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Tech Skills on the Kinghorn January 31, 2009

Posted by Chris Sullivan in Dive Log, Shipwrecks, Technical Diving, Training.
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The following day, on Sunday September 2nd 2007, we joined the recreational divers on the Kinghorn. We spent most of the time on the deck, going through our doff/don and shutdown skills. This time, I did not fail to add some air to my BC to aid in keeping my balance, and didn’t let the Scuba unit get too far from my centre of gravity. The shutdowns were even easier than the time before, and I could go through the whole sequence of shut down and opening of all three valves in less than a minute.

The next exercise was dropping and picking up stage bottles while swimming in one continuous movement. The first pickup was a little awkward with the speed of the current behind us but the second was great.

We also did our SAC check, swimming at a normal pace for 10 minutes back and forth on the wreck. In the current, the upstream legs were much slower than the downstream, but we kept the same pace the whole time to get a good idea of our air consumption.

In the 10 minutes, I used 250 PSI, while maintaining a depth of 80 feet. Calculating the SAC is relatively straightforward. The number of cubic feet consumed is simply the capacity of the tanks, multiplied by the PSI consumed, divided by the working pressure of the tanks. The tricky bit is that on most steel tanks, the working pressure isn’t the rated pressure, but the rated overpressure of the tanks. So instead of 2400 PSI I had to used 2400 plus 10% which is 2640 PSI.

Each tank holds 95 cubic feet at 2640 PSI, so from two tanks the air consumption was 2x95x250/2640 cubic feet. The result? Almost exactly 18 cubic feet consumed, or 1.8 cubic feet per minute. But that isn’t the SAC rate of course, because it hasn’t been adjusted for depth. To convert from air consumption at depth to air consumption at the surface, I need to divide the result by the number of absolute atmospheres of pressure at my depth of 80 feet.

To do this, I need to adjust the gauge pressure in atmospheres to absolute pressure. My gauge pressure was based on 80 FFW (feet of fresh water). You’ll see in the books that to convert feet of depth to absolute atmospheres you add 33 to the depth then divide by 33. That’s for salt water. For fresh water, it’s 34 feet. Now if you’re using a depth gauge or computer that’s set to salt water, you use the salt water formula as your depth reading will be slightly different from your actual depth.

So the pressure at 80 feet was (80+34)/34 atmospheres. That’s about 3.35 atmospheres. If I divide my air consumption at depth by the absolute pressure I get the SAC rate, which is about .54 cubic feet per minute. Not bad, although I think I used to do better when I was in my twenties.

For dive planning, we use SAC rates to estimate how much air we need, or more commonly, how much time we can safely spend underwater to follow the rule of thirds or whatever other measures we’re using. If I estimate my potential errors as depth +/- 1 foot, consumption +/- 25 PSI, tank capacity +/- 1/2 cubic foot, and time +/- 6 seconds then my maximum possible SAC rate is close to .6 cubic feet per minute. This is what I would use for dive planning, although SAC rate should be checked often, as it can vary with conditions, diver fitness, anxiety and so forth.

For your calculating pleasure I’ve constructed a simple spreadsheet using MS-Excel that estimates the SAC rate both using the standard formulas and worst case by taking measurement error into account.



1. SAC in MS-Excel - Scuba Forum - Scuba Diving Forums and Discussion Board - January 31, 2009

[…] spreadsheet handles Imperial measurements only. If you’d like me to do a metric version then please post a comment on my blog. Chris Attached […]


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