SAC Rates

[TSandM]

If you want precision, the timed swimming test is the best way to do it. But precision isn't really critical, as no one (I hope) is calculating their gas supply to the last 100 psi based on their prior SAC rate. So a time-weighted average is probably good enough, especially if it's taken from a dive which isn't too heavily multi-leveled.

 

[DA Aquamaster]

I use two different methods:

1. Timed swim at constant depth:

I swim for 5 minutes or so at a fast cruising speed to get the heart rate up and ensure there is no excess O2 in the tissues, then note th time (wait for the dive timer or computer to click to the next minute) and then note the SPG reading. Continue swimming at that same constant depth for another 5 or 10 minutes, then note the spg reading. At deeper depths you can get by with 5 minutes at shallower depths, you will want a longer swim.

Then crank the numbers through the forumla posted above. This will give you a real world swimming SAC.

2. Note the pre dive and post dive pressures and use the average depth for the whole dive.

This pretty much requires a computer that will track and calculate average depth. Just take the total gas used and the average depth and crank the numbers through the formula above. The advantage here is if you do it every dive, you develop a very good feel for what conditions may increase your SAC so that when you encounter them on a dive, you can increase your conservatism and your reserve.

Both techniques give me very consistent results at about .65 for normal swimming dives. The slightly faster than normal pace in the timed swim seems to compensate for dry suit/BC inflation, etc that occur in a real dive.

My resting SAC is a lot lower, but that number is not real useful for anything other than deco gas planning. Some agencies take a resting SAC times a constant of 1.5, 2.0 or even 3.0 to get a workign SAC but that is pretty much a wild guess.

 

[sam1]

(((Pb-Pe)/Pb)x((Pb/Pw)xV)/T)/((D/33)+1)

Where:
Pb = Beginning Pressure
Pe = Ending Pressure
Pw = Working Pressure of Tank
V = Volume of Tank
T = Time of Dive in Minutes
D = Average Depth of Dive in Feet

A long-winded (and, hopefully, intuitive) explanation of the above:
(Pb-Pe)/Pb is the fraction of the gas in your tank that you consumed
(Pb/Pw)*V is the volume of gas in the tank at the beginning of the dive
Multiplying the two expressions above gives the amount of gas used in the dive
Dividing that by T gives the volume of gas used in the dive per minute of the dive
Finally, dividing the preceding by (D/33+1) adjusts the gas consumed per minute at the average depth of the dive to its surface equivalent.

 

[TSandM]

I swim for 5 minutes or so at a fast cruising speed to get the heart rate up and ensure there is no excess O2 in the tissues

I'm curious about your reason for this. Although it may well be possible to exert sufficiently to incur oxygen debt in muscles, even in the presence of elevated ppO2s, I'm unsure why this would be expected to affect SAC rate, which is determined almost entirely by CO2 production.

 

[SparticleBrane]

Personally I am not a big fan of just giving out answers to people's questions but I don't mind leading people to the correct answer.


I will give you this -- SAC rate is a measure of how much gas you breathe, at depth, over a certain period of time. Expressed mathematically this is "cubic feet per minute per ATA" (volume breathed per time period per depth).


With that being said, you now know you need several variables:

• How much gas you consumed over the entire period of the dive, expressed in cubic feet
• Length of the dive in minutes
• Average depth of the dive, in atmospheres absolute (ATA)

Since our SPGs don't measure in cubic feet, you would need to convert from gas used in PSI to cubic feet, yes?

Length of the dive in minutes fromt start to finish should be easy, right?

Average depth of the dive in ATA is a bit more difficult. As mentioned above, this can usually be gotten from a dive computer but it will usually be expressed in feet, so again you'll have to convert to ATA. Note that by having the average depth as part of the equation, you are mathematically getting rid of the depth changes in the dive -- it's as if you stayed at that one average depth for the entire length of the dive.


So, after finding all these things you should have enough info to calculate your SAC rate, in cubic feet / minute / ATA. We can get more detailed if you need, just ask.

 

[BobinNC]

Bob,

If you have Excel, I'll send you a spread sheet that does lots of stuff, if not:

(((Pb-Pe)/Pb)x((Pb/Pw)xV)/T)/((D/33)+1)

Where:
Pb = Beginning Pressure
Pe = Ending Pressure
Pw = Working Pressure of Tank
V = Volume ot Tank
T = Time of Dive in Minutes
D = Average Depth of Dive if Feet

the K

Kraken,

I received the sheet thank you for your help, and everyone else who added to this as well.

Bob

 

[DA Aquamaster]

I'm curious about your reason for this. Although it may well be possible to exert sufficiently to incur oxygen debt in muscles, even in the presence of elevated ppO2s, I'm unsure why this would be expected to affect SAC rate, which is determined almost entirely by CO2 production.

The reasons are mostly practical. Theoretically, I could hyperventilate and then swim for a minute or so before breathing off the reg at all. If I then limit my time trial to only a minute, I could have a really good SAC. It just makes more sense to use a steady state swimming condition after the O2 and CO2 stabilize, then to start from essentially a resting condition.

Plus it works and is very close to what I actually see in a real dive over the course of the entire dive.

 

[Doc Intrepid]

Bob,

Just remember that SAC is a 'surface air consumption' rate - an average baseline value that you use for dive/gas planning.

To pre-plan how much gas (volume) you'll need at depth; figure out when to turn the dive if your buddy has a different tank than you have; or look at different volumes of decompression gas, (etc.) you will need to convert this value to respiratory minute volume (RMV).

which is likely another thread

K's Excel spreadsheet may have it also, I've not looked at it.

Doc

 

[The Kraken]

Doc,

Not included . . . . waaaaay to many variables.

The formulae in the spread sheet are just some basic calculations derived to assist in dive planning.

the K

 

[Spimon]

I've also made a spreadsheet to calculate SAC and to keep a log of it. Although it's all metric, so may not be useful to the majority of this forum.
It also graphs your individual dive SAC along with your average SAC over and calculates a theoretical dive time based on past averages.
Anyway, if it's any use to anyone it's on my website in the 'tools' section called 'Dive Time Calculator'.

Link to the site:
Simon Yachting - YA/RYA Skipper


- Simon