We spend 20 minutes at 30m, how much air do we use?Īscending from the dive, you have two options when it comes to calculating the air used during the ascent. We spend 15 minutes at 10m, how much air do we use? (SAC = 25 lpm) Lets take a look at a couple of examples: We now know exactly how much air we will use for every minute we spend at a certain depth. Respiratory Minute Volume (RMV)Ĥ0m 5 Bar x 25 LPM = 125 LPM Bottom Phase of the Dive If a diver rushes to the surface with full lung capacity at 20m the lungs would expand a further three times normal with the largest pressure change of all happening at the last ten metres when it would double. An ascent to the surface without exhaling would cause the air already in the lungs to expand further, but with minimal capacity to do so. But when breathing compressed gases, as in scuba diving, the mass of gas in the lungs is increased to fill the normal volume. When a diver surfaces or ascends these figures are reversed. At 20m it is one third, at 30m it is 1/4, at 40m it is 1/5. Therefore total lung volume during a breath hold dive at ten meters is half of what it was at the surface. Boyles Lawīoyle's Law states "For a fixed amount of an ideal gas, kept at a fixed temperature, pressure and volume are inversely proportional."įor every ten meters of descent the pressure increases by one atmosphere absolute (ata or atm). We call this the Respiratory Minute Volume (RMV) and it is also measured in litres per minute(LPM). We can use our SAC rate and Boyle’s law to work out how much air we will breathe at any given depth. use previous logged dives to calculate your SAC rate OR if your computer is downloadable to a supplier piece of software, download and use the application, which in most instances will calculate and show you, your SAC rate. If you do not know your SAC rate, then assume a value of 25 litres per minute, but do not rely on this figure. Your SAC rate is how much air you breath during 1 minute while on the surface, usually while doing gently finning. The next figure you need is your Surface Air Consumption (SAC) rate. To calculate the amount of air in a cylinder you multiply your cylinder size (litres) by the cylinder pressure (Bar).Ĭylinder size x Fill pressure = Available Airġ2L twinset x 210 Bar = 5,040 Litres How long will that last me during the dive? Do I have enough reserve air for my buddy?.How long will that last me during the dive?.Its the answer to the following questions. Remember your maximum dive time will vary with cylinder size and dive depth. Do you just go diving and assume you have enough air for the dive or do you take a minute and plan. Air planning is a key factor when it comes to planning a dive.
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