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Air Calculations

This article will show you how to calculate you air requirements for your dive.


Introduction

An important part of any dive plan is to ensure you have enough air to get you back safely, with a little to spare, just in case.

Average air consumption

The average divers consumes about 24 or 25 litres of air a minute whilst on the surface. The theory behind this is that the average adult lung size is about 6 litres, but when relaxed we only breathe about 25% of this i.e., 1.5 litres. If we breathe about 16 times a minute that equates to (16 x 1.5) 24 litres a minute. This is referred to as the SAC or Surface Air Consumption. In reality this "average" varies enormously, due to the following factors:

  • Smaller people have smaller lungs.
  • Women seem to breath less anyway.
  • Body Mass Index (BMI), the higher it is the more O2 is required.
  • General state of health, the healthier you are the less air you use.
  • Anxiety/relaxed - when stressed air consumption can triple.
  • How much work is being undertaken - Drifting with the current uses the least.

It is important for each diver to know their average air consumption, assuming a typical finning action. More on this later.

Air Consumption at Depth

Due to Boyle's law, as you go deeper, the air you breathe is more compressed so you consume more of it. At 10 meters, air pressure will have doubled and hence so will your air consumption. at 20 meters it will have tripled and so on. If a diver breathes 25 litres a minute on the surface they will be breathing 50 litres a minute at 10 meters and 75 litres a minute at 20 meters.

Whilst underwater additional factors can affect your air consumption:

  • Experienced divers generally breathe less than novice divers.
  • Some divers skip breathe, that is they deliberately pause the breathing after inspiration.
  • Underwater work such as swimming against the current.
  • Cold will increase it.
  • Other factors, mask clearing, pumping and dumping jackets and suits.
  • Air leaks

Calculating SAC

Some dive computers can provide this information when data is downloaded to a PC. If you are nos so fortunate you will have to work it out. A simple method is to undertake a few dives with a square profile (i.e., a single descent to a steady depth and a single ascent) and note the air consumed during each dive. Then calculate the SAC rate using the following process:

Example: A diver with a 10 litre cylinder dives to 20M for 20 minutes and uses 120 BAR.

  1. Calculate the litres of air used, during the dive by multiplying the air consumed in BAR by the size of the cylinder:

    120 BAR consumed x 10 = 1200 litres used during the dive.
  2. Get the average litres per minute by dividing the litres used by the time in minutes:

    1200 / 20 = 60 litres a minute
  3. Compensate for depth - by dividing the average litres per minute by the depth multiplier (3, see right hand column in table below).

    60 / 3 = 20 litres per minute.

    This dive indicates an equivalent SAC rate of 20 litres per minute.
Depth (M)
Convert to
SAC Rate
Convert From SAC Rate
0
1
1
3
0.76
1.3
6
0.62
1.6
9
0.52
1.9
10
0.5
2
12
0.45
2.2
15
0.40
2.5
20
0.33
3
25
0.29
3.5
30
0.25
4
35
0.22
4.5
40
0.2
5

You would need to do this for a few dives and average out the results.

Using your SAC

Once you know your typical SAC rate you can easily calculate:

  1. Whether you have enough air for a dive
  2. How long you can stay on the bottom
  3. Which size cylinder may be the most optimum.

Example: A diver with a 12 litre cylinder charged to 232 BAR wishes to undertake a dive to 30m for 25 minutes and has a SAC of 25 litres per minute.

  1. Work out how much air is available - A 12 litre cylinder pressurised to 232 BAR would give 2784 litres
  2. At 30m (4 BAR) the air consumption would be 4 (BAR) x 25 (SAC) = 100 litres per minute.
  3. To do a 25 minute dive take 25 x 100 = 2,500 litres

Theoretically the diver could do this, but there is very little air in reserve for the unexpected.

Including a Reserve

As is the common practice, a diver may want to keep a reserve of 50 BAR. In this case it is best to extract that 50 BAR from the cylinder pressure figures, before any calculations are made.

Example: A diver with a 12 litre cylinder charged to 220 BAR wants to dive to 25m for as long as possible, and surface with at least a 50 BAR reserve. He has a SAC rate of 25 litres per minute.

  1. Work out how much air is available - Remove and ignore 50 BAR from the cylinder calculations. i.e., 220 (BAR) - 50 = 170 BAR. In a 12 litre cylinder this would give 2,040 litres.
  2. At 25m (3.5 BAR) his breathing rate will be 3.5 (BAR) x 25 (SAC) = 87.5 litres per minute
  3. How many 87.5 (litres per minute) will go into 2040 (his available air) = 23 Minutes

So excluding ascent and descent and stops the diver could dive for up to 23 minutes.

For multilevel dives each level should be calculated separately. Don't forget to include air for ascents, descents and decompression stops.

Use this calculator to identify what air you will need:

Surface Air Consumption (10 to 50 litres per minute)
Depth (0 - 100 Metres)
Duration 1 to 120 Minutes
litres required

 

Use this calculator to identify how much air you have available
Reserve Required (0 to 100 BAR)
Cylinder Size (0.40 - 18 litres)
Cylinder Pressure (1 to 350 BAR)
Available litres

 

The Effects of Temperature

Normally, when a cylinder is filled the gas inside will warm up, this in turn will increase the pressure in the cylinder, the reverse happens when you first drop into the water, due to the drop in temperature, you loose a few BAR on you gauge.

As a general rule, for every change in temperature of one degree Celsius, the pressure in a scuba tank changes about 0.62 of a BAR. For greater accuracy see Charles' Law.

 

 

 

 

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Page last updated on October 16, 2007
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