I sent over an e-mail to a friend of mine that was an attempt to address entry level gas planning. I thought I'd post it here. It's basic and takes a few assumptions, but hopefully some will find it useful.
About SAC rates - Since we talked about it a little yesterday and I was doing mine, I thought Id calculate yours. Its a great tool for gas planning but it is also a great way to track your comfort in the water. As you become more comfortable in the water, more methodical in your movements, develop better buoyancy control, and adopt a more streamlined position you will notice a significant decrease in your consumption rate.
Youll need a little background for it all to make sense so here goes:
Data Needed:
Dive Time 46.4 minutes
Average Depth 21.21 ffw
Start Pressure 3000 psi
End Pressure 400 psi
Tank AL 80
Hopefully that all makes sense.
NWGratefulDiver on the board also has a great article. Much better than mine! Can't seem to find his web site right now. I'll look for it and edit if I find it. Maybe he'll chime in.
Hunter
About SAC rates - Since we talked about it a little yesterday and I was doing mine, I thought Id calculate yours. Its a great tool for gas planning but it is also a great way to track your comfort in the water. As you become more comfortable in the water, more methodical in your movements, develop better buoyancy control, and adopt a more streamlined position you will notice a significant decrease in your consumption rate.
Youll need a little background for it all to make sense so here goes:
Data Needed:
Dive Time 46.4 minutes
Average Depth 21.21 ffw
Start Pressure 3000 psi
End Pressure 400 psi
Tank AL 80
- ATA = Atmospheres Absolute Surface pressure at sea level is 1 atmosphere or 14.7 psi. Every 33 feet or 10 meters you descend underwater you increase pressure by another atmosphere or 14.7 psi. So at 33 feet deep we are at 2 ATAs (one for the normal pressure at the surface and another for the 33 feet of water above us. At 66 feet we are at 3 ATAs, 99 feet 4 ATAs, etc.
- The formula is Depth/33+1 = ATA
- SAC = Surface Air Consumption expressed in psi/min. (people generally confuse this with RMV see below). SAC is dependent on tank size so it is really only useful on tanks of the same size or during your dive to monitor your breathing rate. SAC is calculated by dividing the psi you used by your bottom time and then adjusting for depth in ATAs.
- So you started with 3000 psi and ended with 400 psi. Our run time was 46.4 minutes so you used 56.0 psi per minute. (3000-400=2600/46.4=56.0)
- Now adjust for depth. Our average depth today was 21.21 ffw or 1.64 ATAs. (21.21/33+1) Now divide your psi/min by ATAs and you get your SAC. 56.0/1.6 = 35 psi/min SAC.
- RMV = Respiratory Minute Volume expressed in cubic feet/min. RMV is used to express your consumption regardless of tank volume. This is so you can change out tanks or dive with other divers with different tanks and still be able to plan your gas needs. Its just a little more involved that SAC. First you need to relate cubic feet to psi with regards to the tank(s) in question. This is done by multiplying your tank volume by its rated pressure.
- In the case of an Al 80 it holds 77.4 cu ft of gas at 3000 psi. So each psi is 0.0258 cuft in that tank. (77.4/3000) Theres an easier way but Ill get to that later.
- So if you used 2600 psi then you used 67 cuft. (2600*.0258) Divide cuft used by run time. 67/46.4 minutes = 1.44 cuft/ min.
- Now just adjust for depth just like calculating your SAC. 1.44/1.6 ATAs = 0.9 RMV.
- Tank Factors Heres a quick way to deal with tank volumes at rated pressures for calculating your RMV. Theres really only a hand full of tanks out there that you would dive as primary tanks. Tank factors are just close estimates that make calculating this stuff in your head way easier. I have tank factors for the tanks I use memorized and the rest written in my wet notes.
- AL 80s have a tank factor of 2.5. So to figure how many cubic feet you used on a dive using tank factors you take your psi used, in this case 2600 psi and divide by 100. 2600/100 = 26 and now multiply that by your tank factor. 26*2.5= 65. Not quite as accurate as the method above but certainly acceptable for any gas planning you should be doing. Double 80s have a tank factor of 5.
- The tanks that I was diving with yesterday were double HP 100s. A single HP100 has a tank factor of 3 and doubles have a tank factor of 6. My start pressure was 1900 and my end pressure was 1300. I used 600 psi. 600/100 = 6. 6*6 = 36. I used 36 cuft on that dive. My SAC was 600psi/46.4minutes/1.6ATA = 8.1psi per min SAC. My RMV was 36cuft/46.4 minutes/1.6ATA = 0.485 RMV
Hopefully that all makes sense.
NWGratefulDiver on the board also has a great article. Much better than mine! Can't seem to find his web site right now. I'll look for it and edit if I find it. Maybe he'll chime in.
Hunter