SAC Rate Test

Hello all,

So I have read just about as much as I can on how to calculate SAC Rates.

I have even made a shiny new Excel Spread sheet that does volume of air (In Litres) for a specified Tank size (Litres again, sorry) at various Tank pressures (Bar) and depths (meters) then, based on an inserted SAC Rate, will calculate the time available(in minutes) at given depths (in meters) based on the tank size and tank pressure. (If you cannot work out that I am proud of my project yet then .... ) anyway .. All the work is worth Nara if I do not know what my SAC Rate is, so ...

What is the best method of establishing a personal set of data to calculate the SAC Rate? I have thought that the following scenario would work but would like feedback from the Forum on the "validity" of this method.

1. In order to keep this as acurate as possible, all tests are done on the surface (but in the water).
2. Get into the water with all equipment etc.. as expected for a normal dive.
3. Wait a few minutes (5 should do) for the tank temp to normalise.
4. Note the pressure and start a 10 minute timer.
5. After 10 minutes, Note the pressure again and start the next 10 minute timer.
6. Start finning (hard) by pushing against the boat (or any other relatively fixed object).
7. After (the second) 10 minutes, Note the pressure again.
8. Do the Math

Would this be a reasonable method to accurately determine a working and relaxed SAC Rate?

Thanks in advance

Richard

Hate to burst your bubble, but your SAC rate will vary with each individual dive.

The SAC rate is a dynamic quantative analysis that is based upon many variables such as your excitement level, temperature of water, amount of gear you're carrying, how hard you're working, the current and a host of other things.

In my opinion it's primary use is that of a planning tool to estimate how much gas you may breathe on a dive.

You may have a SAC rate of .35 cu. ft. on a nice warm, clear dive, but on the next dive, one where the visibility is extremely limited, it's cold and you're diving in a current, it may jump up to .75.

In other words, it's not a constant upon which you can always rely when planning a dive.

the K

the K,

My bubbles don't burst that easily.

I absolutely understand that SAC rates vary based on prevailing conditions both inside the body and inside the body of water. This (understanding) is shown by the fact that my suggested procedure makes allowance for both resting and exerted conditions. If nothing else this should show a broad band of SAC expectation for (as you mentioned) planning purposes within a given environment and with specific equipment.

Obviously if one changed any of your conditions (Tropics to Poles), Equipment or method, then the SAC rate is affected, that stands to reason and any insinuation of the contrary is regretted.

Best Regards

Richard

SAC rates, as you know, require a constant depth to do the initial measurement and as mentioned above can vary. The way I determine mine is to measure my consumption over a few dives and I do this regularly. I do dive regularly in very similar conditions and the same equipment so this makes my job easier. The more often you do it the more you will get to know your own consumption. This will give you a range to work within. If I was to dive somewhere new or in new equipment then I would test my consumption again over a few dives.

Riger,

I did the same thing as you're doing, but I put mine in a table that gives the approximate amount of bottom time on a square dive profile with a residual 500 psi remaining.

It is fun to do stuff like that . . .

the K

If your dive computer provides you with an average depth, you can quickly and easily calculate your average surface consumption for every dive.

• Convert average depth to ATA (average depth/33 + 1)
• Divide total amount used by ATA
• Divide by total dive time

Example ... you did a 40 minute dive with an average depth of 33 fsw and used 1600 psi from your cylinder.

1600 / 2 ATA = 800 psi/ATA (e.g. "surface" psi)

800 / 40 minutes = 20 "surface" psi/minute

You can convert this figure to volume by knowing how much volume your cylinder holds per psi (i.e. total volume of your cylinder / working pressure of the cylinder = volume/psi)

If you don't have average depth on your dive computer then you can calculate as you described above ... by taking starting and ending pressures and holding a constant depth for a specified time period. We do this in my AOW class by doing timed swims at a constant depth of 33 fsw ... under both high exertion (swimming fast) and low exertion (swimming slowly).

As noted, your surface air consumption is actually not a number, it's a range between your best-case (resting SAC) and worst-case (working SAC) ... much like the "highway miles" and "city miles" specification for your car. Your actual "mileage" will depend on the conditions and circumstances of every given dive.

What I recommend to my students is that they track air consumption over a number of dives in varying conditions and look for "trends" ... knowing how given conditions affects your breathing (because we're all different in that respect) helps you understand and anticipate your needs when you are planning your next dive.

... Bob (Grateful Diver)

Suunto Cobra and Vytec provide diver with average depth of an actual dive as well as the gas consumed. This info can used to acurately measure SAC over several dives and different conditions.

Riger, I'm not expert diver but your method sounds pretty good to me. It is however a theoretic SAC that you are calculating in some sense but then the entire idea of calculating SAC and predicting gas requirements or dive time based on available gas is pretty theoretic.

I'm rambling, what I wanted to say is, do your experiment and note those values but then also compare it to your typical dive SAC rates as ayeshacantrell said. I see you have a Galileo Sol dive computer. That is a pretty mean piece of electronics and it can probably tell you everything you need to know. For one, it will give your average depth for each dive. Based on this value it is easy to calculate your SAC rate (RMV is actually the correct term - Respiratory Minute Volume). If you do this over a number of dives you will get some sort of an indication of your average, worst case and best case RMV. You will then also start to notice how cold water, exertion or excitement affects your RMV.

The above procedure of course assumes that you have a constant RMV throughout your dive, which you don't. When you see a Manta Ray swimming by you may get excited, resulting in a temporary higher RMV. But for the most part I would think that calculation is probably more useful than your surface experiments. Either way, it should be interesting to note how they compare.

The Galileo Sol is an air integrated computer though so if you have the air integration option then your question is already answered by your computer. It can calculate your RMV throughout the dive which would preclude the need for any experiments or maths.

I think your plan sounds fine. You'll get an idea of how big a difference there is between resting and swimming gas consumption. Even if your SAC rate when underwater doesn't precisely match what you get with this experiment, it will still give you some starting numbers. Doing what Bob describes will let you compare how actual diving matches up with your surface numbers, and if there is a big difference, you can think about what you are doing differently underwater that is changing things (buoyancy swings, or swimming technique, or whatever).

Good for you for being this interested in the ideas!

Experimenting with different methods, right or wrong will yield results you can use provided you repeat them going forward. After all, the real reason to know and understand your SAC rate is better and more accurately plan you dives.

I would suggest not getting too hung up on comparing you SAC rate to others except to again, plan your dives.