How are you computing your SAC?

[The Kraken]

Sharp,

Check your tank pressure just a couple of minutes into your dive, the pressure will have been reduced because of the temperature change. It won't be exact, but it'll be better than recording the pressure of the tank while it was still sitting in the sun.

SAC is not unlike statistics, one can carry it a bit too far and interpolate that which is desired.

Nothing is perfect.

the K

 

[Land Locked]

I was trained using PSI not CFM, anyone else use this formula?
Psi used / Bottom time X surface rate depth
Feet Conv to SR Conv from SR
10 .76 1.30
30 .52 1.91
60 .35 2.82
90 .27 3.73
120 .21 4.64

This is condensed of course, but this is the way I've done
it for years now. So question is, is there a difference between
figuring SAC by PSI or Volume used, is one better than the other?
Or are they equal when it comes to dive planning?

 

[Charlie99]

So question is, is there a difference between
figuring SAC by PSI or Volume used, is one better than the other?
Or are they equal when it comes to dive planning?

PSI numbers are dependent upon the volume of your tank, while cubic foot per minute numbers aren't. A PSI number means nothing unless you also include tank volume and working pressure.

OTOH, our SPGs read PSI, not cubic feet, so there are advantage and disadvantages of each system.

 

[RIDIVER501]

Since this was a wreck dive, I spent most of my dive at about 75 feet. As pointed out here, that doesn't take ascent/descent times into account. It also doesn't take temp/pressure changes into account, but I don't know how to accurately account for that.

I don't know how to take those things into account either.
But does it really matter?
I think we are getting into a hyper-accuracy debate here.

Hyper-accuracy =

measure with a micrometer
mark with chalk
cut with an axe.

Is the extra accuracy by factoring descent/ascent time, temperature change and any other variable we can concot into the math problem going to matter a hill of beans in the grand scheme of things?

In the long run, I think not.

 

[Land Locked]

PSI numbers are dependent upon the volume of your tank, while cubic foot per minute numbers aren't. A PSI number means nothing unless you also include tank volume and working pressure.

OTOH, our SPGs read PSI, not cubic feet, so there are advantage and disadvantages of each system.

Interesting, this is where my grey matter needs a kick. If I know how much
Psi/min I'm using under specific diving conditions and I pre-plan a dive to 80'
knowing what tank I'm diving and the pressure, I take my my SAC Psi/min and
use a multilevel dive plan figuring time and depths. Could I not resonably follow
that plan and know what pressure I will have left during and at the end of the dive?
I even setup my dive computer software to determine Psi/min Sac for me, it does
take inputs for tank type, working pressure, beginning and ending pressure.
I may have to look into converting over to volume calculation and give it a try.
My instructor is a cave diver and he taught me the Psi/min Sac tables.

 

[The Kraken]

I use SAC to determine the capacity of the tank required for a dive based upon a general SAC rate developed for that particular type of dive. I use the SAC for planning.

The PSI/min rate is a great tool to use to determine one's activity at depth or to determine an inordinate usage of breathing gas. I use the PSI/min for monitoring.

the K

 

[Land Locked]

I use SAC to determine the capacity of the tank required for a dive based upon a general SAC rate developed for that particular type of dive. I use the SAC for planning.

The PSI/min rate is a great tool to use to determine one's activity at depth or to determine an inordinate usage of breathing gas. I use the PSI/min for monitoring.

the K

It was a long time ago, but this rings a bell, Psi/Min monitoring the pre-planned
dive. Darn, I've lost a few brain cells over the years!
Question still remains, anyone use the formulas I was trained with for determining
Psi/Sac at surface and depth? Or are these an antiquated way of doing things?

 

[Sideband]

I use SAC to determine the capacity of the tank required for a dive based upon a general SAC rate developed for that particular type of dive. I use the SAC for planning.

The PSI/min rate is a great tool to use to determine one's activity at depth or to determine an inordinate usage of breathing gas. I use the PSI/min for monitoring.

the K

I think we are on the same page. I use my SAC, in cu ft., to figure out how much gas is needed for a particular dive. I figure my turn around point and 'Rock bottom' in PSI. Using PSI/min I can figure approx. times for those points.

Joe

 

[Sideband]

I was trained using PSI not CFM, anyone else use this formula?
Psi used / Bottom time X surface rate depth
Feet Conv to SR Conv from SR
10 .76 1.30
30 .52 1.91
60 .35 2.82
90 .27 3.73
120 .21 4.64

I was given those when I took my Advanced course. I use the wheel though.

Joe

 

[Charlie99]

Interesting, this is where my grey matter needs a kick.

It seems that my other post was too complicated and subtle. PSI style SAC works fine!

To go back and forth from PSI to cu ft style, just apply a fudge factor associated with the particular tank you use. For example, with an AL80, the fudge factor is 38.75psi/cu ft.

(This is the 3000psi working pressure divided by the 77.4 cu ft capacity. I'll usually use either 38 or 40 for ease of calculations).