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How to calculate how long your air supply will last while scuba diving
Greg - 6/26/2013 9:50 AM
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Category: Educational
Comments: 1
How to calculate how long your air supply will last while scuba divingTo determine how long a scuba tank should last while scuba diving, follow the steps below:

1. Determine your SAC Rate with the easy-to-use calculator on DiveBuddy. The SAC Rate is the PSI/min used.

Example: If I dive to 33 feet for 60 minutes and use 2500 PSI of an 80cu/ft tank that originally had 3000 SAC Rate would be 20.8 PSI per minute. So at the surface, 2500 PSI would last me about 120 minutes (ie: 2500 / 20.8).

2. Determine your air consumption rate at your planned depth.

Example: If I know my SAC Rate is 20.8 and I plan to dive to 66 feet (ie: 3 atmospheres of pressure), I would multiply 20.8 by 3 to get an air consumption rate of 62.4 PSI/min at 66 feet.

3. Determine how long your scuba tank will last at your planned depth.

Example: If I dive with a scuba tank that holds 3000 PSI, and I want to leave 500 PSI in the tank when I’m done...that leaves 2500 PSI of usable air. Divide 2500 by 62.4 and you get 40 mins.

So...given my SAC Rate, my planned depth, my available tank air...I should be able to dive to 66 feet for about 40 mins.

Summary (given sample data above):

Tank air time at surface: 120 mins

Tank air time at 33 feet: 60 mins

Tank air time at 66 feet: 40 mins


EskimoBluDay - 7/04/2013 3:37 PM
Some confusion between SAC (Surface Air Consumption) and SRMV (Surface Respiratory Minute Volume). There is a significant difference. SAC is air consumption rate at the surface, at rest. SRMV is a computed value and it varies with a number of factors (work load, stress, temperature, current, etc.) and thus is different from dive to dive. Better ways to calculate either allowable dive length or air required for a particular dive are:

First find out how much air you use in 20-30 minutes while sitting on the couch watching TV and calculate your SAC. Then you can use the following.

(Target Depth / 33) + 1 = Depth Factor

SAC x Depth Factor x Dive Factor = RMV

RMV x Dive Length = Air Required

Useable Air / RMV = Allowable Dive Length

RMV = Respiratory Minute Volume

Dive Factor is an estimate dependant on temp,current, work rate, etc. and varies with each dive. For a normal dive I use 1.5. Higher if it’s cold, in current, with a high work load, stressful,etc.

Some other formulae that are useful:

(Tank) Rated Vol / Working Pressure = cf/psi [for an Al-80 - 77.4cf / 3000psi = 0.0258 cf/psi]

cf/psi x psi used = cf used [breathing from 3000psi to 500psi on an Al80 - 2500 0..258 = 64.5 cf]

cf used / dive length = Raw SRMV

raw SRMV / dep or ave atm = deep or ave SRMV (atm = (depth/33) + 1)

SRMV – Surface Respiratory Minute Volume