Breathing underwater

Most divers ask “how long will my air last?” There are several variables to this but primary factors are how much air you start with, the depth you intend diving to, and your rate of consumption.

Body size is important, and activity underwater and stress levels are also factors. Fitness is not necessarily an huge factor. An unfit overweight diver that moves slowly in a relaxed manner will consume less air than an elite athlete with a high stress level finning inefficiently.

A 12 litre cylinder filled to 200 bar will have 2400 litres of air. If your breathing rate is 20 litres a minute on the surface you could use the cylinder for 120 minutes. The same cylinder at 30 metres will only last 30 minutes.

If you decide to purchase your own cylinder, it’s critical to maintain it

The technical jargon to work out your predicted air consumption per minute is as follows:

The volume of the tank is divided by the breathing rate multiplied by the absolute pressure of the depth at which it is breathed.

Afterwards, you can calculate your realised or actual breathing rate for a particular dive. Take the amount of air you consumed on the dive in bar, and work out how many litres you used using the above information. You can work out how many litres of air you used per minute by dividing the number of litres by the dive time. You can track your air consumption, and tie it to a variety of factors – how you felt on the day, your weighting, how your gear was set up, and of course depth – if you keep proper records in your logbook for each dive.

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Scuba diver, teacher, gadget man, racing driver, boat skipper, photographer, and collector of stray animals

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