MOD Calculator
meters feet
What is the MOD of a gas?
The MOD of a gas mix, also called Maximum Operating Depth, refers to the depth at which a certain gas mix reaches the safe limit for the partial pressure of oxygen (ppO2). For recreational diving, that safe limit is always set at 1.4 bars; for technical dives, the decompression mix can be breathed at a ppO2 of up to 1.6 bars.
Exceeding the MOD of a gas while breathing said gas exposes the diver to an increased risk of oxygen toxicity (more specifically, exposure to a high ppO2 over a short period of time poses a risk of Central Nervous System oxygen toxicity, or CNS toxicity for short). For this reason, when breathing a specific gas blend, its MOD should never be exceeded, and we even recommend having 1-3 meters as a safety margin above the MOD.
The Maximum Operating Depth of air is 56.7m (or 187 feet). When using a nitrox blend, although that blend allows you to stay at a specific depth for a longer period of time, due to its increased Oxygen content, the MOD will be reduced, meaning you are limited to a shallower depth when using Nitrox.
How to Calculate MOD
Calculating the Maximum Operating Depth of a gas comes down to figuring out at which depth the ppO2 will equal the safe limit (which is usually 1.4 atm in a recreational context). The formula for MOD is:
(Meters) MOD = (ppO2/FO2) × 10 -10
(Feet) MOD = (ppO2/FO2) × 33 - 33
The ppO2 in the formula is usually substituted for 1.4, while the FO2 in the formula represents the oxygen fraction, meaning the percentage of oxygen in the gas mix (for instance for air, the FO2 would be 0.21).
Below is a table showing the MOD for frequently used gas blends, for both a maximum ppO2 of 1.4 and 1.6.
| Mix | 1.4 ppO2 | 1.6 ppO2 |
|---|---|---|
| 21% | 56m | 66m |
| 25% | 46m | 54m |
| 28% | 40m | 47m |
| 32% | 33m | 40m |
| 36% | 28m | 34m |
| 40% | 25m | 30m |
| 50% | 18m | 22m |
| 60% | 13m | 16m |
| 70% | 10m | 12m |
| 80% | 7m | 10m |
| 90% | 5m | 7m |
| 100% | 4m | 6m |