A pig flies over a frog in a pond. Illustrative of the mnemonic to remember Dalton's T

If the number of hits this site gets regarding nitrox MOD, EAD, and best mix are any indication, it’s pretty apparent that most of y’all didn’t learn to love the math. The truth is, I didn’t either, but if you take a look at the picture above, you’ll see all you need to remember to determine all your nitrox calculations. At least, by the end of this article, flying pigs will make more sense…

“A pig flies over a frog in a pond.”

The aerial acrobatic maneuver of our porcine friend is the mnemonic that can help you remember Dalton’s T (sometimes called Dalton’s triangle).

A pig flies over a frog in a pond. Illustrative of the mnemonic to remember Dalton's T - this image has labels to help the user understand the nitrox memonic

The “PiG” represents the (P)ressure of the (G)as, usually that’s going to be a number like 1.4.

The FROG represents the FRaction Of the Gas. That would be the percentage of O2 in the gas. For example, Nitrox 50.

The Pond is the atmosphere(s) of pressure. Say 5.5 Atmospheres.

The Dude Abides, but the PIG Divides

Nitrox - A pig flies over a frog in a pond. Illustrative of the mnemonic to remember Dalton's T - this image has labels and symbols that describe the appropriate multiplication or division to determine nitrox meausrements.

Using Dalton’s T

If we know any 2 numbers above, we can determine the third number by multiplying or dividing.

You don’t need to remember specifically the formulas for Best Mix, MOD or EAD because as long as you remember the our friends Pig and Frog, you’ll be able to work out the rest.

Maximum Operating Depth (MOD) for Nitrox:

MOD refers to the maximum depth at which a particular Nitrox mixture can be safely used. The primary concern is avoiding oxygen toxicity, which can lead to seizures and other severe health issues. The MOD is determined by the partial pressure of oxygen (PPO2) in the breathing gas.

To determine the MOD (at sea level) we know the percentage of nitrox we are using, and the partial pressure of oxygen that we do not wish to exceed. A very simple recreational diving example is the MOD of Nitrox 32.

As a recreational diver, you were probably trained that 1.4 is the maximum PPo2 for planning, so now you know the PiG (1.4) and the FROG (.32).

You also know from the spiffy header above that the PiG is always divisive. So any time the PiG figures into your planning, you must divide. The pig divides.

So 1.4 / .32 = 4.375 (the pond)

The pond is atmospheres, right? To get depth from ATA, we subtract one ATA (the atmosphere at the surface).

So 4.375 – 1 = 3.375

To get depth from ATA, you can just move the decimal one place to the right (if that’s easier to remember) or multiply the ATA * 10.

Either way, 3.375 is 33.75 meters.

If you want imperial measurements, multiply 3.375 * 32.81.

If you want to know why Nitrox 32 is so popular, determine the contingency depth for that mix (use a PPo2 of 1.6) and you’ll see why it’s so ubiquitous in rec. diving.

The formula for MOD is: ���=1(���2−0.01)MOD=(PPO2​−0.01)1​

Understanding this equation is crucial for every Nitrox diver to ensure they stay within safe limits and enjoy the benefits of extended dive times.

Nitrox Equivalent Air Depth (EAD):

EAD is a concept used to equate the partial pressure of nitrogen in a Nitrox mixture to that of a standard air mixture at a given depth. This allows divers to use standard air dive tables or dive computers to plan their dives with Nitrox accurately. Some agencies and divers want to keep their EAD at or less than 100 feet.

For students it really would have been easier to call EAD, END (equivalent Nitrogen depth) because that’s what we are really trying to determine.

To determine EAD, again we know the PPo2 (1.4) for the pig, and we know the fraction of the gas. In this case, let’s use Nitrox 28 at a depth of 45 meters.

The TDI Nitrox manual states, “The EAD for any nitrox mix, at any depth, can be manually calculated,
by first creating a ratio where the fraction of nitrogen in nitrox is divided by the faction of nitrogen in air, then multiplying that ratio by the ambient pressure expressed in m / ft, then converting that figure back to depth.”

Does that make your brain hurt? If you’re a visual learner, the answer is probably, “Yes!” So let’s go back to our friends Pig, Frog and Pond.

In this case, the Frog is the fraction of Nitrogen and the depth is the Pond, so we have to convert depth to ATA.

Nitrox 28 has (for our purposes) 72 percent nitrogen (.72).

45 Meters depth is 5.5 ATA.

Since we know Pond (5.5) and most of Frog (.72), we do not know Pig. Since the pig isn’t here to divide, we must multiply (see the graphic above), but first we have to convert back to air (since we are converting equililent AIR depth after all).

So we take the fraction of nitrox (.72) and divide it by the fraction of nitrox in air (.79)

.72/.79 = .92

So the frog becomes .92 and the pond is still 5.5 ATA

.92 * 5.5 = 5.06 ATA or just over 40 meters.

Another example:

Nitrox 40 at 30 meters:

(.60 / .79) * 4 = about 3 ata or about 20 meters

The formula for EAD is: ���=((���2−0.01)×10.79)0.21EAD=0.21((PPO2​−0.01)×0.791​)​

Mastering the calculation of EAD is vital for divers to make informed decisions about their dive profiles and manage their nitrogen exposure effectively.

Best Mix:

Finding the Best Mix involves determining the optimal Nitrox blend for a specific depth. While this is not nearly as commonly calculated today, because we use standard gasses, it is still easy enough to determine.

In this case we want to know the Fraction of the Gas (FROG) where we know our (PiG) maximum PPo2 (1,.4) and the depth in ATA (Pond).

So if depth = 45 meters and PPo2 = 1.4 what’s the best gas? The pig divides….

1.4/5.5 = .254

If I wanted to know the best mix for 57 meters, it would be 1.4/6.7 or about .21…

All of the above is just a visual person’s guide to nitrox calculations. Some of the mixes and depths above could get you killed if you dive them without proper training, and this article is about math, not risk management, so please dive within your training and your physiological limits.

If you want to learn how to do dives outside the recreational limits, you can call, text or email us, or just check out our homepage for additional options.

This is where the art of blending gases intersects with the science of diving. The goal is to balance the benefits of increased oxygen content with the limitations imposed by oxygen toxicity.

Divers can use online calculators or specialized dive computer functions to identify the Best Mix based on planned dive depths. By understanding the factors influencing the Best Mix, divers can enhance their safety and maximize the advantages of Nitrox.


In the world of scuba diving, knowledge is paramount for safety and enjoyment. Nitrox diving, with its potential benefits, adds an extra layer of complexity that demands a solid understanding of the associated mathematics. Learning to love the math behind Nitrox MOD, EAD, and finding the Best Mix is not just about equations; it’s about empowering divers to make informed decisions that contribute to safer and more enjoyable underwater experiences. So, embrace the numbers, sharpen your skills, and explore the depths with confidence!

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