Calculating Sac Rate in Excel

[Capt Jim Wyatt]

but..use the max depth to get the best, most accurate number.

 

[Spectre]

Capt Jim Wyatt once bubbled...
but..use the max depth to get the best, most accurate number.

I don't intend to argue. We're talking mathematics here. How, mathematically, can using max depth be more accurate than average depth. Likewise, how does using max depth give you a more conservative number?

Think rationally here. Two dives. one very square, at 99. One multi-level, averaging 66 feet, max depth 99. You're trying to tell me that the most accurate method of calculation is using 99 feet for _both_ dives.

That's not only wrong, but dangerous advice.

 

[Silverback]

I'm not an insturctor here, and I've only just recently gotten into the area of mixed gas diving, but I think what may be wrong here is the fact that the wrong terminology is being used. I don't believe using the max depth won't give you the "most" accurate SAC, but will generate the more conservative value... The deeper you are the more gas you'll used with all things being equal.

Averaging you depth for decent and ascent, and then combining them with the max depth is a good way to be closer on obtaining a SAC, but to do this 100% accurate, you would need to calculate every stage of the dive on all three planes.

????

 

[Capt Jim Wyatt]

is correct. Its pretty simple stuff really.

The point I was trying to make is that it is better/more accurate NOT to do depth averaging while calculating SAC rates.

Seems to me we are talking apples & oranges.

I hope others do not feel I am giving out dangerous advice as spectre seems to think.

 

[Soggy]

Capt Jim Wyatt once bubbled...
The point I was trying to make is that it is better/more accurate NOT to do depth averaging while calculating SAC rates.

How is it more accurate?!

I hope others do not feel I am giving out dangerous advice as spectre seems to think. [/B]

Sorry, but I'm with Spectre on this one. Unless you are doing a dive specifically to calculate SAC (which isn't realistic) and are maintaining an EXACT depth, the SAC rate should be done at your average depth. If you are dividing the dive into two pieces, bottom, and ascents/descents and want to only use the bottom portion of your dive for calculating SAC, you should use the MINIMUM BOTTOM DEPTH. Otherwise, you end up with a number that is far too liberal, useless, and dangerous for gas contingency planning.

In Jeff's example above, which is realistic given a slow ascent and an NDL bottom time, calculating rock bottom with a .47 cft/min SAC and a .63 cft/min SAC will give you wildly different numbers and in the event of an OOA emergency, either you or your buddy (or both) might end up dead because you planned with the wrong numbers.

 

[adder70]

Capt Jim Wyatt once bubbled...
but..use the max depth to get the best, most accurate number.

Actually, if you have a multi level dive and use the max depth, you will underestimate your SAC. This could result in a dangerous situation if you use a series of multi level dives to calculate SAC and use this value to plan a square profile dive.

You can plan assuming you will always be at max depth and get a conservative dive plan, but assuming max depth for the duration of a multi level dive when calculating air consumption will result in underestimating SAC, which can be dangerous if you plan a dive that requires all of your air (dives into overhead or "return to ascent line" in heavy seas and/or current).

 

[Spectre]

Capt Jim Wyatt once bubbled...
Seems to me we are talking apples & oranges.

I hope others do not feel I am giving out dangerous advice as spectre seems to think.

I believe we _are_ talking apples to oranges, and I'm very confused as to how one would get a "higher and more conservative number" by using max depth.

Perhaps Jim is intending to remove the resting portions from the dive, and therefore coming up with a more accurate 'at work' rate. But that's not as simple as just taking the max depth.

Take a time and pressure reading at the start and finish of your working portion of your dive. That removes the more relaxed ascent and decent. Now your looking at _just_ the working porition of the dive... which is going to be the conservative number you would be using for the dive.

However I still dispute that using the max depth of that portion of the dive errors it lower than it actually is. Average depth [for the portion of the dive where you are trying to calculate out your SAC rate], is mathematically the correct SAC rate.

I find many people suggesting using your MAX depth to calculate out SAC rates, and _then_ suggesting using those calculated out SAC rates to calculate gas reserves for ascents. This is what I feel is dangerous advice. Mathematically you _will_ end up with a lower sac rate using the max depth with a constant breathing rate then using average [most accurate] or minimum depth [most conservative]. The most _dangerous_ thing in _using_ calculated rates for your dive plans is if the rate you are using is on the low side.

Like Jim says, you can't use a number that you calculated over a few dives. The more dives, the better. I calculate my rate on every dive, and when they appear to be anomilous with my expected rate, I note why.

 

[Capt Jim Wyatt]

on the Adolphus Busch and the divers stay at one depth the entire dive so we can calculate their SAC rates.

What is unrealistic about that?

 

[Uncle Pug]

And then I check that against what I figured.

If you keep a running rough average of your dive in ATA and then multiply that times minutes you have ATAminutes. Divide your gas consumed by that to get your rough RMV/SAC.

If I do a 60 minute dive that averages out to 2.5 ATA using 1500 psi I know that my consumption is on the order of 10 psi/minute.

If I am diving a single LP104 I know that every 100 psi = 4 cf and my consumption in cf/min is .4

Now if the dive was one that involved just floating around I would say that goes into the resting SAC bin for future reference.

If it happened to be a dive where I was covering a lot of ground finning I would put that in the working SAC bin.

More important that figuring out your SAC is figuring out why you are figuring it out.

After some time of doing this I would have a fair idea of what my resting and working SACs were for planning purposes (and would inflate them just a touch.)

 

[Spectre]

Ok. Both Jim and I are talking about _two_ different things. Jim isn't talking about max depth as much as the working portion of the dive.

If you want to come up with an accurate, and safe, SAC rate to use for planning, you need to calculate your working rate. Using a resting rate for gas reserves isn't going to help you, since how do you know you'll be resting during the resting portions of the dive?

Jim is talking about removing the ascent and descent from the calculation... which is _definately_ correct. If you leave in the resting portions of the dive, you end up with a SAC rate that gives you something smattered between your resting and your working rate. You're working rate will be higher, your resting rate will be lower.

That smattering between your working and resting rates is what Jim was [unfortunately not effectively] stating to be less accurate.