Calculating air consumption SAC / SCR / RMV

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Fishkiller

Contributor
Messages
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Location
Mesa Arizona, The all beach no ocean state.
# of dives
100 - 199
Well

I downloaded my Dive computer to my PC and started looking at begining and ending presures. I then started doing my math, and calculators are a kewl invention, I don't have to take off my shoes anymore. :) but I degress.

I was wondering what your air consumption rates were??

mine averages 29 psi per minute total dive time.
 
Hmmm....figuring that out does require math, doesn't it? I can honestly (and sheepishly) say I have never actually determinded what my air consumption was..guess I probably should. My dives usually aren't "extreme" so I've never really worried about it..but it would be interesting to know.

Now, when you figured it..did you have to take different depths into consideration? Or just average for X number of dives at an average depth? Or..do you have an air integrated computer that does that for you?
 
but Maths was never my strong point...and with Scubabunny mentioning X i am getting flashbacks of algebra - bring me out in a hot sweat here.

Is there any easy way of working it out?
 
Knowing your PSI per minute is not terribly useful and it's not a true air consumption rate. A true air consumption rate is useful.

Air consumption is a personal aspect of diving. In other words, it varies from diver to diver. Even more, it varies for the same diver from dive to dive. Factors which will affect your air consumption rate are: work load, physical fitness, recent illnesses, water temperature, depth and comfort level. A useful method of calculating your personal air consumption rate involves finding your Respiratory Minute Volume (RMV). This is more useful than other methods because you can use it with any size tank. To find your RMV, descend to a comfortable depth. Relax, note your tank's pressure and sit quietly for 10 minutes. Again note your tank's pressure. Next, repeat the process while swimming normally for 10 minutes. At this point you know how many PSI/10 minutes you use at a specific depth for a specific tank both swimming and at rest. This is easily converted to cubic feet. Divide the total volume of the tank by the working pressure of the tank. This results in a tank factor expressed as cu ft/1 PSI for this particular tank. Multiply the tank factor by the PSI used then divide by 10 for each of the figures, PSI used resting and PSI used swimming. This is your RMV for that depth. Now convert it to a surface figure. Take your RMV at depth and divide by the ATMA of your depth. (D+33)/33=ATMA. RMV at depth/ATMA = RMV. Do this for both resting and swimming. You now have two baseline figures. To figure your air consumption at any depth, merely take your surface RMV and multiply by the ATMA of the depth you are planning.

Pounds per square inch (PSI)
Depth (D)
Atmosphere absolute (ATMA) ATMA = (D+33)/33
Tank factor is different for each type of tank (F)
Total tank volume (V)
Working pressure of the tank (W)
PSI used (P)
Respiratory minute volume (RMV)

V/W=F
(F*P)/10 = RMV at depth
RMV at depth/ATMA = RMV


WWW™
 
Well.....ermmm......Walter...thanks for that simple ( :all: ) explanation......<whimpers>
 
I'll stick with Butch's idea! - oh my god...i'm agreeing with butch!!!!
 
Abby,

Go through it slowly. It's actually quite simple. No one is asking you to memorize formulas. Look at the whole thing at once and it might seem like a job. Take it one step at a time and it's easy.

WWW™

 
Walter: That is a very accurate way to determine your breathing rate.


I like my method take beginning pressure minus ending pressure divide by number of diving minutes..
not as accurate but gives you a general guidline.


My Aeris 500AI gives me tank pressure at the beginning of my dive(below 5ft) and ending pressure (when I go above 5ft)
most of my dives have been shallow 40 footish. though on my few deeper dives 60 ish the average works out the same..




PS I still want to stay longer in the water
 
https://www.shearwater.com/products/teric/

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