Calculating air consumption SAC / SCR / RMV

[ABQdiver]

This whole SAC/SCR thing has me a bit confused. Now I realize that using air at "your" rate is something you not be ashamed about and with practice, planning and other measures that rate may be cut.
That said, the math and figures shown by Spectre are confusing, What is ATA? Can you show the formula to make it clearer?

I have a formula in my logbook that shows SCR computed by:

(PSI divided by Time) 33 or 34 for freshwater
---------------------------------------------------------
Depth + 33 or 34 for freshwater


I have guessed in the past dives I've done that this could not be correct because your depth could change frequently and thus your math be off. Would a average depth be sufficient? or a combination of depth and time for each plane (or depth) of the the dive profile be more accurate? The formula I show does not seem to work accurately.


I recognize that knowing your air comsumption is another tool for a "good" dive plan and would like to know more details.
TIA

 

[Spectre]

ABQdiver once bubbled...
That said, the math and figures shown by Spectre are confusing, What is ATA? Can you show the formula to make it clearer?

ATA = Atmospheres absolute. That's the whole concept of Boyles law, where volume X at 1 ATA [the surface] is X/2 at 2 ATA [33 fsw or 34 ffw (feet salt water and feet fresh water respectively).

The importance of atmospheres is that it effects your air consumption... e.g. if you breathe 1 cuft/min on the surface [or 1 ATA], you breathe 2 cuft/min at 2 ATA [33 fsw], 3cuft/min @ 3 ATA [66 fsw] etc, etc, etc...

I have guessed in the past dives I've done that this could not be correct because your depth could change frequently and thus your math be off. Would a average depth be sufficient?

If you are looking for an overall SAC rate, then yes, average depth would work fine. You can test this by calculating out the SAC rate for a 15 minute dive at 60 ft, and then calculate out the SAC rate for a dive that is 5 minutes at 90 ft, 5 at 60, and 5 at 30. The rates will be the same.

So walking through my example:

A 5 minute descent to 50 fsw. That's an average of 25 fsw for 5 minutes. 25 fsw is 1.75 ATA * 5 = 8.79.

This assumes a constant descent rate.... and therefore the average depth would be 25 fsw. 25/33+1 converts depth to ATA [or (25+33)/33]. So 25 fsw is 1.76 atmospheres absolute. [actually 1.7575, and I rounded wrong on my original post]

So using Boyles Law, you'd breath 1.76 times the amount of gas on the surface. If you multiply that by 5, you end up with 8.79 [58/33 * 5]. Which means that in 5 minutes at 25 fsw, you'd breathe the equivalent of 8.79 minutes on the surface.

15 minutes @ 50 fsw. 15 * 2.51 = 37.73

Converting 50 fsw to atmospheres is (50+33)/33, or 2.5151. Multiplying that by15 minutes at 50 fsw, results in the equivalent of breathing on the surface for 37.73 minutes.

The other two are for the ascent [5 minute ascent, so it's the same as the descent], and a 3 minute safety stop at 15 fsw.

Once you have converted the dive profile to a constant surface unit, you can then use that to calculate how much air per surface unit you used.

Edit: Started falling asleep while typing... I'll read it over in the morning to make sure I didn't screw up above...

 

[JimC]

This is how I calc my SAC for a dive.

First you need a few numbers:

Time
Average Depth (SWAG or down load from CPU)
In Gas
Out gas
Tank Capacity (when full)

Step One - Calculate cf/psi
(Do this once for the tank)

Tank volume in cu
------------------------ = cu/psi
Tank Rated PSI


Step Two - Calculate psi/min @ depth

Gas In - Gas Out
---------------------- = psi/min
Bottom Time


Step Three - Calcualte psi/min @ surface

psi/min (@depth)
-------------------------------- = psi/min
(Average Depth / 33) + 1


Step Four - Calculate SAC


psi/min * cu/psi = SAC


=======================================
Example:

A 30 min dive with an average depth of 33 feet in salt water. Using a Catalena S80, PSI going in was 2800 and out was 2000.

cu/psi = 77.4 / 3000 = 0.0258
psi/min @ depth = (2800-2000)/30 = 26.6667
psi/min @ surface = 26.6667 / ((33/33) +1) = 13.3333
SAC = 13.3333 * 0.0258 = 0.344

So your SAC rate for this dive is 0.34 cu / min. Which you can use to help plan your next dive. The psi/min @ surface can also help you estimate how much longer your tank will last when you look at your PSG.

 

[ABQdiver]

Thanks to both Spectre and JimC,

I think I'm finally getting the concept. Obviously there are more than one way to obtain the figures. But the general idea is starting to make sense.

One thing that does puzzle me is the volume of air in a given tank. Both examples spoke of using a 80cf tank and gave different pressures or volumes of air. The pressure would be obvious to me, just by looking at your gauge. But the volume of air is a little more obtuse.
If you know your tank size.. say a alum 80, and your pressure....say 3125psi then is there a way to calculate the volume of air for that particular time, under those conditions?


This is great! I love learning new things

 

[Spectre]

ABQdiver once bubbled...
If you know your tank size.. say a alum 80, and your pressure....say 3125psi then is there a way to calculate the volume of air for that particular time, under those conditions?

You need to know the tank rating. JimC is correct with the 77.4 for a Catalina 80 [I said 77.7, which is incorrect].

So if the tank is 77.4 at 3000 psi, that would mean .0258 cu ft per psi. So 3125 would be 80.625 cu ft.

If your talking, say, a luxfer aluminum 100 [99.3 @ 3300 psi], then it's 99.3/3300 * 3125 = 94 cuft.

Since your diving in new england, make sure you take your initial pressure reading after you jump in the water... gas expands the warmer it is, so while you might think you have 3125, as soon as you hit the cold water it might drop to 2900....

[although it doesn't hurt to use the higher PSI, as that gives a little buffer into your numbers...]

 

[jeffsterinsf]

The "ideal gas law" lets you know how much gas is in a tank.

PV = nRT

Sounds nasty, but as long as the temperature (T) isn't much different than 68 deg. F (relative to a chilly absolute zero at -460 deg. F), it says that the amount of gas (nR) in your tank (V - a constant, assuming your tank isn't growing or shrinking) is proportional to the pressure (P).

pressure in tank
---------------------- * rated capacity = gas in tank
rated pressure

Note that "rated pressure" here is to mean the pressure at which the tank holds the rated capacity. Some tanks have different "working pressures," so look at the tank specs carefully.

The temperature changes have a small effect, a 25 degree difference is about 5% (5% of (68+460) = 26.4 deg. with 68 deg. F being "standard" temperature of "STP"). You might have seen it in action if your tanks were filled "too quickly," heating the gas. As the tank cools, you would see the pressure in the tank fall slightly.

If you're looking to compute SAC, and there is just a couple hundred PSI left in the tank, taking the reading after a cold-water dive wouldn't make a huge difference . You can always let the tanks warm up to ambient during your surface interval or ride home and take the measurements then, so that the starting and ending readings are taken at the same temperature. IMHO, the inaccuracies in computing SAC from an entire dive (e.g., different activity levels at different depths) are probably bigger than the post-dive temperature-induced errors in computing SAC.

This link
http://www.diverlink.com/gear/tankspecifications.htm
has been suggested in the past as a place to get generic tank data. You might also check the manufacturer's web site.

http://www.luxfercylinders.com/americas/product_info/scuba/specifications.htfx.htm
lists the common Luxfer S080 as 77.4 cuft @ 3000 psi, not 77.7 as listed on diverlink. Then again, if you're cutting things within 1/2%, I'd be worrying...

 

[bonzjivar]

I have been cetified for 25 years but have always been pretty good at sucking a tank dry in record time. What can a guy do to improve SAC?

 

[DiverBuoy]

If you got certified 25 years ago and haven't dove since - there's your problem. You didn't really indicate your experience level. But let's assume you made an ave of 1 dive per year and have around 25 dives total experience. If your diving has long gaps like this in between events it's equivalent to starting over each time. All the anxiety is back and the thrill that prevents you from really relaxing.

Like a fine wine you take it in "taste it a little then swallow". Or more accurately breath in 3 seconds breath out 6 seconds (or 4 and 8 if you're really relaxed). Exhale completely (but don't press). Continue until you get into a rhythm, which in turn further relaxes you.

And dive MORE and REGULARLY - the annual club med visit won't do it - as far as increasing bottom time.

Finally for those people who still just suck air - get a much bigger cylinder.

 

[Drew Sailbum]

1. Master buoyancy control. Get that precise weightless feeling going and you will reduce your work load and oxygen needs. Slight cchanges in lung volume should be your primary means of making minor adjustments in depth.

2. Trim out properly. Take the time to do a weight check right to know the exact minimum amount of lead you need. Don't take more than is necessary. Shift that lead around on your body until you are in a slightly-head-up position. Secure all dangling gear to be as streamlined as possible.

3. Practice, practice, practice. Get out and dive as much as possible. It makes an immense difference.

 

[Paco]

There are a whole lot of variables when it comes to air consumption but I agree that practice is the key.