How much gas does your dive need?
The gas a dive needs is three things multiplied: your surface consumption rate (SCR, also called SAC), the litres you breathe in a minute at the surface; the pressure at your average depth; and the time. Divide by the tank’s litres and you have bar, which is what your gauge reads. This episode does it step by step on one example dive, checks the plan against the dive as it went, and then puts the reserve, minimum gas, on top: the dive plus the reserve must fit the fill.
What is said
Planning a dive: twenty metres, thirty-five minutes. Will your tank cover it, with a reserve left? Work that out before you get in.
The gas a dive needs is three things multiplied: your surface consumption rate, SCR, the litres you breathe in a minute at the surface; the pressure at your average depth; and the time.
Our example: twenty metres average depth, thirty-five minutes, an eleven point one litre tank, a rate of fifteen litres a minute. Step one, the pressure: twenty over ten, plus one. Three atmospheres. Step two: fifteen litres a minute, times three. Forty-five litres a minute at twenty metres.
Step three: times thirty-five minutes. Fifteen hundred and seventy-five litres. Step four, into bar: divide by the tank's litres. A hundred and forty-two bar.
Check it against the dive as it went: a hundred and forty bar used. The plan said a hundred and forty-two bar. It works. Then the reserve, minimum gas: what two divers need to get up from twenty metres, the deepest point of this dive: sixty bar. Together, two hundred and two bar: more than the tank holds. The rule: the dive plus the reserve must fit the fill. Here: fewer minutes, or a bigger tank.
Turn Pressure does this for your dive: the gas, and the minimum gas. Free on the App Store. diver.today.