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Using an Al 80 and assuming that a direct ascent to the surface is possible my KISS method is to ascend at a pressure equal to ten times the depth in feet +300.
e.g. 100 foot dive,ascend at 1300 psi (100X10 +300 )
If you ascend to ,say, 70 feet then you can stay there until pressure drops to 1000 psi and so on.
Using an Al 80 and assuming that a direct ascent to the surface is possible my KISS method is to ascend at a pressure equal to ten times the depth in feet +300.
e.g. 100 foot dive,ascend at 1300 psi (100X10 +300 )
If you ascend to ,say, 70 feet then you can stay there until pressure drops to 1000 psi and so on.
No spreadsheet required!!
thats an interesting process and that is for rock bottom air psi to ascend to the surface for 2
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that is for rock bottom air psi to ascend to the surface for 2
Yes.
Wont hold true if you have a truly awful SAC rate,and probably best to add some more conservatism below 100 feet (or get a bigger tank) but I like the simplicity of it.
Thanks to everyone for the input. Sorry if this has been gone over before. When calculating SAC rate is it best done in shallow water, or just averaged at different points in the dive?
Thanks to everyone for the input. Sorry if this has been gone over before. When calculating SAC rate is it best done in shallow water, or just averaged at different points in the dive?
Shallow or deep doesn't matter since depth is normalized out of the equation.
Note that you need an accurate reading of average depth. Many computers will provide that information if you download the log. Otherwise, note your gas supply, swim at a CONSTANT depth for a known period of time (like 20 minutes), and note your remaining gas.
Agree that the best way to measure SAC is from a computer or logging device such as a Sensus Pro. That will give you an accurate SAC for the entire dive which is what you need for planning.
Doing a timed swim at ,say,33 feet when you are relaxed and neither ascending nor descending will tend to give a low result.
Well, this question (about where to calculate it) immediately points out one of the weaknesses of the number. The best way to calculate SAC rate is to get to a known depth (say 33 feet) and swim at a steady rate for five minutes, and then use those numbers to figure SAC. The problem with this is that this number isn't very useful for planning a whole dive, as it's rare that you will swim steadily through a whole dive (cave diving excepted!)
For real purposes of tracking and planning, I've found using the average depth for the dive (from my computer/gauge) and the total gas consumption and time, gives me a number that incorporates the times I was swimming, the times I was sitting and looking at something, and the reduced gas use on ascent. If I were planning a dive quite different from my ordinary reef-swimming "look at the fishies" dive, I'd have to adjust my expected gas consumption accordingly.
As you accumulate dives and do these calculations over and over again, you'll find trends. I know, for example, that my gas consumption goes up when you put a reel in my hand. I know I use less gas on a warm water drift dive. I know what doubles versus singes does to it. This just comes out of having a bunch of data points and spotting trends.
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My dive journal can be read here, and a current dive blog HERE
Calculating your SAC based one only a few dives does not represent much except to establish a base line.
I record my SAC rate on every dive but just to see what, if anything made a significant change to it. For example, when I am diving from a boat here in SoCal with warmer than usual water, the vis was great and we had little current or surge, my SAC was above the baseline. Not so when beach diving in colder water with 8’ vis and fighting a medium current.
The bottom line is I like to see the difference so I can better predict my dive time based on real data. However, dive time is controlled more by who you are diving with. Sometimes I’m the limiting factor, sometimes it is my buddy.
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Dave
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