I did a dive yesterday with a local group here and this was a long drift dive with strong current. The DM heading this dive wanted me to switch my tank 80cu for one of his 117cu steel. So my question is how do I calculate the air I would of taken if I'd used my 80cu for this dive?
Started at 3400 and finished at 1100.
I'm pretty sure I would of drain it but I'm still curious.
Thanks,
As others have pointed out, the capacity of the tank and the model number can be a little misleading. For example, a Worthington HP119 actually holds 123 cu.ft. of gas at 3442 PSI, a HP100 holds 99.5 cu.ft. at 3442 and a Catalina AL80 holds 77.4 cu.ft. at 3000 PSI.
There is a little information missing from your post. The tank was rated at 117 cu.ft. but at what pressure? If it was 117 @ 3500 PSI then each PSI is 117 / 3500 or 0.33 cu.ft / PSI. So at 3400 PSI it might not have been full. So you really had ~113 cu.ft. On the other hand, if the tank working pressure is 3442 PSI you had 115 cu.ft. to start. Let's say the tank working pressure is 3442 (worst case scenario). In this case you started with 115 cu.ft. of air and ended with ~37 cu.ft. of air. Total air used was 115 - 37 or 77.6 cu.ft. of air. Since an AL80 holds 77.4 cu.ft. of air, you would probably be doing a CESA or hoping on your buddy's octo if you went with an AL80.
Were you diving the drift off the Lillie? It is a fun dive. I usually use my HP119 for that dive.
If you want to calculate how much air you would need for any given dive you'll need to do a little work. Take a dive to 33 feet. Record your tank pressure and do a normal dive for say 10 minutes. Note your tank pressure after 10 minutes. If you stay at 33 feet you are at 2 ATA. Every breath you take at 2 ATA is twice the volume you take at the surface. If you used 15 cu.ft. in 10 minutes then you used 1.5 cu.ft. / minute at 2 ATA. Your surface Air Consumption rate (SAC rate) would be 0.75 cu.ft. / minute. At 33 feet you would use 1.5 cu.ft. per minute, at 66 feet you would use 2.25 cu.ft. / minute, at 80 feet you would use (80 / 33) + 1 * 0.75 cu.ft. / minute. General formula is ((depth / 33) + 1) * SAC_rate.
Lillie drift is around 80 feet. If my SAC rate is 0.75 then I'm going to be using 2.57 cu.ft. / minute. A 30 minute dive will require approximate 80 cu.ft. (2.57 * 30 = 77.1 cu.ft.). I want a safety margin of one third. So two thirds for the dive and one third for safety. This means multiple the air needed for the dive by 1.5. If I need 80 cu.ft. for the dive then I want to take 80 * 1.5 or 120 cu.ft.
Lowering your SAC rate means you can carry less air *BUT* you want to carry enough air for you *OR* your buddy. If your buddy is an air hog, you need to carry more air because you are his backup.
