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Maximum Operating Depth (MOD) and Scuba Diving


Photo of a scuba diver and a big yellow sponge

Divers who dive deep on air, as well as those who use nitrox, trimix, or pure oxygen, should calculate a maximum operating depth (MOD).

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What Is a Maximum Operating Depth?:

A maximum operating depth (MOD) is a depth limit based on the percentage of oxygen in a diver's breathing gas.

Why Should a Diver Calculate a Maximum Operating Depth?:

Breathing high concentrations of oxygen can cause oxygen toxicity, which is usually fatal when diving. The concentration (or partial pressure) of oxygen in a diver's breathing gas increases with depth. The higher the percentage of oxygen, the shallower the depth at which it becomes toxic. Divers calculate a MOD to be sure that they do not descend beyond the depth at which the oxygen in their tank may become toxic.

In What Situations Must a Diver Calculate a Maximum Operating Depth?:

A diver should calculate a MOD whenever he dives using enriched air nitrox, trimix or pure oxygen. Technical divers who engage in deep air diving must also calculate MODs. A scuba diver who breathes air and who remains within recreational dive limits need not calculate a MOD for his dive. In fact, on most recreational dives the maximum depth will be limited by factors such as the no-decompression limit, narcosis, and the experience level of the diver instead of the MOD.

How to Calculate a Maximum Operating Depth:

1. Determine the percentage of oxygen in the your scuba tank.
If you are diving on air, the percentage of oxygen in your tank is 0.21. If you are using enriched air nitrox or trimix, use an oxygen analyzer to determine the percentage of oxygen in your scuba tank.

2. Choose a maximum partial pressure of oxygen for your dive.
Most scuba training organizations recommend that divers limit the concentration of oxygen (called the partial pressure of oxygen) to 1.4 ata. However, a diver may lower or raise this number depending upon the type of diving and the purpose of the breathing gas. In technical diving, for example, pure oxygen is frequently used at partial pressures higher than 1.4 ata for decompression stops.

3. Calculate your maximum operating depth using this formula:
{(maximum partial pressure of oxygen / percentage of oxygen in tank) - 1} x 33 ft

Example: Calculate the MOD for a diver breathing 32% oxygen who plans to dive to a maximum oxygen partial pressure of 1.4 ata.

Step one: substitute the appropriate numbers into the formula.

{ (1.4 ata / .32 ata) - 1 } x 33 feet

Step two: do the simple arithmetic.

{ 4.38 - 1 } x 33 feet

3.38 x 33 feet

111.5 feet

In this case, round the 0.5 decimal down, not up, to be conservative.

111 feet is the MOD

Cheat Sheet of Maximum Operating Depths for Common Breathing Gasses:

Here are some MOD's for common breathing gasses using a partial pressure of oxygen of 1.4 ata:

Air . . . . . . . . . . . 21% Oxygen . . . . MOD 187 feet
Nitrox 32. . . . . . 32% Oxygen . . . . MOD 111 feet
Nitrox 36. . . . . . 36% Oxygen . . . . MOD 95 feet
Pure Oxygen. . . 100% Oxygen . . . MOD 13 feet

Putting a Maximum Operating Depth into Use:

While understanding how to calculate a MOD is great, a diver must also make sure that he stays above his depth limit during a dive. One good way for a diver to ensure that he does not exceed his MOD is to use a dive computer that can be programmed for nitrox or mixed gases. A computer can be programmed to beep or otherwise notify the diver if he exceeds his MOD or partial pressure limits.

In addition, a diver using enriched air or other mixed gas should also label his tank with the MOD of the gas inside. If the diver accidentally exceeds the MOD written on his tank, his buddy may notice the written MOD and alert him. Writing the MOD on the tank, along with other information about the gas the tank contains, also helps to prevent a diver from mistaking the tank for one filled with air.

Now you can calculate a maximum operating depth for a breathing gas containing any percentage of oxygen. Safe diving!

Hint: To read more diving articles, use the tabs at the top to browse general categories, or go to the about.com scuba homepage and check out the blue list of topics on the left.

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