nitrox MOD/TOD

[JRScuba]

SSI Nitrox Course book page 3-11 States: It is important that every Nitrox cylinder be clearly labeled as to its FO2 content and the Maximum Operating Depth ( MOD ) for that FO2. It should be clear who analyzed the cylinder and when they did so. Ideally, the MOD markings should also indicate what limiting PO2 value they are for.

I was also trained to check the FO2 when I picked up the cylinders. This way you know the FO2 you have, instead of finding out at the wrong time you have the wrong FO2 when it is to late.

Some divers I know have their own oxygen analyzer to insure the are getting the right fill.

 

[H2Andy]

Are people being taught to use PPO2 of 1.4 these days when calculating MOD... ?

yes. 1.4 is used for MOD (both PADI and TDI used this in their classes i took)

1.6 is for deco, when you are not excerting yourself

 

[PerroneFord]

My IANTD Adv Nitrox info says:

At the Advanced Nitrox Diver Level:

1.6 bar/ata is used to calculate the maximum operating depth (MOD) of a mix

1.5 bar/ata is the Recommended Limit or Target Operating Depth (TOD)

1.4 bar/ata is used where there is work or cold on a dive

Copyright IANTD/IANTD, Inc, 1996-2002 slide #32

Not sure if there have been updates since 2002 in their materials. I use 1.4 as my MOD on my doubles.

From my experience:

IANTD uses the term TOD for your target operating depth for the dive. Which could be anything up to 1.5 PPO2 and is adjusted for conditions or physical factors. IANTD uses MOD as 1.6 PPO2. (This was from Advanced Nitrox and Technical Diver courses, I have not taken the IANTD basic Nitrox class so at that level it may be different.)

PADI uses MOD at 1.4 PPO2 and Contingency Depth = 1.6 PPO2.

DSAT uses MOD for bottom gas/working phase - 1.4 PPO2

I've found most (if not all) of the dive operations I've used, use only 1.4 PPO2 for recreational nitrox diving.

 

[rawls]

Dave...I dive on NC wrecks mostly, so I can confirm the shop I think you are talking about uses MOD and not the contingency or target plug ins...It's straight 46.2/O2%minus 33...The reason is because, although they get alot of technical divers, they don't set up for deco diving unless there is a trip planned specifically for such. There are ususally a mix of recreational divers and some technical divers. Since the majority of divers on these trips are not nearly as far advanced as you are...for safety purposes, they are going to use the MOD of 121fsw for an EANx30...Hope that helps a little to explain why she said..."it's 121"...Pretty fastidious wasn't she Still hope you enjoyed your dives...

 

[wedivebc]

My IANTD Adv Nitrox info says:



Not sure if there have been updates since 2002 in their materials. I use 1.4 as my MOD on my doubles.

Thanks Perrone, I took my original nitrox training through IANTD and that is probably where I came up with the TOD concept. I wonder why you call it MOD when referring to your doubles though

...for safety purposes, they are going to use the MOD of 121fsw for an EANx30...Hope that helps a little to explain why she said..."it's 121"...Pretty fastidious wasn't she Still hope you enjoyed your dives...

I really enjoyed the dives even though I hardly saw the Papoose because I was too busy looking at all the sharks
We also did the U boat, the Spar and a wreck I don't have the name of, It was a large shallow wreck closer in than the others and maxed out at about 80ft. Water was greener and colder there (more like home).
I would dive with those guys again (hopefully when less people are on the boat)

 

[PerroneFord]

Thanks Perrone, I took my original nitrox training through IANTD and that is probably where I came up with the TOD concept. I wonder why you call it MOD when referring to your doubles though

Well, no one else I know uses TOD, so I just use the common moniker. With deco bottles I would also call it MOD, but I'd be using 1.6 ATA.

 

[Thalassamania]

The snake oil never seems to stop.

MOD is a term coined by NOAA. Everyone else should have enough respect to leave the definition alone (1.6 by the way) and simply say, but we think that it is a significantly less risky if you don't exceed 1.4 (or whatever) and call that Target Depth, Planned Depth, etc.

All this B.S. leaves the incompletly trained diver (and today that's virtually every diver who's reading has not strayed outside of the major agency's pubs) thinking that they will instantly be struck dead if they see an FO2 of 1.41 and confused when talking to tother divers who have bought different snake oil.

 

[jonnythan]

It doesn't really matter what MOD is "supposed" to mean when most divers are taught something different, does it?

 

[H2Andy]

Thas, always so charmingly humble

but i'd suggest reading the NOAA manual before pontificating.

NOAA defined "MOD" as "not to exceed 1.6" (look at section 6.3(B) of your link)

in other words, you can calculate MOD with 1.4 if you like. or 1.2. or whatever.

some people use 1.4, some people use 1.6

thus, the question "what is the MOD of this mixture" is an incomplete question.

the complete question is, "what is the MOD of this mixture at 1.6 ATA?"

 

[Thalassamania]

I'd say that it does matter, especially since divers are not taught the same think (I think that was the basis of this thread).