Do you use your SAC for anything? POLL

[4sak3n]

SAC stands for Surface Air Consumption. It is your breathing rate realated up to how much you would be breathing on the surface. Depth does not affect your SAC (beyond the indirect effects like stressing you out and increasing WOB ... which is minor).

 

[BoltSnap]

SAC stands for Surface Air Consumption. It is your breathing rate realated up to how much you would be breathing on the surface. Depth does not affect your SAC (beyond the indirect effects like stressing you out and increasing WOB ... which is minor).

You are exactly right. It is other "instantaneous" factors that would change the Gas Consumption rates while U/W. This is why an AI computer would be needed U/W to figure out the real remaining gas time during the dive. SAC rate calculations are only "guesstimates" not exact planning tools.

 

[billmas]

You are exactly right. It is other "instantaneous" factors that would change the Gas Consumption rates while U/W. This is why an AI computer would be needed U/W to figure out the real remaining gas time during the dive. SAC rate calculations are only "guesstimates" not exact planning tools.

Hi Burhan,

Yes, you're right but to be a little pedantic.... the prior SAC rate calculations from previous dives are fairly precise when calculated point by point during the dive, but when using those figures for planning, yes they do become guesstimates or just guidelines.

Ie, as a newly qualified OW diver, I was averaging 25 l/min, peaking at around 50l/min when stressed and settling to 11 l/min when relaxed. These have dropped the more i have dived.

For my planning now (having learnt to relax and not get stressed) I assume 20l/min when calculating general dive plans, but up to 50l/min for calculating worst case scenarios (ie getting out on a pony). You can run these figures through a fairly simple Excel sheet to see what effect it has at what depth for various tank sizes, SAC rates, rates of ascent etc.

I've dug out two dive logs which I know were quite square profile, my Excel prediction for dive times on the two are 40 and 39 minutes. Actual dive times are within a minute for each dive.

My DC, an Air integrated Atom 2 - constantly gives me "Air Time Remaining" (taking into account SAC, current depth, ascent rate, stop time, stop depth, and a surface reserve to be left over). It runs quite accurate for me.

If anyone would like the Excel sheet, PM me your email address, it is fairly straightforward (but it is in metric and I'd need to annotate it though!).

Kind Regards
Bill

 

[BoltSnap]

billmas,

Wouldn't be wonderful if there is a real SAC/AC rate modeling tool where you can start by graphing your dive and adding your the various factors you anticipate during the different dive depths. Excel starts with number and does graphs but I am talking about starting with a graph and then entering the numbers.

BTW I sent you my e-mail address. Thanks!!

 

[LakeCountyDiver]

WOW I started a simple thread to see what "kind" of divers use there SAC rate and got a lot more thorwn out there then I thought.

I did learn that is is good to have a few different SACs to apply to different dives and to ALWAYS be a little conservative.

 

[billmas]

billmas,

Wouldn't be wonderful if there is a real SAC/AC rate modeling tool where you can start by graphing your dive and adding your the various factors you anticipate during the different dive depths. Excel starts with number and does graphs but I am talking about starting with a graph and then entering the numbers.

Hi Burhan,

Yes I could see that working... set up a dive profile by clicking/dragging over a chart, then overlay your predicted SAC rate ontop, allowing it to be edited up/down for any part of the dive.

You could look at the effect of diving 5m shallower for parts of the dive - how much extra dive time it would give you etc.

Where I can see that being particularly useful is for example looking at "what if" scenarios...ie what if for example all of the following happened at once...
- my buddy ends up on my octo at 30m
- we start an immediate free ascent
- both of our SAC rates double due to the additional stresses / loadings of above

I would personally be planning for something like 100 l/min combined SAC as a worst case scenario. This would show what reserves we need to keep in hand as a bare minimum for any given depth.

As a computer programmer - I might have a look one day, but not enough time at the moment

BTW I sent you my e-mail address. Thanks!!

Yes, email sent.

Kind Regards
Bill

 

[BoltSnap]

Bill,

That is exactly the idea I have. It has to be metric AND imperial though

 

[TSandM]

I really don't think there is any point (except to amuse oneself) in trying to bring minute-to-minute precision into SAC rates. The whole idea is to have a generally useful tool for prediction. If I am trying to calculate my gas consumption to the liter to see if it will fit with the tank I'm carrying, I'm cutting things WAY too close. When computing safety reserves, we use fudge factors that are probably ridiculously conservative in a lot of cases, but I don't want to be there for the time that they aren't.

Divers should be smart enough to know that, if their usual SAC rate is calculated on a simple, low-exertion reef dive, that a high current dive or a dive in heavy surge is going to mean higher gas consumption, and just apply a general adjustment for predicted gas needs.

None of this is that accurate, or precise!

 

[TSandM]

I really don't think there is any point (except to amuse oneself) in trying to bring minute-to-minute precision into SAC rates. The whole idea is to have a generally useful tool for prediction. If I am trying to calculate my gas consumption to the liter to see if it will fit with the tank I'm carrying, I'm cutting things WAY too close. When computing safety reserves, we use fudge factors that are probably ridiculously conservative in a lot of cases, but I don't want to be there for the time that they aren't.

Divers should be smart enough to know that, if their usual SAC rate is calculated on a simple, low-exertion reef dive, that a high current dive or a dive in heavy surge is going to mean higher gas consumption, and just apply a general adjustment for predicted gas needs.

None of this is that accurate, or precise!

 

[billmas]

Hi TSandM,

I really don't think there is any point (except to amuse oneself) in trying to bring minute-to-minute precision into SAC rates. The whole idea is to have a generally useful tool for prediction.

The only accurate way to measure SAC is with a point by point calculation. Without access to digital dive logs off a DC, previously it would just be calculated on max depth and time, or some guesstimate of average depth. Doing that can give wildly different numbers for a square versus V profile dive.

I do entirely agree - SAC is a useful predictor for dive planning, but not the be all and end all.

If I am trying to calculate my gas consumption to the liter to see if it will fit with the tank I'm carrying, I'm cutting things WAY too close. When computing safety reserves, we use fudge factors that are probably ridiculously conservative in a lot of cases, but I don't want to be there for the time that they aren't.

Absolutely, my dive planning involves running two figures, one for what I expect to do, and one for worst case scenarios if the brown stuff hits the whirly thing. I plan reserves around the worst case scenario.

Divers should be smart enough to know that, if their usual SAC rate is calculated on a simple, low-exertion reef dive, that a high current dive or a dive in heavy surge is going to mean higher gas consumption, and just apply a general adjustment for predicted gas needs.

I think they should also be aware of what peak SAC they can reach. Ie, my first ever ocean dive, backwards roll off a moving boat, two at a time, I hit 70l/min (almost 3cu.ft per min) but with no physical exertion (I'd been sitting on my backside on a boat for the past 45 minutes). Without the minute by minute SAC I would never know about the 70l/min - just an elevated average for the whole dive.

None of this is that accurate, or precise!

Indeed. However the more accurate the numbers we feed in to our predictions, the less innacurate/imprecise those predictions are likely to be.

I suspect there is some fixation on SAC rate, specifically people trying to lower it as if it is the measure of a good diver (just as there can be with how much weight people carry). That is a bad thing and not particularly helpful.

Kind Regards
Bill