MOD is the acronym for Maximum Operating Depth, You will start hearing about it when you take your EANx (nitrox, enriched air…) course, but the truth is that every single mix, even the air you breathe have a maximum operating Depth. Lets take a closer look
Your lungs are naturally designed and improved to work in the current atmospheric conditions. At one atmosphere you are breathing air that is in between 19 to 21 % oxygen. This amount of oxygen is more than enough for living ( you may not know it but at a cellular level our fuel is oxygen, everything we make is get energy out of a oxidation reaction) make efforts and even running a marathon if you are fit enough.
The way SCUBA diving equipment works is by equalizing the pressure to the exterior, you may (or should) know this already. This have an effect on the air and our lungs and in general in our whole body, the effect itself is not bad or good, but the fast changes of conditions can be, since this changes get reverted inside our body.
The mother of all the mess and the key to understanding this, PO2 is the oxygen partial pressure. In recreational diving and professional diving the richness of individual component gases of breathing gases is expressed by partial pressure. Which is this formula
partial pressure = (total absolute pressure) × (volume fraction of gas component)
Since in the title says understanding, lets simplify.
It is the fraction of the gas in 1 atmosphere that you breath of oxygen (in regular air 0.21 out of 1)
Knowing what the Po2 is; the next step is to know human body limits, we can stand 1.6 Po2. After this, O2 (oxygen as we breathe it) becomes toxic. Even at this 1.6 Po2 the lungs get irritated and depending of the sensibility it can be painful, so everyone agreed to use 1.4 Po2 as a contingency limit for working conditions (deco stops are a different thing) which makes our air breathable up to:
1 atm 0.21
6,7atm 1.40 which in salt water equals to 56.7 meters deep and in sweet water equals to 66.2 meters
MOD is a calculation that doesn’t take in account any nitrogen or oxygen narcosis, which means that even thought oxygen is physically breathable you may not want to go to 56 meters on this kind of mix. Be always a responsible diver and dont go beyond your certification allowed depth. All this systems are thought to make diving safe for everyone. Dead divers don’t make any good to anyone.