Equivalent air depth
Encyclopedia
The equivalent air depth (EAD) is a way of approximating the decompression
requirements of breathing gas
mixtures that contain nitrogen
and oxygen
in different proportions to those in air, known as nitrox.
The equivalent air depth, for a given nitrox mix and depth, is the depth of a dive when breathing air that would have the same partial pressure
of nitrogen. So, for example, a gas mix containing 36% oxygen (EAN36) being used at 27 metres (88.6 ft) has an EAD of 20 metres (65.6 ft).
Working the earlier example, for a nitrox mix containing 64% nitrogen (EAN36) being used at 27 metres, the EAD is:
So at 27 metres on this mix, the diver would calculate their decompression requirements as if on air at 20 metres.
Working the earlier example, for a nitrox mix containing 64% nitrogen (EAN36) being used at 90 feet, the EAD is:
So at 90 feet on this mix, the diver would calculate their decompression requirements as if on air at 67 feet.
are suitable for use with these kind of calculations. At 27 metres the Bühlmann 1986 table (0–700 m) allows 20 minutes bottom time without requiring a decompression stop. While at 20 metres the no-stop time is 35 minutes. This shows that using EAN36 for a 27 metre dive can give a 75% increase in bottom time over using air.
Decompression sickness
Decompression sickness describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurization...
requirements of breathing gas
Breathing gas
Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...
mixtures that contain nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in different proportions to those in air, known as nitrox.
The equivalent air depth, for a given nitrox mix and depth, is the depth of a dive when breathing air that would have the same partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of nitrogen. So, for example, a gas mix containing 36% oxygen (EAN36) being used at 27 metres (88.6 ft) has an EAD of 20 metres (65.6 ft).
Calculations in metres
The equivalent air depth can be calculated for depths in metres as follows:- EAD = (Depth + 10) × Fraction of N2 / 0.79 − 10
Working the earlier example, for a nitrox mix containing 64% nitrogen (EAN36) being used at 27 metres, the EAD is:
- EAD = (27 + 10) × 0.64 / 0.79 − 10
- EAD = 37 × 0.81 − 10
- EAD = 30 − 10
- EAD = 20 metres
So at 27 metres on this mix, the diver would calculate their decompression requirements as if on air at 20 metres.
Calculations in feet
The equivalent air depth can be calculated for depths in feet as follows:- EAD = (Depth + 33) × Fraction of N2 / 0.79 − 33
Working the earlier example, for a nitrox mix containing 64% nitrogen (EAN36) being used at 90 feet, the EAD is:
- EAD = (90 + 33) × 0.64 / 0.79 − 33
- EAD = 123 × 0.81 − 33
- EAD = 100 − 33
- EAD = 67 feet
So at 90 feet on this mix, the diver would calculate their decompression requirements as if on air at 67 feet.
Dive tables
Although not all dive tables are recommended for use in this way, the Bühlmann tablesBühlmann tables
The Bühlmann decompression algorithm is a mathematical model of the way in which inert gases enter and leave the body as the ambient pressure changes. It is used to create Bühlmann tables. These are decompression tables which allow divers to plan the depth and duration for dives and show...
are suitable for use with these kind of calculations. At 27 metres the Bühlmann 1986 table (0–700 m) allows 20 minutes bottom time without requiring a decompression stop. While at 20 metres the no-stop time is 35 minutes. This shows that using EAN36 for a 27 metre dive can give a 75% increase in bottom time over using air.