SAC rate

[navy85]

Here is a website that may be of interest: Surface Air Consumption Rate. Gets a little technical...talks about Boyle's Law, Charles' Law, stuff I learned 20 years ago and really didn't use much - until I started diving

Uncle Pug once bubbled...
You can figure your SAC rate in cubic feet by converting psi to cf specific to the tank size you were using.

If you use different tanks (yes, I am new and don't own any tanks yet) - in terms of volume, that is - wouldn't you have to determine your SAC in cu ft/min to be able to compare (since pressure and volume are inversely proportional - Boyle's law)? If you use the same size tanks all the time it doesn't matter.

 

[Devils_advocate]

I found a decent calculator on-line. If you want it just PM me and I'll e-mail it to you.

 

[salty]

has anyone noticed their SAC rates go up when using trimix with +20% He mixes?

 

[kramynot2000]

This is a great thread because I've been wondering about this. So, is this right, if I did a shallow dive and burned 2750 psi in 55 minutes at an average depth of 33' then using Uncle Pug's formulas, I divide 2750 by 110 (55 times 2 ata) which equals 25 psi per minute on the surface.

I'm diving an HP 80 which has 80 cuft at 3500 psi so each 43.75 psi equals 1 cuft.

So, my SAC rate is 25 divided by 43.75 which equals .57 cuft per min, right?

It actually becomes kinda neat starting understand this stuff.

 

[Uncle Pug]

kramynot2000 once bubbled...
So, my SAC rate is 25 divided by 43.75 which equals .57 cuft per min, right?

Now that you have figured out how to ascertain your SAC you need to figure out how to use it for gas management.

Round your SAC to .6 and plan a 30 minute square profile dive to 70'. You are diving off of a live boat so you can ascend anywhere for pickup.

How much gas will you need in cf for this dive and how much should you hold in reserve for you and your buddy to make a safe ascent in an OOA both sharing your reserve.

It would be nice to know what the reserve is in psi.

What is the reserve required in psi if you are using:
HP80 (3500psi)
HP100 (3500spi)
LP104 (2640psi)
AL80 (3000psi)

 

[Big-t-2538]

Braunbehrens once bubbled...

All this stuff is 1000 times easier with the metric system...unfortunately I also use the imperial system.

damn simplicity of the base 10 system....who ever thought that was better than just randomly grabbing numbers out of old ways anyway....stupid simplistic europeans....

 

[JohnStrr]

To make a long and complicated story short the Masons (our founding fathers and numerous past presidents are a good example) believe that the imperial system is based on Divine numbers and distances derived from sacred geometry, most of which was taken out of Egypt in the ancient past (note the pyramid on the dollar bill). They make the laws, thus we keep the less than user friendly but very symbolic Egyptian measurement system.

 

[DA Aquamaster]

I will normally figure my SAC over a 10 minute swim at a good clip at a fixed depth. This normally gives me a SAC of around .5 by mid season. (.53 last time I checked)

The advantage of this is that I never do a whole dive with that level of physical activity and the SAC rate then used in planning is very conservative. On a working dive I will still double this for plannng purposes, but to be honest the dive ends if the predetermined pressure to end the dive is reached even if the anticipated bottom time is not achieved.

I don't usually bother figuring a SAC at rest as it is not a number you want to rely on, but I do always note my PSI when I start my ascent and my PSI remaining at the surface.

I know for example that I can ascend from 130 ft with a 1 min stop at 65 ft , a 1 min stop at 30 ft, a 3 min stop at 20 ft, and a 1 min stop at 10 ft and expect to use 350 psi. It's good information to have for every dive and over time allows me to make an accurate estimate of the minimum air I need to surface without cutting into my 1/3 reserve.

 

[runvus4]

Small nit-pick clarification...

SAC/SGC rate is PSI per minute and specific to the tank that you measure it in.

RMV is the CuFt/min that is tank independant.

 

[DA Aquamaster]

salty once bubbled...
has anyone noticed their SAC rates go up when using trimix with +20% He mixes?

Higher gas consumption is one of the downsides of Trimix. The good news is that it also slips though otherwise low performing regulators better than air.

runvus4 once bubbled...
Small nit-pick clarification...

SAC/SGC rate is PSI per minute and specific to the tank that you measure it in.

RMV is the CuFt/min that is tank independant.

The term SAC rate is pretty much synonomous with the old SAC rate calculator which was just a circular slide rule that let you input depth, time and PSI used to obtain your SAC rate in PSI/min. It's still a handy tool to have tucked in the logbook and it is too bad they don't make them anymore.

The SAC rate calculator just used PSI for convenience as it eliminated having to convert the psi used to a unit of volume. The price paid for that convenience however was that the result was tank specific, which in an era when everybody dove with either a steel 72 or an AL 80 was not a serious limitation. I'd argue that is still the case for most rec divers, who 90% of the time will be strapping on an AL 80. The additional advantage was that dive planning could be done in PSI and the turn around and end pressures were already in PSI. It was much nicer for the math challenged.

Dive planning in cu ft, is fine but in the end you have to convert back to PSI to know what you need to know in the water. So you can convert before or after but until you get an SPG that reads in cu ft, you will still have to convert to or from PSI somewhere in the process.

Sometimes a term gets to be too closely associated with a particular item and the traits of that particular item then become synonomous with a whole range of similar items. Much like the tendency for many people to refer to all personal water craft as "Jet Skis", SAC is associated with the SAC Rate Caluclator and consequently with PSI.

But, the association with the SAC Rate Calculator aside, surface air consumption is still surface air consumption no matter what units you choose to use. Similarly, speed is still speed whether you refer to it in miles per hour, kilometers per hour, or furlongs per fortnight. To say it is incorrect to use the term SAC rate with units of volume is, in my opinion, beyond nitpicky.

RMV is perhaps the new "tec" approved term but is not any more right than SAC and in some respects promoting the term RMV seems to be right up there with reinventing the wheel. Besides, when you say "Surface Air Consumption", people know exactly what you mean - the term is intuitively obvious and there is a great deal of value in that.