Many divers routinely check their surface consumption rate (SCR) in psi, which is a good measure of breathing efficiency. However, if you calculate your SCR in cubic feet per minute (cu. ft./min.), you'll have a functional number that will allow you to estimate how long a given cylinder of air will last you at any depth. Getting Started As with any SCR calculation, you first need to establish a base consumption figure by conducting a test run. Thirty-three feet for 10 minutes is a good choice and makes the calculations a little easier. Watch your submersible pressure gauge, compass, depth gauge and watch or timer. Better yet, use a gas-integrated dive computer and all but the compass will be contained in one instrument. Kick out five minutes and back five minutes at roughly your normal dive pace. This is also a good time to check your navigation skills. Math Class Now you're ready to convert psi to cubic feet. The SCR equation can be expressed as: SCR = (1-P2/P1) x Vf/Aa/Tm Where P1 = pressure in psi at beginning of test dive P2 = pressure in psi at end of test dive Vf = actual volume of air in tank in cu. ft. Aa = pressure in absolute atmospheres Tm = time of dive in minutes Step 1 — Establish your Vf, the volume of air in cubic feet that you started with. This is easy if you know the volume of your tank and it is full (see "Tank Chart"). Keep in mind, most standard aluminum 80 tanks hold 77.4 cu. ft. at 3000 psi. To find the actual volume of a less-than-full tank, divide the actual pressure by the full pressure and multiply by the volume in cubic feet of a full tank. EXAMPLE What is the beginning volume of a standard aluminum 80 at 2800 psi? (2800 / 3000) x 77 = 72 cu. ft. Step 2 — Calculate the cubic feet of air used during your test run. The process is similar to Step 1: Divide the ending pressure by the beginning pressure and multiply your answer by the starting tank volume in cubic feet. Subtract that figure from the starting volume to get the volume of air used during the dive. EXAMPLE How many cubic feet of air did you use if you began your test dive with 2800 psi and finished with 2300 psi? (2300 / 2800) x 72 = .82 x 72 = 59 cu. ft. remaining 72 - 59 = 13 cu. ft. used during the dive Step 3 — Convert to cubic feet per minute for your SCR. EXAMPLE Your test dive was 10 minutes at 33 feet. Since 33 feet is two atmospheres of pressure, divide cubic feet of air used by two, and then divide that by the number of minutes. You end up with your SCR in cubic feet per minute (cu. ft./min.). (13 / 2) / 10 = 6.5 / 10 = .65 cu. ft./min. at the surface So What? The Practical Application Congrats, you passed math class. What do you do with this knowledge? Now that you know your SCR, it is possible to estimate the length of time a tank will last you at a given depth. Step 1 — Divide the volume of any tank by your personal SCR: 77 cu. ft. / .65 cu. ft./min. = 118 minutes Step 2 — Divide by depth in atmospheres: 118 min. / 3 Aa (rounded) = 39 minutes (assuming a 60-foot dive) You now know that on a dive to 60 feet, a full aluminum 80 cylinder would last you about 39 minutes. Since you must allow time for descent, ascent, a safety stop and a reserve of 300 to 500 psi, a bottom time of no more than 30 minutes would be practical for you. Such specific information can help you more accurately plan your dives, decompression stops, and select a dive buddy, if you dive with one. Conversion Chart: SCR to Minutes at Depth   SCR (in cu. ft./min.)   .3 .4 .5 .6 .7 .8 .9 20 159 119 95 79 68 59 53 30 134 100 80 67 57 50 44 40 116 87 69 58 49 43 38 50 102 77 61 51 44 38 34 60 91 68 55 45 39 34 30 70 82 62 49 41 35 31 27 80 75 56 45 37 32 28 25 90 69 52 41 34 29 26 23 100 64 48 38 32 27 24 21 110 59 44 35 29 25 22 19 120 55 41 33 27 23 20 18 130 52 39 31 26 22 19 17 Approximate number of minutes a full, standard aluminum 80 will last. All partial minutes rounded down. Tank Chart Catalina Cylinders Make & Model Materials Cu. Ft. PSI C100 A 100 3300 C60 A 60 3300 C80 A 77.4 3300 S40 A 40 3000 S53 A 53 3000 S63 A 63 3000 S80 A 77.4 3000 Faber Cylinders Make & Model Materials Cu. Ft. PSI #24045 S 45 2400+ #24065 S 65 2400+ #24080 S 78 2400+ #24095 S 95 2400+ #24108 S 108 2400+ #24120 S 120 2400+ #30060 S 60 3000+ #30072 S 72 3000+ #31080 S 80 3180+ #31100 S 100 3180+ #31120 S 120 3180+ Asahi Make & Model Materials Cu. Ft. PSI ST100 S 100 3500 ST120 S 120 3500 ST80 S 80 3500 Luxfer Make & Model Materials Cu. Ft. PSI 050 A 48.4 3000 063 A 63 3000 072 A 72 3000 S040 A 40 3000 S080 A 77.7 3000 S092 A 89.8 3200 S100 A 100 3300 S808 A 80 3000 S80N A 77.4 3300 Pressed Steel Make & Model Materials Cu. Ft. PSI LP-80 S 80 2400 HP-100 S 100 3500 HP-120 S 120 3500 HP-65 S 65 3500 HP-80 S 80 3500 LP-104 S 104 2400+ LP-120 S 120 2400+ LP-95 S 95 2400+ Chart Note: Steel tanks with a + symbol must be filled to the allowed 10 percent overfill to be at full capacity.