SAC Rate

[twaldo]

Ok, maybe I'm inept at searching the forum, but I couldn't find anything that was specific to SAC....

I realize that your SAC rate is variable just like your heart rate, but what is an average SAC for cave divers? Just did a round trip dive from P1 to Olsen and calculated my SAC at .58 which I think is kind of high but made the entire trip just below 3rds.... Starting pressure of 3600 in LP85's (turn of 2400) and ending pressure of 2000 (made the turn before I hit 3rds). This is only the 2nd time I've calculated my SAC and I want to make a habit of it....

The biggest question I have is, my computer does not give me an average depth so I calculated based on my deepest which was 70' for :63 minutes. What should an average depth for that dive be? (again variable based on diver) just looking at averages...

The next question is, why does it seem that I use more gas going out than I do on the return??

Thanks,

Tim

[PfcAJ]

A .58 is fine. Anything between .5 and .7 I would consider pretty normal.

Do you mean you use more gas going in than on the way out?

[twaldo]

Yes... today for example started at 3600, made it to Olsen and probaly 150' on return before hitting 3rds(2400)... then only used a TOTAL of 800 (both tanks) on the remaining swim out ending my dive at 2000 psi...

Tim

[PfcAJ]

Flow was at your back, you probably 'zen-ed out' a little, and with less gas in the tanks, that means you're pushing less mass through the water and your BC is inflated less, which means less profile to push through the water.

[twaldo]

having someone say it, it suddenly makes sense... except the part about flow.... P1 has flow?

[tbone1004]

also that sac rate is quite wrong because your average depth was nowhere near that deep, which makes your SAC lower. Your average depth was probably close to the 40's. If you're computer doesn't give you avg depth your only course of action is to mark PSI and time on a flat section of a cave. Something like the first thousand feet of twin, or even the bottom of JB or Little River are good indicators, though JB and LR have high flow so you have to factor pulling in and coasting out.

If I am able to "cruise" at my preferred kick speed, which is 50fpm, then my SAC stays down around 0.4-0.45, if I have to kick to keep up with people it jumps up to about 0.6. If you have access to a pool it also works since it is true zero flow.

Yes P1 has flow, it's subtle, but it is there. If you have a buddy who has a computer with avg depth that would be better because then you get the total dive average.

[h2ojoe]

I averaged 48' last time I was there.

[twaldo]

I averaged 48' last time I was there.

Thank you... based on that mine went up a bit to .66, I'll have to check with my dive buddy he had a petrel which he said gave an average depth for the dive. I've been to LR twice and I know my SAC was through the roof on both dives... first would have maybe made it to bottom of chimney on 3rds and the 2nd made it about 60' past chimney on 3rds... NOT FAR at all.
I LOVE my Nitek3 dive computer, but I'm starting to see the advantage of a Petrel.....

Tim

[FW]

having someone say it, it suddenly makes sense... except the part about flow.... P1 has flow?

It did the Thanksgiving weekend. Not like Ginnie, but I could tell the difference going/coming. We did the Well passage, and it was noticeable syphon, so I turned us early.

[RN]

also that sac rate is quite wrong because your average depth was nowhere near that deep, which makes your SAC lower.

Shallower depth actually makes the RMV (SAC as it's being called here) higher not lower.