What is your SAC/RMV rate? (and metric questions)

[TexasKaren68]

If her computer is calibrated to fsw, then a reading of 33' on the computer is 1atm or 2ata. your recommendation of dividing by 34' to get the atm increase is erroneous. OTOH, if her computer reads in ffw, then 1 atm = 34ffw and the depths should be divided by 34 to get the atm increase.


Your method of calculation would result in some very interesting SAC rates on my typical multilevel dive of 120', 70 minutes on an AL80. ..... less than 1/4 cfm, even assuming that I drained the tank empty rather than having 800 or 1000psi left as normal.

Even the crudest mental estimate of average depth would be better to use than a max depth that you were at for only 1 minute.

Thanks for all the info.

I used 33' in my calculations even though I was in fresh water. I figured my computer was telling me I was at 33' even if I was really at 34' so it works out OK. I used average depth as well, not max depth.

I know most of the conversion factors, having lived in Europe for a number of years, so things like temperature and distance are pretty easy. Volume on the other hand is something I didn't really have to figure out except when filling the gas tank in my car.

Knowing that an "average" tank holds about 200 bar helps a bit. I guess I am going to have to create a few more columns on my spreadsheet to convert what I know in imperial numbers (my SAC rate for example) into something I can use if I go to Australia

 

[Frosty]

Hi Karen
I can't help you with the other stuff but dive tanks here are usually 11.5litre ally or 12.0 litre steel which I'd be rather suprised if they are actually different from your bottles.

A nice easy conversion -10 feet is three meters,20 feet is six meters, thirty feet is nine meters.fourty feet is 12 meters fifty is 15m and fixty is 18m
I know that isnt accurate but it does seem to be dive industry accepted standard. your OW limit is sixty feet and in meters ours is 18 meters

My gauges are in psi But 3000bar is 200psi and 1500bar is 100 and 750 is 50
So you multiply or divide by 15

 

[TexasKaren68]

Karen, I guess the best thing I can say is that the average man runs somewhere between .5 and .7 cfm as an experienced, efficient diver. Women run a little less. It's not useful to know what MY consumption rate is; I've been rather unusual in that, even in the beginning. Chasing after the consumption rate of someone smaller or much more efficient than you are will only lead to skip breathing, and skip breathing is dangerous. I'm almost certainly a CO2 retainer, and that isn't a good thing in certain diving settings (like deep caves) so it's nothing to emulate.

What's important is to keep following your own numbers, and expect to see them fall -- although it's important to regard the numbers in the light of the dive. Dives where you swim, or work, or are task-loaded with new equipment or procedures, will have higher consumption rates than relaxed floats in familiar conditions.

Oh no worries, I'm not trying to match anyone else's consumption rate. I just had no idea whether my rate was super high or average, or just crazy

I only have four dives to go by where I documented the info I needed, so hopefully in a couple weeks I'll have a better idea of how I'm doing. I have two dives scheduled in my lovely local river a week from tonight and then 10 dives in Florida in a couple weeks (Five two-tank trips over three days ).

 

[Blackwood]

always use my max depth and bottom time for calculating my SAC rate, even if I only spent a min out of 30mins on a dive at max depth. It's just me, I'm not too picky with my math.

I like reading your posts, so please don't plan a dive using a max-depth-full-BT SAC rate.

This has nothing to do with being picky. It has to do with being correct and having somewhat consistent numbers that are applicable to dive planning. You consume gas at the time weighted average depth.

Say you do a 100' dive for 30 mins and drain an 80CF tank.

Max-depth-full-BT SAC rate is roughly .7CFM/ATM.

Average depth for a realistic multilevel dive may be 50 feet (say 100 for 5, 90 and 80 for 1, 70 and 60 for 3, 50 40 and 30 for 4 and 20 for 5).

Actual SAC rate is some 50% higher at roughly 1CFM/ATM.

And beyond underestimating your actual gas needs, your numbers will be dependent on profile and thus follow no particular trend unless you are always diving the same profile.

This could explain the apparent drop from 1 in the pool (square profile without much time spent ascending, thus max depth and average depth are roughly the same) to .4 shortly after in open water (where max depth may have been significantly deeper than average depth).


If you don't want to compute your time weighted average and instead use max depth, you can do so by noting your pressure when you arrive at DMAX and again when you leave DMAX, and calculating based on time spent at DMAX, not full runtime.

 

[clpkab]

... I guess I am going to have to create a few more columns on my spreadsheet to convert what I know in imperial numbers (my SAC rate for example) into something I can use if I go to Australia

Not "if" - "WHEN"

 

[diveprof]

Karen, I guess the best thing I can say is that the average man runs somewhere between .5 and .7 cfm as an experienced, efficient diver. Women run a little less. It's not useful to know what MY consumption rate is; I've been rather unusual in that, even in the beginning. Chasing after the consumption rate of someone smaller or much more efficient than you are will only lead to skip breathing, and skip breathing is dangerous. I'm almost certainly a CO2 retainer, and that isn't a good thing in certain diving settings (like deep caves) so it's nothing to emulate.

What's important is to keep following your own numbers, and expect to see them fall -- although it's important to regard the numbers in the light of the dive. Dives where you swim, or work, or are task-loaded with new equipment or procedures, will have higher consumption rates than relaxed floats in familiar conditions.

I fully agree. The important thing is to make sure the air supply last from the beginning to end of the dive (with a reserve of course) Sure, you can track numbers. However, focusing on your SAC rate is not the best path to more efficient air consumption. Rather focusing more on your bouyancy control, proper weighting and streamlining of gear, and avoiding "happy hands" (using them to stay off the bottom or for propulsion) are the factors that lead to a better SAC rate.

 

[Thiad]

And do take heart about things like temperature, as a few people said. My warm water dive SAC rates make me smile, but I got really bummed comparing those to what my usual rate is in bad vis at 50 degrees.

But it's bad vis and 50 degrees! It took me a bit to get my head around that and stop beating myself up. My dive profile shows this big bump in the beginning - that initial splash where you catch your breath and work to get where you're going. After that, it settles down. It's definitely something to be conscious of, but I'm not as concerned about it today as I was in the past. You just go with the flow and remember to breath!

 

[g1138]

I like reading your posts, so please don't plan a dive using a max-depth-full-BT SAC rate.

This has nothing to do with being picky. It has to do with being correct and having somewhat consistent numbers that are applicable to dive planning. You consume gas at the time weighted average depth.

You're absolutely right. I haven't done a lot of dives deeper than 60ft. Nor have I done any dives with a 1min bottom time at max depth followed by the remainder of a dive at a shallower depth.
I didn't think about that.

If my SAC rate seems way too good or different I usually look at my whole dive profile to see what's up. I know it's happened once and I used an average depth from my Sensus unit that one time.
In hindsight, I should have definitely clarified in my original post.

 

[ajduplessis]

The calc for your SAC/RMV have been covered very well in the posts. My figures are between .42-.49 (12-14 lpm) depending on depth and workload and even lower on deco.