Air consumption


You are planning to scuba dive more actively. The few advices will help you to take some decisions.

Take the time to read the following. Think about it. You have your own free will. At the end, you will decide.

It seems that my cylinder goes empty so fast what should I do?

Measure your air consumption!

The best way to proceed is to make tests in a controlled environment. First, be sure to be in shape, rested and relativly calm without any stress. If you can, make the tests in a swimming pool, its ideal. Otherwise choose a lake with a bottom of 15’ depth. Gear up with all your usual equipement. If it is hot outside, relax, breathe, hydrate yourself and dive into water if necessary. It is essential to keep the same equipment during the whole test. If you can, rent a dive computer which can measures the cylinder pressure or otherwise try to rent a digital pressure gauge or one with several subdivisions marks. Don’t forget to have a pencil and a slate to carry underwater to take notes. Have time clock.

To start, descend to 10’ deep. While at that depth, maintain your buoyancy. Note the initial pressure. While staying at that depth, breathe normally without forcing your respiration in any way. Don’t try to conserve your air. Breathe simply without moving for 10 minutes. Take note of the pressure at the end.

Next, you have to swim. Always at the same depth. Note the initial pressure. You will have to swim in order to move like during a dive. Avoid to swim in breathless way. Then swim for a 10 minutes period. At the end note the final pressure.

How to calculate the air consumption?

First let’s say you have for example the following data:

Cylinder volume: 80 cubic feet

Cylinder volume: 3000 psi

Initial pressure (rest test): 2600 psi

Final pressure (rest test): 2100 psi

Depth (rest test): 10 feet

Initial pressure (swim test): 2000 psi

Final pressure (swim test): 1200 psi

Depth (swim test): 10 feet

Firts calculate the tank factor

Tank factor= No of cubic feet / Tank fill pressure

Tank factor = 80 / 3000 = 0.027 cu. ft/psi

Calculate the RMV (Respiratory Minute Volume) at rest

Delta pressure at rest = 2600 - 2100 = 500 psi

Volume used = 500 psi ∗ 0.027 pi. cu /psi = 13.3 pi. cu.

RMV = 13.3 pi. cu. / 10 min = 1.33 pi. cu per minute

Calculate the RMV while swimming

Delta pression whilke simming = 2000 - 1200 = 800 psi

Volume used = 800 psi ∗ 0.027 pi. cu /psi = 21.3 pi. cu.

RMV = 21.3 pi. cu. / 10 min = 2.13 pi. cu per minute

How to utilize these data to evaluate the air volume necessary?

Plan your dive!

Let’s say you are planning the following dive: 20 minutes at 30 feet deep followed by a 10 minutes at 60 feet. After, you will ascend to surface.

We use the RVM at swim to do the calculations. Let’s calculate the absolute pressure at 30 feet, we obtain Pabs = (30/33) plus 1 atm = 1.9 atm and at 60 feet we obtain Pabs= (60/33)plus 1 atm= 2.81 atm.

Calculate the volumes needed, we obtain Volume total = (Pabs1 ∗ Duration1) plus (Pabs2 ∗ Duration2) = (1.9 ∗ 20) plus (2.81 ∗10)= 39.9 plus 28.1 = 68.0 cubic feet. Now add the volume of air needed to respect th third rule, we get Vol total = 68 ∗ 3/2 = 102 cubic feet. A 102 or a 112 cubic feet cylinder should do the job.

Note: please consult a dive manual or your dive instructor in order to make more precise calculations for greater safety. Other factors might be involved like your dive equipment (thermal protection), your buoyancy (more or less lead), the water temperature, the water current, your physical condition. All these factors must be taken into account while preparing your dive plan. If you can, repeat the measurements of your RMV at rest and at swim. Also take note that dive practice will increase your RMV.

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