Air consumption rates.

[Kharon]

I guess I'm not sure I understand what you are saying. I believe over sufficient dives an established SAC should be a good number for gas planning... but of course you should build in a buffer as conditions, temp, current, etc will obviously effect your average SAC.

What I am trying to say is that the calculation of the actual rate of gas used during a dive is more valuable (at least to me) than a calculated SAC that has to have fudge factors re-applied re. planned depth, etc.

As an example, it looks like the median SAC based on the dive plots you've shown is around .75, that seems like a good starting point for gas planning and then if you know there will be lower temp or more workload you can add more buffer or if you know it's warm water no current you can add less buffer.

Actually, with a mean of 1.3 cf/min of actual gas usage, using 1.3 cf/min is a much more accurate (for me) starting point. If I use 0.75 and expect to get 86 minutes out of 77.4 CF (with 500 psi remaining) I'm going to be disappointed. If I use 1.3 cf/min I'll most likely be fairly close to 49.6 minutes (leaving 500 psi in the tank). Actual dive times for this data set are mean 50 min, mode 51 min, and median 52 min - no where near 86 minutes.

the tight grouping of all your other points looks like fairly consistent SAC over different dives so unless all of those dives were identical conditions (temp, current, finning) I'd say that's a fairly consistent SAC you have

Actually, almost all the dives were under virtually identical conditions (the high outliers were abnormally high current - don't know what the low outlier was). The only variable was dive length, max depth, and average depth. The first two plots were done to eliminate any of that as a possible controlling factor. The third plot, if SAC was a good indicator of actual amount of gas used, should show a correlation very close to 1::1 - it does not.

My point is that if you want to get a good estimate of how much gas you use on average, calculating the actual amount of gas used on a dive is far more accurate than the artificial SAC/RMV. Don't take my word for it. Go back and do the calculations on your dives and see if the amount of gas actually used matches your SAC. I'll bet it won't.

 

[Kharon]

it appears when you calculate by hand you always get values ~30% higher than your computer?

I don't understand. Where are you getting 30% from? When I divide SAC into actual gas used I get an average of 1.9 - that's 90% higher than the SAC. Am I doing that wrong?

 

[scubadada]

Kharon,

I don't have the slightest idea what you're doing but you sure have a lot of time to do calculations. Perhaps we just have a problem with vocabulary or communication, that's often the case.

My RMV is 0.37 cu ft/min, I dive for 60 minutes at an average depth of 60 ft. I use 63 cu ft of gas from my AL80 (77.4) that was filled to 3000 psi. I have 14.4 cu ft of gas left and surface with 560 psi. You do realize that my RMV is calculated when I download my AI computer from cylinder size, working pressure, psi used, average depth, and dive time.

I don't see anything artificial about my RMV. I don't understand your SAC and actual gas used discussion, I guess I don't really have to.

Good diving, Craig

 

[The Kraken]

Just make sure to use the AVERAGE dive depth when computing one's SAC rate.
Keep in mind that some computers quit calculating after one ascends above 13-15 feet or so.
Keeping one's reg in one's mouth while waiting to do a ladder ride adds significantly to the amount of gas consumed.

 

[Skeptic14]

So now I see where our miscommunication/misunderstanding is arising...

For some reason you consider SAC (which you say is artificial) different from calculating the actual amount of gas used on a dive (by hand I assume?).

When I say my SAC was .59, that is based upon my computer's calculation and I get that when I download my dive log after the dive. And if I used my average depth from my dive log and do the calculations myself (dive time, cylinder size, start pressure, end pressure) I come up with virtually the same number. These are the same thing, just one was calculated by the computer and the other by hand.

What are you referring to as SAC which you say is artificial?

 

[Kevrumbo]

What I am trying to say is that the calculation of the actual rate of gas used during a dive is more valuable (at least to me) than a calculated SAC that has to have fudge factors re-applied re. planned depth, etc. . .

My point is that if you want to get a good estimate of how much gas you use on average, calculating the actual amount of gas used on a dive is far more accurate than the artificial SAC/RMV. Don't take my word for it. Go back and do the calculations on your dives and see if the amount of gas actually used matches your SAC. I'll bet it won't.

No! You misunderstand fundamentally how to utilize SAC Rate: By definition, the units are in volume/time per depth in ATA, or in Imperial units, cu ft/min per depth in ATA.

Therefore your "planned depth" is not a "fudge factor" as you incorrectly claim; your actual gas consumed in cubic feet is derived by summing the product of your SAC rate, time at each depth during the dive, and that depth in ATA. Do you understand??? Cubic feet = (cubic feet/minute*ATA)(minute)(ATA)

All your plots & graphs above are a "Red Herring" mathematical construct & analysis, and are irrelevant because you do not understand the basic principle on how to correctly utilize the concept of RMV/SAC.

 

[Freewillow]

Sac is volume/time. By définition, the SAC IS @ 1bar since it is @THE surface.

 

[Kevrumbo]

Sac is volume/time. By définition, the SAC IS @ 1bar since it is @THE surface.

Your units for Surface Consumption Rate is Volume/Minute * ATA, by Definition.

It is UNDERSTOOD by Convention when referencing to Surface Consumption Rate (or SAC/RMV) that you are referring to the normalized value at the surface at 1 ATA: Hence for example 0.5 cuft/min * ATA multiplied by 1 ATA is equal to a SAC RMV of 0.5 cuft/min; Whereas 0.5 cuft/min * ATA multiplied by 4 ATA is a Depth Consumption Rate of 2 cuft/min at 100' (4 ATA). . .

 

[admikar]

Your units for both Surface Consumption Rate AND Depth Consumption Rate are in Volume/Minute * ATA, by Definition.

It is UNDERSTOOD by Convention when referencing to Surface Consumption Rate (or SAC/RMV) that you are referring to the normalized value at the surface at 1 ATA: Hence for example 0.5 cuft/min * ATA multiplied by 1 ATA is equal to a SAC RMV of 0.5 cuft/min; Whereas 0.5 cuft/min * ATA multiplied by 4 ATA is a Depth Consumption Rate of 2 cuft/min at 100' (4 ATA). . .

You're stuck on semantics. When I plan a dive, I use my SAC rate* planed depth, and get same numbers as you would.
Square profile to 30 meters on a 200 bar steel 15 l. My SAC is 16 liters/min.
Simplified plan, since I'm at work: 200 bar*15l-50 bar*15l= 2250 usable liters of air. My consumption on depth 16*(1+3)= 64 l/min.
My dive time 2250l/64l/min= 35 min of bottom time.That's in "normal conditions". Work, cold, current, I'll check my SPG more, and dive accordingly.

 

[Kevrumbo]

You're stuck on semantics. When I plan a dive, I use my SAC rate* planed depth, and get same numbers as you would.
Square profile to 30 meters on a 200 bar steel 15 l. My SAC is 16 liters/min.
Simplified plan, since I'm at work: 200 bar*15l-50 bar*15l= 2250 usable liters of air. My consumption on depth 16*(1+3)= 64 l/min.
My dive time 2250l/64l/min= 35 min of bottom time.That's in "normal conditions". Work, cold, current, I'll check my SPG more, and dive accordingly.

Semantics of a practical objective explanation of the units of measurement involved in correctly applying the concepts of Surface Consumption Rate to a Depth Consumption Rate.

Here's an even more simplified method of tracking your gas usage based on your example of a Sac Rate of 16 liters/min with a 15 liters/bar cylinder:

16 liters/min * ATA divided-by 15 liters/bar equals approx 1 bar/min * ATA pressure Sac rate.

At 30 meters (4 ATA), this becomes a pressure Depth Consumption Rate of 4 bar/min. So for an interval of 10 minutes at a depth of 4 ATA, you expect to consume 40 bar of gas and your SPG to show 160 bar remaining (200 bar starting pressure minus 40 bar equals 160 bar); another 10min interval for an elapsed bottom time of 20 minutes, and you know & confirm that your SPG reads 120 bar remaining (160 - 40 = 120 bar); another 10min interval for an elapsed time of 30 minutes, and you know & confirm that your SPG reads 80 bar remaining (120 - 40 = 80 bar); and finally for a 5min interval for an elapsed bottom time of 35 minutes, and you know & confirm that your SPG reads 60 bar remaining (80 - 20 = 60 bar).

However I would have as a Minimum Gas Reserve of 70 bar, more conservative than your 50 bar (and additionally carry an O2 deco bottle and switch to it at 6m); and start the ascent earlier at 30 minutes elapsed bottom time with my SPG showing 80 bar remaining. . .

The point is your SPG reads in units of pressure --not volume. Why not convert your volume Sac rate for your particular tank to a pressure Sac rate of 1 bar/min * ATA, an easier value to figure Depth Consumption Rate with?