I though this message chain from a fellow diver on one of my message boards might be interesting to our dive team… as it raises an interesting point that some people might find surprising….
The messages are in first to last order.
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Hi Mike
Did I seen you on the Loughgarry today?
Chris
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Hi Chris,
Yep I was on the wreck on Saturday morning... vis was poor though... I obviously missed you!!
Where were you hiding out?
Mike.
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There was a rib with 2 guys on it in DUI suits. One was me.
I didn't dive the Loughgarry, I have been diving on the NW wall earlier in the morning. Beautiful dive, well worth it if you are up again.
The Loughgarry was eventful though, one of our divers surfaced, shouted "I've missed stops and I need more air" then goes under. We had to lower a CCR bailout bottle full of 80% and then wet notes with "MOD 9m" in big letters. I only let out 6m of line just in case. He surfaced and asks "What does MOD 9 minutes mean?" OMG!!!
Well done with Cave 1. Do you not have any photos?
Chris
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That got me thinking – does everyone know what MOD is?
Maximum Operating Depth of “x” meters.
Simply put this is the absolute maximum depth you should breath the gas contained in the cylinder. It has nothing to do with time!
The MOD is normally down to the interplay between the % of Oxygen present in the gas and its partial pressure at depth.
For a deco gas the “resting” Partial Pressure of Oxygen (PPO2) should never exceed 1.6, a “working” PPO2 of 1.4 should never be exceeded.
Breathing a decompression or any diving gas for that matter, deeper than it’s stated MOD “X” will expose the diver to significant risk of Oxygen Toxicity which unfortunately results in either a convulsion or a possible black out. In either case unless you are very lucky to have a highly attendant dive buddy it could result in a “Death by Drowning” certificate from your local coroner.
So how does one work out the MOD of a gas mix?
You need to know two things:
- The % of Oxygen in the gas mix. Always make sure to use a good quality O2 analyser and ensure that you have analysed the gas yourself – no short cuts! Do not take anyone else's “word” for what's in a cylinder you are going to dive.
- Your target PPO2 – accepted maximums are: Deco = 1.6 (resting) / Working = 1.4
You now divide the intended PPO2 by the decimal percentage of the analysed gas mix.
This number is in Atmospheres Absolute (ATA) which includes the one atmosphere at sea level. To convert this to a maximum depth you must take away the 1 atmosphere for sea level and multiply the remaining number by 10 in order to convert it to meters depth rather than pressure. (remember each 10 meters of depth = 1 atmosphere)
This is best demonstrated by a simple working up of an example, the variables in this example are as follows:
- Desired maximum working PPO2 of 1.4
- Analysed gas mix with confirmed 32% O2
- What is this mixes MOD?
Answer = 32% Nitrox mix has a MOD of 33meters (I am rounding back the .7 for conservatism)
So if you see any cylinder with a MOD “x” sticker this means that it is not to be breathed below this depth.
Finally divers may notice that some cylinders are labelled with just a depth… for example my 50% O2 deco mix just has a “21” sticker on it on both sides. There is no “MOD” prefix.
This means the same thing… therefore any cylinder that just carries a depth number is to be treated exactly as an MOD and that number is the maximum operating depth for that cylinder – do not breath off of it at a deeper depth.
It is industry recognised that you should never exceed the following PPO2 levels:
- Deco (resting) PPO2 of 1.6 maximum
- Diving / working PPO2 1.4 maximum
- Many people recommend using a PPO2 of 1.2 or 1.3 for Working dive gases as an extra safety precaution.
Exceeding these PPO2 guidelines leaves a diver in a high risk category for a possible Oxygen Toxicity convulsion.
** Never dive a breathing gas without appropriate training **
