SAC Calculations

[Fastmarc]

Ok, thanks all.
Unfortunately my computer doesn't give me a average depth reading, but I should be able to use the graph to do the calculation based on the info given above.

 

[mstevens]

The RMV is generally expressed as the PSI used per minute.

Respiratory minute volume is the product of tidal volume and respiratory rate. The units used must, therefore, be expressed in terms of volume rather than pressure. Common units in medicine (and in scientifically literate societies) would be liters or milliliters (per minute). In the US and a few other countries, cubic feet (per minute) might be used. For example, in one study average RMV in anesthetised males was 130 mL/Kg of body weight, and 113 mL/Kg in females. For an "average" 70 Kg male that's an RMV of 9.1 L/min or 0.32 cf/min.

SAC is merely RMV, which does not vary by depth, corrected for pressure at depth, since the volume of air once let out of the cylinder does vary by depth.

The only reason PSI/minute might be useful is that pressure gauges display, well, pressure and not volume. At constant depth the rate of pressure drop per minute will allow you easily to calculate time remaining at that depth.

Less wordily, RMV can't be expressed in terms of pressure over time (PSI/min) but at constant depth is proportional to it.

 

[The Kraken]

Good point, MS, I stand corrected!

I truly appreciate you making that clarification!

Some of my points were not expressed well, I must admit. And I do apologize for my errors.

(If there were a "Foot in Mouth" smilie, I'd put it here)

I do realize that pressure is not a measure of volume.

I think it is because of the fact that the SPG is deals with pressure that the PSI/min consumption rate is recommeded while diving.

the K

 

[cerich]

The heck with all these calculations get the NAUI SAC rate Calculator, looks like a wheel, easy to use and cheap.

NAUI does require that Scuba Diver Students calculate their SAC rate, I know that lots of instrs "forget" this but it is a standard.

 

[Fastmarc]

NAUI does require that Scuba Diver Students calculate their SAC rate, I know that lots of instrs "forget" this but it is a standard.

You are right, I do remember covering it now, though I didn't remember initially, but a call to my instructor quickly jogged my memory.
I did a dive yesterday that allowed me to check my rate as it's depth was reasonable constant. I got a figure of 0.4 .

 

[CD_in_Chitown]

The thing to remember is that people tend to be a bit sloppy when
talking about "SAC" rates. They tend to leave off units which is actually
very imporant. They also some times report SAC rates in terms of
pressure and sometimes in terms of volume.
For example, the units of SAC rates that people report could be:
BAR/MIN
PSI/MIN
Cubic Ft/MIN
Liters/Min

You may also see another acronym of "RMV",
Respiratory Minute Volume. This is a surface air usage base on volume
not pressure. You get values with units such as:
- Cubic Feet/Min
- Litres/Min

You're description of RMV does not seem correct to me. Remember that SAC is an abbreviation for surface air consumption and is measured in pressure/time. Your RMV is how much gas you're using in a minute and I thought factored in depth by the SAC rate plugged into the equation (at least that is how I learned).

Your points on the equations posted are accurate, I think there is some typos in some of the posts.

For the OP I encourage you to discuss the formula's and their implications on your diving with an instructor or mentor if you're not gifted with mathematical accumen. (No offense intended, there was no way I could get this reading it the first time.)

To add to the explanations for finding your own RMV I would encourage you to calculate the whole dive instead of a 10 minute segment. You're breathing will be different for the period you're testing than for the rest of the dive, I'd wager.

 

[The Kraken]

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

 

[Humu-out-of-Water]

Thanks for all the imput and discussions on this thread... THIS is why I tell my (high school) students they need math in school.:brain:

 

[CD_in_Chitown]

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 not how I learned it. I learned SAC was based on PSI used and could be conveyed as PSI/MIN. This number needs to be factored with your average depth to determine your RMV for the dive, that is how to apply the depth factor. (Had to check my notes). Then your RMV is conveyed in a volume/time equation that computes to (~.4-.7 Cu ft/min) depending on activity for me.

Thus your RMV calculated based on the conditions of a previous dive and useful as a planning tool, you can determine Cu ft of gas required and apply to any size or pressure tank.

 

[Sideband]

You're description of RMV does not seem correct to me. Remember that SAC is an abbreviation for surface air consumption and is measured in pressure/time. Your RMV is how much gas you're using in a minute and I thought factored in depth by the SAC rate plugged into the equation (at least that is how I learned).

Your points on the equations posted are accurate, I think there is some typos in some of the posts.

For the OP I encourage you to discuss the formula's and their implications on your diving with an instructor or mentor if you're not gifted with mathematical accumen. (No offense intended, there was no way I could get this reading it the first time.)

To add to the explanations for finding your own RMV I would encourage you to calculate the whole dive instead of a 10 minute segment. You're breathing will be different for the period you're testing than for the rest of the dive, I'd wager.

CD,
SAC takes into account your depth, time and change in cylinder pressure for a specific tank style ie. AL80, and gives results in PSI/Minute. If you change to a ST95 the results for the AL80 are worthless. RMV includes making a conversion of PSI/minute to CFt/minute and can be carried over from one tank style to another. If you divide your tank volume (ie 80cubic feet) by the rated pressure (3000psi) you get the tank factor which you can multiply by your SAC to get RMV.

Joe