SAC Average or Max Depth?

[=C=]

Hello,

I started diving with a new Tusa IQ computer and it logs my average depth on a given dive. I've started using this to compute SAC... Do you think I giving up a margin of saftey or am I being more percise when it comes to my gas consumption

What do you use and why?

edit Hmmm bad math... by using the average I'm pushing my SAC higher.... right? The average would mean I had to consume the same amount of gas at a smaller depth. Blah mondays.

 

[Deefstes]

You are being more precise. I don't see why this could be considered a safety risk. I also don't see the point of calculating your SAC based on your maximim depth if you've only spent 30 seconds at that depth but 30 minutes at half that depth. It just makes sense to me that you should calculate your SAC based on your average depth.

 

[spectrum]

I use the average depth from my computer. It's a real number and I can get it from every dive. By using this my SAC rate gives me a picture of my range and what sort of dives cause it to swing.

Personally the deeper I go the slower I seem to breathe so if anything I'm building in a reserve.

The big benefit as I said is that you know your rate based on a history, not a few spot checks.

Pete

 

[Colliam7]

I started diving with a new Tusa IQ computer and it logs my average depth on a given dive. I've started using this to compute SAC... Do you think I giving up a margin of saftey or am I being more percise when it comes to my gas consumption ... What do you use and why?

I think you are being more precise, IF you also have precise gas measurments. A couple of thoughts. Since SAC is Surface Air Consumption, air consumption rates at various depths need to be normalized to surface values, based on depth, and therefore pressure. This can be tedious to do over a given dive. However, depending on the sampling algorithm of your computer, if it calculates an average depth based on frequent repetitive sampling of depth every so many seconds (for example, Suuntos sample every 20 seconds as I recall), an average depth can be a useful parameter for calculation of SAC, when coupled with total dive time and total gas consumption.

One recommended approach for initially calculate a SAC rate is to establish neutral bouyancy a fixed, relatively shallow depth (e.g. 25 feet), and swim slowly and consistently at that depth for at least 10 minutes (more is better), recording your air pressure at the beginning and at the end of the time interval, so you can precisely estimate the amount of air (e.g. cf) used during that measured time interval. Then, the total air consumption (divided by time, to give a 'per minute' value) is normalized for depth, to give a SURFACE air consumption value.

 

[=C=]

....One recommended approach for initially calculate a SAC rate is to establish neutral bouyancy a fixed, relatively shallow depth (e.g. 25 feet), and swim slowly and consistently at that depth for at least 10 minutes (more is better), recording your air pressure at the beginning and at the end of the time interval, so you can precisely estimate the amount of air (e.g. cf) used during that measured time interval. Then, the total air consumption (divided by time, to give a 'per minute' value) is normalized for depth, to give a SURFACE air consumption value.

that is what I was looking for... Thanks Colliam

 

[NadMat]

you can also download the computer info to Tusa's PCLogbook and enter the tank and gas start/end info and it will calculate it for you. You can see the difference between gas usage in these two dives from this weekend. First dive was following a new diver who was a bit negative and swimming to maintain depth ( if she slowed she would sink ). after she got neutral and slowed down, gas usage for second dive was much lower just hovering and gliding.

here are pdfs of printout from PCLogbook software.

 

[don Francisco]

Air consumprion is directly proportional to depth plus 32fsw (1 atmosphere) so, all things considered, using the average depth and total air consumption will give you a reasonable value for SAC.

In reality, some folks seem to have slightly higher SACs at deeper depths, others at shallower depths, probably due to personal changes in breathing patterns at various depths.

In using the SAC for dive planning, be sure to allow reserves to cover situations where gas consumption will be higher than expected, such as currents, or stress.

Note that while SAC is a linear function and averages can be used, Nitrogen absorbtion is a far more complex equation, so tables or a computer are needed for this calculation.

Last thought in answer to your question. Having and using more information will always tend to make you safer, unless you use the data to game the system, and dive to the very fringes of calculated limits. This is true of NDL, SAC, and MOD calculations. Use whatever data you have optimize your dive plans, but allow yourself room for error or surprises.