SAC Calculations

[The Kraken]

OK, my last post on the matter:

psi = pounds per square inch > that is a measure of force
cfm = cubic feet per minute > that is a measure of volume over time

We consume in volume, not force.

Excuse me, do you happen to have 350 psi of milk?

the K

 

[theskull]

OK, my last post on the matter:

psi = pounds per square inch > that is a measure of force
cfm = cubic feet per minute > that is a measure of volume over time

We consume in volume, not force.

Excuse me, do you happen to have 350 psi of milk?

the K

Yes, but in SCUBA the psi tells us how much volume of air is in a tank relative to its size and service pressure.

Excuse me, do you happen to have an Al 80 with 3000 psi I could use for this dive?

theskull

 

[limeyx]

Since the SAC rate is the "amount" of air one consumes at the surface, would it not be measured in volume since it is not under pressure (other than 1 ATA)?

I've always understood the SAC to reflect the cubic feet/min aspirated by a person.

Perhaps I'm wrong, but how can one measure the psi of a person breathing the earth's air?

While submerged, one can determine the volume of air used by observing the reduction in psi of the tank.

But it would seem to be easier to learn one's customary reduction in psi relative to a given tank/depth factor.

the K

That's always how I've calculated SAC (and seen it calculated)
cft/min (or whatever the metric equiv is -- liters/min ?)

Very little point measuring psi/min due to diff tank sizes. The PSI/min needs to be measured only as in input to the SAC calculation, but doesn't mean much on its own.

 

[Sideband]

That's always how I've calculated SAC (and seen it calculated)
cft/min (or whatever the metric equiv is -- liters/min ?)

Very little point measuring psi/min due to diff tank sizes. The PSI/min needs to be measured only as in input to the SAC calculation, but doesn't mean much on its own.

Sure it does. Once you are on your dive do you know how many cubic feet of air are in your tank at any given time? Doubtfull. You do know how many PSI are in the tank though because we have guages that read in PSI.

Joe

 

[CD_in_Chitown]

Sure it does. Once you are on your dive do you know how many cubic feet of air are in your tank at any given time? Doubtfull.

Sure you do, if you know the cuft/100PSI its easy.

Is it useful? I don't usually make changes to the dive plan that require re-calculating gas during the dive, and I wouldn't be able to source a slate big enough to work it out on the fly.

 

[Packhorse]

Sure it does. Once you are on your dive do you know how many cubic feet of air are in your tank at any given time? Doubtfull. You do know how many PSI are in the tank though because we have guages that read in PSI.

Joe

Your SPG is like a fuel gauge in your car. Its only there to tell you when you are going to run out.
It does not tell you how long it will last. The only way to work this out is using it to caculate your MPG or SCR.
Most Air Intergrated computers will calculate remaining air/dive time based on amount of air used during that dive and current depth and tank pressure, just like a cars trip computer will calculate distance to empty. This is ofcourse much more usefull than just a SPG or fuel gauge.

 

[Sideband]

If I know that I use X PSI /minute and am a given number of minutes into a dive then I can look at my guage and tell if I am on target for the dive I planned or if I am breathing heavier than planned.

CD,
Good point. I've never calculated that before. I'll give it a shot.

Joe

 

[limeyx]

Sure it does. Once you are on your dive do you know how many cubic feet of air are in your tank at any given time? Doubtfull. You do know how many PSI are in the tank though because we have guages that read in PSI.

Joe

Once I am diving I really dont care how many cft are in the tank. What I do is:

- Everything underwater is PSI (How many PSI to get to the surface, how many PSI I need to turn the dive on, how many PSI I should use every 5 mins). When I am underwater I really dont care too much about how many cft I have -- you just need *some* way to measure it and since the SPG is in PSI, that seems a good way. I can always convert to cft if I really want to by multiplying the # of 100 PSI's by my Tank Factor.
- SAC is always in cft/min -- anything else is useless, as I have many different tanks that I like to use. PSI/min only makes sense for SAC if you only ever dive one kind of tank (and if your buddies do too)


I think that certain people (agencies?) seem to make things too complex by going for huge precision, rather than math that can easily be done in the head to come up with a "close enough" answer.

There seems to be tables and tables and lists and lists for everything, when a few simple rules and some mental arithmetic will do.