How are you computing your SAC?

[The Kraken]

Well, didn't want you to think I was rude.

About a week or so ago. Came as a great surprise and shock to me!

Thanx . . .

the K

 

[RIDIVER501]

I generally have a pretty good idea of my average depth, but I didn't see any average depth or info on dive profile posted by Sharpenu.

So how did you figure out the average depth number for Sharpenu's dive?

I didn't. I just plugged in the numbers he provided from his post and made no assumptions about it.

 

[akscubainst]

Dave, Take a look at the spread sheet I posted earlier in the thread. on the 1st page you can put your real dive stats in for your last 5 dive to get a rough running avg of your SAC rate.

I just plugged in my last five dives and came up with a .39 avg.
Do the dives, plug in the numbers, get your results.

The SS is pretty cool, I added it to my keep file.

To fill it out would suggest that I logged my dives. I'll have to download my dives from last weekend and see what it comes up as. I did 6 dives with openwater students. My SAC was probably double normal....

The PSI per min thing is pretty cool. I know at 15' on a Steel 93 I use 2-4psi per breath according to my computer.

dave

 

[MXGratefulDiver]

you need the computer to tell you that? You don't know that you spent the bulk of your dive at x' depth and use that as a best guess?

There is a difference between an educated guess on a data to give you a resonable idea of what you did and picking any old number to give yourself the "Best"/most desireable answer.

come on. I did 2 dives last night and know where the bulk of my time was spent on both those dives. I don't need to plug my vytec into a computer for it to tell me where my avg depth was for the dive.

The spread sheet I put together will use what ever number you put into it. You are making the assumption it has to be a certain number. it doesn't.

if you has the super-whammy-dyne computer that would give you avg depth, you probably wouldn't need the spread sheet I have provided because you SWD computer would probably give you SAC rate too.

If you want accurate numbers, this won't work. Even if you're doing a square profile, ascent and descent time will skew your "best guess" somewhat ... and if you're doing multilevel dives, it can be skewed quite a lot.

If you don't have a computer that calculates average depth, you can get a better read on your SAC rate by taking starting and ending pressures during a period of your dive when you are at a relatively stable depth, and basing your calculations on the psi used during that portion of the dive.

... Bob (Grateful Diver)

 

[RIDIVER501]

Thanks, I added the PSI used at depth to help with gas management.
As said before this is just a tool to help in planning. If you use it on the conservative side for dive planning you it can help you plan for approximately how much air you need for a dive or it can indicate that you need to revise you plan becuase you mightnot have enough air to make the dive.

 

[RIDIVER501]

Thanks Bob,
I appreciate the input. it is just a tool to help. provided for all to use, or not use, as they see fit.
I have found it helpful in planning gas management for tec dives. I use it conservatively and that usually give me a wide margin for error.

Far better to have too much gas at the end of the dive then too little.

Here is the original thread from awhile ago where I got the idea and the equations to put together that spread sheet
http://www.scubaboard.com/showthread.php?t=81388

 

[akscubainst]

Sample of how V-planner uses your SAC after plugging it in:

180’, 20
(20)
110 0(28)
100 0(23)
90 1(24)
80 1(25)
70 1(26)
60 1(27)
50 1(28)
40 3(31)
30 3(34)
20 2(36)
15 9(45)

Based on my SAC rates:

180, 20
94.8 cu ft Trimix
16.5 cu ft Nitrox
9.8 cu ft Oxygen
121.2 cu ft TOTAL

I did this dive and only actually used 77 cu ft of Trimix so I'm a little better than I thought.

Dave

 

[JasonH20]

<< Alex, I don't believe that the manufacturers have simplified the formula by assuming that 1 bar = 1atm, but instead the difference is that they use the more widely accepted definition of 10msw being exactly 1 bar, not the 10 m = 1atm definition that you have chosen. >>

I was just quoting the author of Scubase (also an "Alex") at that point. All I can tell you from my own calculations is this:

There is a slight difference betwenn how Suunto Dive Manager and Scubase do the calculation. Scubase adds 14.7 psi to the tank working pressure to account for atmospheric pressure at the surface; SDM does not.

None of this matters much unless you are anally-retentive about the details, which I guess I am.

So last night I manually calculated my RMV from the recorded data on a dive compared to what SDM did for me automatically. SDM came up with .54, I came up with .56. I'd say in the grand scheme of things SDM is close enough for me.

 

[The Kraken]

This has been an excellent thread.

the K

 

[sharpenu]

The whole point I was trying to make here is that average depth by computer is more accurate than "guessing" your average depth, which is what I have been doing. My point is, at the end of the day, guessing at your average depth is still just a guess.

I don't know how your spreadsheet gave you .56 for that dive, if you used the numbers as given without modification.

Using 1500 pounds of a 3500 pound 120 foot tank means you used 51.4 cubic feet of gas. You used that gas (assuming square profile) at 75 feet or 3.27 ata. That is the equivalent of using 15.71 feet of gas at the surface. Divided by 33 minutes leaves you with a SAC of .476 cfm.

Since this was a wreck dive, I spent most of my dive at about 75 feet. As pointed out here, that doesn't take ascent/descent times into account. It also doesn't take temp/pressure changes into account, but I don't know how to accurately account for that.