Air Consumption

Learning Objectives: Correctly calculate the surface air consumption rate in either psi/min. or ft 3 /min. given: depth, time at depth, and amount of air used. Correctly calculate the air consumption rate for any depth, given the consumption rate for any other depth. Correctly calculate air consumption when the size of the cylinder used is changed.

Main Points Calculating gas consumption rates at the surface and at depth. Converting surface consumption rates (SCR) to depth consumption rates (DCR). Converting depth consumption rates (DCR) to surface consumption rates (SCR). Converting gas consumption rates from psi/minute to cu. ft./minute Converting gas consumption rates from cu. ft./minutes to psi/minute

Calculating Gas Consumption Rate at Depth (DCR) Calculate gas consumption rate at depth using the following formula –Gas used/ time Example: A diver at 66' in the ocean breathes 750 psi in 10 minutes. What is the divers air consumption rate at depth (DCR)? Answer: 750 psi / 10 minutes = 75 psi per 66' S.W.

Converting DCR to Surface Gas Consumption Rate (SCR) Convert a DCR to SCR using the following Formula: DCR / Pressure atm Remember the formula for converting depth to pressure is Seawater Patm = (depth + 33) / 33 Freshwater Patm = (depth + 34) / 34

Example 1 If a diver has a DCR of 75 psi/minute at a depth of 66 fsw, what is that diver’s SCR? Answer: DCR / Patm 75 psi per minute /[( )/33)] 75 psi per minute / 3 atm 25 psi per The Surface

Converting SCR to DCR When converting a SCR to a DCR use the following formula: DCR = SCR x Pressure

Example A diver has a known SAC of 25 psi per the surface. What is that diver's 78 fsw? Answer: DCR = 25 psi per min. x [(78+33)/33] DCR = 25 psi per min. x 111 / 33 DCR = 25 psi per min. x 3.4 DCR = 85 psi per 78 fsw.

Converting consumption Rates from psi/minute to cu ft/minute Example: A diver has a known SCR of 25 psi/minute using an Al. 80. What is the diver's SCR in cubic feet per minute? Answer: Determine ft3 / psi 80 ft3 / 3000 psi =.027 ft3 / psi SCR psi/min x ft3 / psi (25 psi/minute) x (.027 ft3 / psi ) =.675 ft3 / minute

Check your answer 3000 psi / 25 psi min = 120 minutes 80 ft3 /.675 ft3/minute = minutes

Converting from psi/min to cu ft/min and from SCR to DCR A diver, using an Al. 80 filled to 3000 psi. has a known SCR of 20 psi/minute. In ft3/min, What is the diver’s breathing 57 fsw?

Converting from psi/min to cu ft/min and from SCR to DCR continued Step 1 Convert the SCR from psi/min to ft3 /min 80 ft3/3000 psi =.027 ft3/psi.027 ft3/psi x 20 psi/min =.54 ft3 / min Step 2 Convert SCR to DCR.54 ft3/min. x[( )/33].54 ft3/min. x 2.7 DCR = feet SW

Converting from ft3/minute to psi/minute When converting from ft3/min to psi/min first determine how many psi/ft3 Formula Psi when full / ft3 when full 3000 psi/63 = psi/ft3

Converting from ft3/minute to psi/minute A diver using an AL 63 filled to 3000psi has a known SCR of.55 ft3/min. Using the same cylinder, what is the diver’s SCR in psi/min. Answer: Determine psi/ft psi / 63ft3 = psi/ft3 Multiply known SCR x psi/cu ft SCR =.55 ft3/min x 47.6 psi/ft3 = psi/min

Converting from ft3 /min to psi / min Converting DCR to SCR A diver using an AL 63 filled to 3000psi has a known DCR of.8 ft3/min at 27ffw. Using the same cylinder, what is the diver’s SCR in psi/min.

Step 1 Convert from ft3 / min to psi / min 3000 psi / 63 ft3 = psi / ft3.8 ft3 / min x psi / ft3 = 38.1 psi / 27ffw Step 2 Convert DCR to SCR 38.1 psi / min / (27ffw + 34ffw)/34ffw 38.1 psi / min / 1.79 atm = psi / min SCR

Converting air consumption rates and tank sizes Convert Consumption Rate to to ft3/min Determine ft3/psi for new cylinder Determine psi/min for new tank size Example A diver has a known SCR of 25 psi/min using AL 80. What is the SCR using an Al 63? Answer 80 ft3 / 3000 psi =.027 ft3 /psi x 25 psi/minute =.675 ft3/min Psi/FT3 for AL 63 = 3000 psi/63 ft3 = 47.6 psi/ft psi / ft3 x.675 ft3 / minute = 32 psi/minute

Have We Covered Calculating gas consumption rates at the surface and at depth. Converting surface consumption rates (SCR) to depth consumption rates (DCR). Converting depth consumption rates (DCR) to surface consumption rates (SCR). Converting gas consumption rates from psi/minute to cu. ft./minute Converting gas consumption rates from cu. ft./minutes to psi/minute

Can You Correctly calculate the surface air consumption rate in either psi/min. or ft 3 /min. given: depth, time at depth, and amount of air used? Correctly calculate the air consumption rate for any depth, given the consumption rate for any other depth? Correctly calculate air consumption when the size of the cylinder used is changed?

Last Thoughts Most divers have a good intuitive understanding/guesstimate of how much bottom time they can get based on cylinder size, depth and exertion level. As you get into advanced diving activities it is important to have a more accurate understanding of your gas consumption rates and gas supply requirements. Knowing our gas consumption rates in cu. ft./minute allows you to select the appropriate size cylinder for the task.