Volume of Air in the Bottle
AIR WE TRANSPORT
It depends on two factors:
- Bottle capacity.
- Pressure at which the bottle can be loaded (working pressure).
These two factors are part of the data set that come engraved on the bottle and are specified for its construction.
The value indicating the internal capacity is the volume of air it contains when it is “empty”, that is, at atmospheric pressure (1bar). This value is expressed in liters .
The value indicating the working pressure refers to the maximum pressure at which the bottle can be loaded into the filling. This value comes normally expressed in bar , but in some bottles comes expressed in psi (pound/square inch).
Therefore, the volume of air transported is expressed:
MAXIMUM VOLUME OF STORED AIR = INTERNAL CAPACITY X WORKING PRESSURE
If we have a 12L bottle with a working pressure of 220bar, the volume of air contained in it at the end of the filling will be:
12L x 220bar = 2640L
It should be not forgotten that the pressure after some time will decrease , because the temperature of the bottle decreases after filling . This decrease in pressure can be verified by simply reading the pressure gauge by mounting the regulator on the bottle before diving. Under these conditions the air available in the bottle will be indicated by the expression:
AVAILABLE AIR VOLUME = INTERNAL CAPACITY X PRESSURE REFERRED TO ON THE PRESSURE GAUGE
If we have a 12L bottle and assuming that the pressure value read on the pressure gauge before the dive is 195bar, the available air volume will be:
12L x 195bar = 2340L
AIR DURATION WE TRANSPORT
The duration of the air we transport depends on the total volume of air in the bottle and the consumption of liters per minute at a certain depth. Talking about the duration of the air we transport is the same as talking about autonomy , which we will address in Module 15 – Basic Diving Planning.
DURATION = VOLUME OF AIR TRANSPORTED ÷ AIR CONSUMPTION TO THE DEPTH WE FIND OURSELVES
With an internal capacity of 12L, working pressure of 220 bar and depth to -27.5m we have a duration of:
Volume transported = 12L x 220bar = 2640L
Consumption at -27.5m = 25L/min x 3.75bar = 93.75L/min
Therefore, Duration = 2640L ÷ 93.75L/min = 28.16 min