Methods of measuring basic gas consumption for recreational divers (1) SAC Rate.
Note: For this discussion I am assuming that were diving in the (average) ocean, and not in the eye of a hurricane or under the worlds tallest high pressure system, so when I say the Surface Im assuming sea level on an average day, where 1 atmosphere is about 33 feet of sea water or 10 meters of sea water or 1 bar or 14.7 psi or 760mm Hg or 29.92 Hg hopefully this will avoid the picking of some (but Im sure not all) nits.
Gas (Air, or Nitrox or Trimix) consumption can be measured in several useful ways, depending on the type diving you do.
If you always dive the same size tank (an AL 80, for example) then there is no need to make any conversions beyond what you can read on your gas gauge your Submersible Pressure Gauge (SPG) and then correct that for depth. The term we usually use for this type measurement of gas consumption is the Surface Air Consumption Rate, or SAC Rate, and it is expressed in how much pressure gets used per minute at the surface.
Its called the Surface rate because, since gas consumption varies directly with pressure (depth), we need some place to start and (hopefully) we all start on the surface J
Calculating it: The formula is
Gas Consumed
Time X Pressure
Where Gas consumed is in psi or Bar, Time is in minutes, and Pressure is in Atmospheres
(1) You can make a direct calculation of SAC Rate if you like, by simply swimming around on the surface while breathing on SCUBA, and noting how much your pressure gauge drops per minute swim around for 10 minutes and use 300 psi/20 bar, your SAC Rate is 30 psi/min or 2.0 bar/min.
(2) Or you can swim around at a particular depth and then apply a pressure correction. To keep the math easy its a good idea to pick a depth thats a whole atmosphere, for example 33 feet/10 meters, which would be a total pressure of two atmospheres one at the surface plus one more for the water depth. If you swam around at 33 FSW/10 MSW for 10 minutes and used 600 psi/40 bar, then youd have used 60psi/4.0 bar per minute at 2 ATA. Now, since the pressure is double what it is at the surface, each lungful of gas has twice as many molecules as it would at the surface, and to correct our consumption rate to what it would be at the surface we therefore have to divide by 2 and we come up with 30psi/min or 2.0 bar per minute again.
(3) A third way to calculate your SAC rate is to use some of the neat information you can get off your computer, if you have one. Most modern computers will tell you your dive time and your average depth, which you can then use along with the information from your SPG to calculate your SAC rate. Lets say your computer tells you that your dive time was 32 minutes, and that your average depth was 50 feet/15M (thats 2.5 ATA)*. Looking at your SPG, you see that you have 500 psi/35 bar left, and when you started the dive you had 3000 psi/200 bar, so you used 2500 psi/165 bar. Plugging the formula with those numbers we get:
2500psi or 165 Bar
32 min X 2.5 ATA
Which yields a SAC rate of 31 psi/min or 2.1 Bar/min
(4) Yet another way to calculate your SAC rate is to let George do it. Most modern Air Integrated computers have an automatic SAC rate calculation built right into them, and you can read it right off the computer
* Converting depth to pressure in ATA
(1) for feet of sea water: ATA=(depth/33)+1
(2) for meters of sea water: ATA=(depth/10)+1
(for math purists, my extra parentheses in the formulae are for clarity)
Rick
Note: For this discussion I am assuming that were diving in the (average) ocean, and not in the eye of a hurricane or under the worlds tallest high pressure system, so when I say the Surface Im assuming sea level on an average day, where 1 atmosphere is about 33 feet of sea water or 10 meters of sea water or 1 bar or 14.7 psi or 760mm Hg or 29.92 Hg hopefully this will avoid the picking of some (but Im sure not all) nits.
Gas (Air, or Nitrox or Trimix) consumption can be measured in several useful ways, depending on the type diving you do.
If you always dive the same size tank (an AL 80, for example) then there is no need to make any conversions beyond what you can read on your gas gauge your Submersible Pressure Gauge (SPG) and then correct that for depth. The term we usually use for this type measurement of gas consumption is the Surface Air Consumption Rate, or SAC Rate, and it is expressed in how much pressure gets used per minute at the surface.
Its called the Surface rate because, since gas consumption varies directly with pressure (depth), we need some place to start and (hopefully) we all start on the surface J
Calculating it: The formula is
Gas Consumed
Time X Pressure
Where Gas consumed is in psi or Bar, Time is in minutes, and Pressure is in Atmospheres
(1) You can make a direct calculation of SAC Rate if you like, by simply swimming around on the surface while breathing on SCUBA, and noting how much your pressure gauge drops per minute swim around for 10 minutes and use 300 psi/20 bar, your SAC Rate is 30 psi/min or 2.0 bar/min.
(2) Or you can swim around at a particular depth and then apply a pressure correction. To keep the math easy its a good idea to pick a depth thats a whole atmosphere, for example 33 feet/10 meters, which would be a total pressure of two atmospheres one at the surface plus one more for the water depth. If you swam around at 33 FSW/10 MSW for 10 minutes and used 600 psi/40 bar, then youd have used 60psi/4.0 bar per minute at 2 ATA. Now, since the pressure is double what it is at the surface, each lungful of gas has twice as many molecules as it would at the surface, and to correct our consumption rate to what it would be at the surface we therefore have to divide by 2 and we come up with 30psi/min or 2.0 bar per minute again.
(3) A third way to calculate your SAC rate is to use some of the neat information you can get off your computer, if you have one. Most modern computers will tell you your dive time and your average depth, which you can then use along with the information from your SPG to calculate your SAC rate. Lets say your computer tells you that your dive time was 32 minutes, and that your average depth was 50 feet/15M (thats 2.5 ATA)*. Looking at your SPG, you see that you have 500 psi/35 bar left, and when you started the dive you had 3000 psi/200 bar, so you used 2500 psi/165 bar. Plugging the formula with those numbers we get:
2500psi or 165 Bar
32 min X 2.5 ATA
Which yields a SAC rate of 31 psi/min or 2.1 Bar/min
(4) Yet another way to calculate your SAC rate is to let George do it. Most modern Air Integrated computers have an automatic SAC rate calculation built right into them, and you can read it right off the computer
* Converting depth to pressure in ATA
(1) for feet of sea water: ATA=(depth/33)+1
(2) for meters of sea water: ATA=(depth/10)+1
(for math purists, my extra parentheses in the formulae are for clarity)
Rick
