Calculating Maximum Operating Depth (MOD)
Maximum Operating Depth (MOD) is the maximum depth at which a given gas mixture can be safely breathed while staying within the recommended partial pressure of oxygen (PO2) limits. Exceeding the MOD increase the risk of oxygen toxicity, which can be life-threatening.
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Basics of Using NitroxHelp calculating your MOD?
Enter FO2 and PO2 and we’ll calculate MOD and EAD down to 130 ft.
Interactive MOD CalculatorUnderstanding Partial Pressure and Dalton’s Triangle
Dalton’s Law states that the total pressure of a gas mixture is the sum of the partial pressures of its component gases. This relationship is often visualized using Dalton’s Triangle, a simple tool used to solve gas-related diving calculations. The three variables in Dalton’s Triangle are:
- Fraction of Oxygen (FO2) – The percentage of oxygen in the gas mixture.
- Ambient Pressure (ATA) – The absolute pressure at a given depth.
- Partial Pressure of Oxygen (PO2) – The pressure exerted by oxygen in the mixture.
Need help calculating your MOD?
Enter FO2 and PO2 and we’ll calculate MOD and EAD down to 130 ft.
Interactive MOD calculatorTo calculate any of these values:
- \( PO_2 = FO_2 \times ATA \)
- \( FO_2 = \frac{PO_2}{ATA} \)
- \( ATA = \frac{PO_2}{FO_2} \)
MOD Formula
To determine MOD, rearrange the P equation:
\[ MOD (fsw) = \left( \frac{PO_2}{FO_2} - 1 \right) \times 33 \]
Where:
- PO2 is the chosen maximum allowable PO2 (commonly 1.4 ATA for recreational diving, 1.6 ATA for contingency).
- FO2 is the fraction of oxygen in the gas (e.g., 0.32 for EAN32).
- 33 fsw is the conversion factor for Pressure in saltwater (1 ATA = 33 fsw).
Note: In freshwater, the conversion factor is 34 fsw per ATA, which results in slightly deeper MOD values. However, the difference is negligible for recreational diving so just stick with 33 for both saltwater and freshwater.
Example Calculation
For EAN32 (32% oxygen) with a PO2 limit of 1.4 ATA:
\[ MOD = \left( \frac{1.4}{0.32} - 1 \right) \times 33 \]
\[ MOD = (4.375 - 1) \times 33 \]
\[ MOD = 3.375 \times 33 \]
\[ MOD = 111 \text{ fsw} \]
Common MODs for Popular Nitrox Mixes
| Gas Mix | MOD (PO2 = 1.4 ATA) | MOD (PO2 = 1.6 ATA) |
|---|---|---|
| 21% (Air) | 185 ft | 215 ft |
| 32% | 110 ft | 130 ft |
| 36% | 95 ft | 110 ft |
Contingency Planning
- If you exceed MOD: Immediately ascend to a shallower depth to reduce PO2 exposure.
- Monitor for symptoms of oxygen toxicity: Early signs include twitching, dizziness, nausea, or vision disturbances. Seizures can occur in severe cases.
- Avoid exceeding planned MOD in strong currents or variable conditions. Plan depth conservatively.
Real-World Applications
- Dive Computers & MOD: Most dive computers track PO2 and will alarm if MOD is exceeded. However, divers should always be able to calculate MOD manually for proper gas planning.
- MOD in Dive Planning Software: When planning nitrox dives, software like MultiDeco or Subsurface will calculate MOD automatically, but understanding the math ensures safer and more informed diving.
A Practical Takeaway
If you're new to nitrox, take a moment to work through these calculations yourself. It’s easy to rely on a dive computer, but knowing the numbers gives you real control over your safety. Personally, I prefer EAN32 with a PO2 limit of 1.4 ATA, as it provides a good balance of depth and extended bottom time while staying within safe oxygen limits.
Understanding MOD and Dalton’s Triangle is essential for nitrox divers and anyone working with mixed gases. Proper gas planning ensures a safer and more controlled dive experience.