When diving Enriched Air Nitrox there are two things that you have to be careful about. The first is the maximum operating depth which is linked to CNS, and the second is oxygen exposure (Pulmonary Toxicity).
How to calculate the Maximum Operating Depth
In a previous post about partial pressures I touched upon the maximum operating depth but not how to calculate it. There are two depths that you calculate after you have analysed your cylinder. The maximum operating depth (MOD) and the contingency operating depth (COD). The MOD is calculated at a PO2 1.4 ata / bar and the COD is calculated at PO2 1.6 ata / bar. The COD is always a little deeper than the MOD, this is to give you a little wiggle room so that in case you go below your MOD you should still be OK.
Now to calculate the MOD or the COD we need to know the blend of EANx that we plan to use. For example there are two common blends NOAA1 which is 32% and NOAA2 which is 36%. We can use either of the following formulae depending on whether you are working in feet or metres.
Lets work out the MOD and the COD for NOAA1 which is 32% Nitrox in both feet and metres. If you look at the formulae for the MOD the only difference between the formula for feet and the one for metres is what you multiply by.
So we take the partial pressure of oxygen at the MOD which is PO2 1.4 ata / bar and we divide this by the blend of Nitrox that we are using 32% (as a decimal so it would be 0.32). So we do 1.4 divided by 0.32, then subtract 1 from this and multiply the answer be either 33 or 10 depending if you are using feet or metres respectively.
MOD in Feet = ((1.4 / 0.32) – 1) * 33 = 111.375 feet
MOD in Metres = ((1.4 / 0.32) – 1) * 10 = 33.75 metres
It’s a straight forward calculation that you must always do when you check the fill in your tank. However, most dive computers will tell you the MOD and the COD when you put the blend of Nitrox into them. So you can avoid the really complex maths. lol. Remember to mark the MOD on your tank and the percentage of the Oxygen so that you and/or the other divers can easily see it.
Ok but my computer says it’s less than this. Why?
You might find that when you put the blend into your dive computer it says that the MOD is slightly less than this. For example the SUUNTO brand of dive computers are more conservative and as such calculate their MOD as if you were using 1% more Oxygen in the blend. (Don’t quote me on this, it is from personal experience from using my computer and and comparing the calculations). I would rather have a computer that was conservative and helped keep me safe rather than one that put me in danger. Err on the side of caution is always a good option.
What happens if I go below the MOD or the COD?
You should try not to go below the MOD but you should definitely not go below the COD. The risk of Central Nervous Shockincreases rapidly and you could suffer a convulsion. Please stay about the MOD.
Central Nervous Shock
Going below the COD means that you are entering the realm where oxygen is becoming toxic. There are symptoms that may or may not appear before a CNS convulsion happens these are:
- visual disturbances (including tunnel vision)
- twitching or muscles spasms (particularly in the face)
- euphoria or anxiety
Some of these may occur, you may get none of them. The worry about CNS is that you will probably convulse and spit your regulator out. There is a good chance that you will drown if this happens.
This is caused by prolonged exposure to high partial pressures of oxygen. So the longer that you are exposed to partial pressures of oxygen greater than PO2 1 ata / bar, you risk injuring your lungs. So to avoid this, we must track our exposure. To do this we can either use our computers to track the exposure or we can use tables. My SUUNTO D9tx will actually tell me how much as a percentage I have used of my available oxygen exposure, this is really handy. My Vyper just had a bar that increased with your exposure. Remember read the manual to your dive computer and follow the manufacturers recommendation.
The most important thing to note is that you should not exceed 100% exposure in a 24 hour period. If you do this you risk reaching toxicity. It is recommended that you be conservative and do not exceed 90% wherever possible. If you reach 100% of your allowable exposure, wait 12 hours before your next dive. If you go below the contingency depth treat the exposure as 100% and do not dive for 12 hours.
So it’s really easy to work out your exposure. All you need to know is the maximum partial pressure of the oxygen on the dive and use the DSAT Oxygen Exposure Table. The table (shown below but blanked out because you should buy one) is very easy to use. You take the partial pressure of the oxygen and then go down until you find the closest time and then read across to the right and this gives you the exposure as a percentage.
You make a dive where the partial pressure of oxygen is 1.12 ata / bar for 37 minutes. We need to round the partial pressure up to the nearest which is 1.2 ata / bar. We then go down and find the closest number of minutes, because 37 is not in the table we round up to the next time available which is 42 minutes. Then reading across to the right tells us that the exposure is 20%.
It’s that simple, just remember to keep track of your oxygen exposure between each dive.
Please note the information in the blog is for illustration purposes only. You must get proper training from a qualified diving professional. Only certified Nitrox divers should dive using gas blends with oxygen higher then 21%. Most diving agencies have courses that you can take.