Air integrated vs. SPG, a small study

[Scared Silly]

I am not using the Suunto DM but DiveLog which is an OS X based app. After downloading the data I enter the tank size and the working pressure and able to get my SAC rate. I know Mark (the developer of DiveLog) chimes in once in a while so perhaps he can shead some light on this. But if I were to venture a guess it is doing this in a differential fashion based on the sample spacing then averaging. This would give the most accurate value.

Now about those scientits - I can clearly see where the problem is you are mixing wine with windex whilst looking at your wife and trying to work.

 

[lamont]

Well, I don't know about all this...

I know that I've worked out 200 psi / 10 mins / ata to be 0.75 SAC for a single E8-130. I took a newbie diver to EUP over the weekend and at the 10 min mark she had gone through 300 psi, and at the 20 min mark she had gone through another 300 psi, so I knew her SAC rate was in the "normal" range rather than anything hooverish enough for me to get worried about. We worked it out afterwards and I think she was at around 0.67 or so. I don't know if she was closer to 0.60 or 0.75, but it was all good...

 

[Packhorse]

Direct from SDM

To calculate Surface Air Consumption (SAC), Suunto Dive Manager first determines the average depth of the dive. It is the average of all depth reading shown on the depth graph in the dive log entry.

It next determines the total amount of air used on the dive. Dive Manager uses information you provide about cylinder size and about pressure gauge readings. Enter this information on the Air consumption tab in the properties window. Dive Manager then calculates the total volume of air used for the dive.

Dive Manager then determines the volume of gas used per minute and adjusts the result to sea level from the dive's average depth. Finally, it adjusts the result from ambient water temperature to zero degrees Celsius.

The result is a volume of gas per minute at Standard Temperature and Pressure (STP). Because Dive Manager uses volumes at STP, you can directly compare SAC numbers from one dive to another, regardless of depth and water temperature. Dive Manager reports SAC in metric or imperial units. Metric units are Standard Liters per Minute (SLM), and imperial units are Standard Cubic Feet per Minute (SCFM).


Im not sure of the bennifits of calculating this way. I for one never dive in 0deg water let alone always dive at 0degs.

 

[TSandM]

The adjustment to 0 Celsius almost certainly explains the discrepancy between the values I'm getting and what the Cobra used to tell me.

That doesn't, however, explain why the values the Cobra came up with seem to be so much more consistent with my overall gas consumption than the ones I'm calculating. I remain confused.

 

[gcbryan]

The adjustment to 0 Celsius almost certainly explains the discrepancy between the values I'm getting and what the Cobra used to tell me.

That doesn't, however, explain why the values the Cobra came up with seem to be so much more consistent with my overall gas consumption than the ones I'm calculating. I remain confused.

Perhaps if you post how you are figuring your SAC rate including the required info someone can catch the mistake if there is one....many heads better than one and all that.

 

[fire_diver]

I would think tank-air temperature change could be a much bigger
factor than gauge accuracy for your SAC calculations,
especially for you folks that dive the northwest where there
can be a large difference between air and water temperature.

With a 40 temp change, the 3000 PSI you started with is
around 8% lower or only 2760 or that 95 you started with
was really only an 87.

--- bill

You bring up an SAC factor I hadn't thought about yet. I check my tanks befor each dive to make sure that I have 3000(or close) to begin. I know that when I get down under a couple thermal-clines, the temp drop will effectively reduce my usable gas at depth. But what I have since failed to account for is the re-warming of the bottle at the end of the dive. I just calculated the SAC by start psi=3000, end psi=xxxx.

I haven't ever waited around for the tank pressure to come back up, then take my ending psi reading. If I'm thinking right, this should result in a lower actual SAC rate by a little bit?

FD

 

[runamonk]

While I would love to use an air integrated computer, the thought really scares the crap out of me.

See I'm a programmer by trade so I know that people make mistakes, no matter how good the coder, something always sneaks through that didn't get thought about. I don't want to be the guy who finds the weird occurance at 90' LOL.

 

[TSandM]

Sac: Pstart-Pend/dive duration = psi/min consumption

psi/min / (av depth/33 + 1) = psi/min corrected to 1 atm

psi/min (1atm) x (tank volume/working pressure) = cu ft/min at 1 atm

Sorry it took several lines, but I don't have mathematical notation on the keyboard down.

Using this formula, my average SAC is about .4 to .45.

What's confusing me: I normally dive an LP 95. My typical dive uses about 1500 psi out of a tank. 1500 psi out of a 95 cu ft tank (WP 2640) is about 53 cu ft. By that quick calculation, I should be able to get two dives safely out of my doubled 72s, with a reasonable reserve.

But if I take the SAC of .45, multiply it times an hour's dive time, times an average depth of 46 feet (pretty typical Cove 2 dive), I come up with 65 cubic feet, which is clearly something I CAN'T get twice out of the 72s.

I have a lot of measurements to support all these numbers, which is why I'm confused.

 

[cool_hardware52]

Sac: Pstart-Pend/dive duration = psi/min consumption

psi/min / (av depth/33 + 1) = psi/min corrected to 1 atm

psi/min (1atm) x (tank volume/working pressure) = cu ft/min at 1 atm

Sorry it took several lines, but I don't have mathematical notation on the keyboard down.

Using this formula, my average SAC is about .4 to .45.

What's confusing me: I normally dive an LP 95. My typical dive uses about 1500 psi out of a tank. 1500 psi out of a 95 cu ft tank (WP 2640) is about 53 cu ft. By that quick calculation, I should be able to get two dives safely out of my doubled 72s, with a reasonable reserve.

But if I take the SAC of .45, multiply it times an hour's dive time, times an average depth of 46 feet (pretty typical Cove 2 dive), I come up with 65 cubic feet, which is clearly something I CAN'T get twice out of the 72s.

I have a lot of measurements to support all these numbers, which is why I'm confused.

TSandM,

If I had to guess..... I'd say your .4 to .45 Sac with a 95 is in error. What you know with certainty is 1500 psi is consumed for a typical Dive, what is more difficult to track is actual average depth. If it's actually shallower than you think your SAC will be higher.

I'd say dive the doubles a few more times to get comfortable, and see if your raw "per dive" cuft consumption compares with the ~53 cuft used with the 95's.

This is probably old news, but your SAC will rise with doubles. I still say the Testing one's SAC with a small slung bottle, swiming at a fixed depth is the most reliable.

You need bigger tanks, the best breather in the team needs the biggest bottles


Tobin

 

[TSandM]

Interestingly, I did the calculation for last night's dive with the 72s (using the average depth reading from the Mosquito, which is what I have been doing all along), and came up with a SAC of .41. The numbers may be wrong, but they are awfully consistent.

Bigger tanks would require a hand truck to get ME to and from the water . . .