick mark. :biggrin:

 

[TSandM]

The answer to this thread seems to be that nobody would deny a buddy just because of SAC, but I fear the corollary may be that quite a few would deny a buddy if they started throwing this much math around.

Not me! Throw that much math around, and you go on my short list of "would love to dive with" people, along with Lamont

 

[do it easy]

Of course to get better data one would use a compression chamber and use good flow rate and volumetric measurments at various pressures with a person doing light exercise for an exact time period. Then the results would really mean something. This was just something I noticed in the data.

Mike

I think a better way would be to do the same dive at different depths and temps- say swim 200' at various depths and compare your SAC that way.

 

[mikerault]

do_it_easy,

I was suggesting a method that eliminates human error as much as possible. If the exact volume of air a person is using is measured under the controlled conditions in a chamber then the numbers will be accurate. Doing it underwater where depth needs to be maintained and you are dependent upon less than accurate gages to get the delta pressures, you are introducing many sources of possible error into the calculations.

 

[do it easy]

do_it_easy,

I was suggesting a method that eliminates human error as much as possible. If the exact volume of air a person is using is measured under the controlled conditions in a chamber then the numbers will be accurate. Doing it underwater where depth needs to be maintained and you are dependent upon less than accurate gages to get the delta pressures, you are introducing many sources of possible error into the calculations.

If you conduct the "dives" in a chamber than the SAC rate will have nothing to do with diving, just "air comsumption while doing light exercise under pressure." How would you convert your dry number to a wet number?

I would rather have an inaccurate number that was measured while actually diving since it will probably be a better estimate. My SAC rate is predictable/stable enough for dive planning purposes.

 

[Ice Mike]

Some are short because I was using up a tank, a dive buddy called the dive, or other reasons that probably didn't affect the SAC.

Some where stressed, others were not (hence averaging the numbers using linear approximation to smooth these outlayers).

Of course to get better data one would use a compression chamber and use good flow rate and volumetric measurments at various pressures with a person doing light exercise for an exact time period. Then the results would really mean something. This was just something I noticed in the data.

Mike

Ok kicked out the dives shorter than 20 min. Assumed they were dives called short for one reason or another and not representative.
Did a quick and once again very dirty analysis for both time and depth. Time remains the key factor on your sac.

See contourplot for graphical representation: both deep and shallow you are capable of a good sac. But short dives are not capable of a good sac.

To get better data:
1. Make sure that SAC is never a reason to cut a dive short. (Bring enough air).
2. Make many more dives (ideally hundreds of dives)
3. Start logging other things like water temp
4. Always use the same equipment (don't switch between wet and dry)
5. Always dive with the same buddy
6. measure current at dive location (if applicable)
7. analyse the data in chronological order

maybe then a relationship can be found between your sac and other factors.

For now there really arent any relationships apparent. (The fact that you think you see a relationship in excel means nothing. Humans are programed to see relationships.
It is sound statistical analysis that proofs or disproofs relationships. Not really possible in excel, I use minitab. Disclaimer: I did not use sound statistical analysis (it's late, I am not good enough in it yet and I want to get paid for doing the analysis ))

Back to original question:

I will only avoid a high sac if, the following criteria are met:
1. the person with higher sac is an insta buddy
2. on a very very special dive
3. which costs me more than average money to make

So a high sac insta buddy on normal dive at normal cost would not be a problem. But on that once in a lifetime dive.....

 

[Ice Mike]

Not me! Throw that much math around, and you go on my short list of "would love to dive with" people, along with Lamont

I knew it! Deep down women love math !

 

[Ice Mike]

I think a better way would be to do the same dive at different depths and temps- say swim 200' at various depths and compare your SAC that way.

Yep this would be my preferred way.

Have a good think about what you are trying to proof/disproof and then design some experiments and carry them out in a statistically sound manner. (Ie repeating measurements etc.)

You will then most likely find that depth isn't that relevant (within rec limits).

Things like workload, temperature, stress are more important for your sac. But I don't think that you really need to do the experiments to figure that out.

 

[Ice Mike]

I would rather have an inaccurate number that was measured while actually diving since it will probably be a better estimate. My SAC rate is predictable/stable enough for dive planning purposes.

Agree

And even if you could complety descripe the SAC relationship. The dive will never be exactly to plan. So saying something about your sac for a future dive will always be an estimation.