Diving Nitrox with Computers?

[DevonDiver]

One thing I find intersting is that the OP is asking specifically about computers for nitrox dives. Why not all dives?

Probably because MOD is a mystery to non-nitrox trained divers... and the OP (and many others) have never even contemplated diving deep enough that the MOD for 21% would be an issue...

Is MOD A.) unimportant for air,

For recreational divers, with an abaolute recommended max depth of 40m, yes.

B.) easier to remember than for nitrox, or

I doubt it differs. But you wouldn't mark an MOD on your tank for an air-fill.

C.) is it deeper that you'll go?

I reckon this hits the nail on the head.

Plan your dive including a "this is as deep as I'll go" parameter. Regardless of whether it's most severely limited by deco, gas supply, gas contents, etc., remember the most severe limit.

Yep... that's the max depth used in dive planning. Not the max operating depth used in conjunction with ppo2.

As others have said, if you were going deep enough to worry about the ppo2 of air then you would know how to plan that dive. If you didn't then Darwin's Law covers it.

 

[syntaxerrorsix]

Max Depth is always part of the plan. But MOD (Max Operating Depth), a different term, specifically refers to the characteristics of a gas being used for diving.

For recreational divers using air, the MOD is so far below the recommended limits for diving that it would be fruitless to include that factor within a dive plan. I mean...come on... 185'... just for the sake of mentioning it?!?

Whatever guy, clearly you have something to prove today, I'll let you have it.

 

[DevonDiver]

I don't particularly care about winning any debate....but I do care that novice divers might read inaccurate or misleading 'facts'.

If you were taught to brief the MOD on every dive, so be it. I bet there are a lot of confused faces on the dive boat?!?

Instructor: Ok guys, today we are diving to a max depth of 60ft, with a max bottom time of 40 minutes. Our rock bottom is 1200 psi and our max operating depth is 185ft. Everyone clear?

Diver 1: Urrmmm... you said the max depth was 60ft?!? I am not comfortable diving to 185ft!

Instructor: No, thats the max operating depth. The max depth is 60ft.

Diver 2: Eh? So how flipping deep ARE we going?

Instructor: 60ft. But our max operating depth is 185ft.

Diver 1: But I am an AOW diver.... my max depth is 100ft.

Instructor: Thats the max recommended depth. The max operating depth is 185ft. The max depth is 60ft.

....you get the idea.... clever stuff..... carry on briefing it...


Oh...by-the-by... if I ever had to brief a diver what their own MOD was, then I wouldn't be getting in the water with them!

 

[Blackwood]

MOD isn't important for air because you won't come close to it at any sort of recreational dive. If you are planning dives that might exceed 1.4 for EAN21 You have a lot more to worry about than the toxicity of the oxygen component.

Probably because MOD is a mystery to non-nitrox trained divers... and the OP (and many others) have never even contemplated diving deep enough that the MOD for 21% would be an issue...
For recreational divers, with an abaolute recommended max depth of 40m, yes.
I doubt it differs. But you wouldn't mark an MOD on your tank for an air-fill.

I reckon this hits the nail on the head.
Yep... that's the max depth used in dive planning. Not the max operating depth used in conjunction with ppo2.
As others have said, if you were going deep enough to worry about the ppo2 of air then you would know how to plan that dive. If you didn't then Darwin's Law covers it.

FWIW, I'm trying to frame this in the context of the OP.

That's the answer I predicted, hence the "... if C."

Why is the CNS derived limit harder to remember than the other limits?

 

[Charlie99]

If you were taught to brief the MOD on every dive, so be it. I bet there are a lot of confused faces on the dive boat?!?

Instructor: Ok guys, today we are diving to a max depth of 60ft, with a max bottom time of 40 minutes. Our rock bottom is 1200 psi and our max operating depth is 185ft. Everyone clear?

Or even better

Instructor: Ok guys, today we are diving the wreck to a max depth of 60ft, the sand at the bottom is 80', max bottom time is 40 minutes. Our rock bottom is 1200 psi and our max operating depth is 185ft. Everyone clear?

 

[Splitlip]

I normally dive hard bottoms, so usually never an issue. I plan accordingly, have my own tanks.

One time however, I did a wreck dive with an over rich mix. I misread the stats on the site and the bottom exceeded the MOD for my mix. I was very much aware of my MOD without needing a reminder.

I may have set an alarm for max depth on my computer, but it did not matter. It crapped out when the wreck came into sight. I finished the dive (as well as the second of the day) using the depth gauge feature in my Citizen's Aqualand. I now dive with 2 computers.

(anybody who does this regularly remember how much a PITA doing those calcs are? I ended up buying 32 and 36 % PADI tables as backups to the backup computer. LOL

 

[RTee]

The FO2% is there to tell you if you have entered the values correctly or not. It is very common for people to be diving nitrox and leave the computer set to 21%. Max depth is there since some people still dive tables and you need that bit of information. I agree that it would be nice if some of the fields were user settable. That may have to wait for version 2.0.

I also have been diving a Vyper for two years. Not only is there an alarm that can be set but I am pretty sure that if you approach the MOD of your mixture, the PO2 will start flashing (not sure if you are going to get the beep as well as I am away from home and cannot consult the documentation).

One thing you need to be careful...it will always revert back to 21% after a certain time so in fact you could set it up for 32% and then if you get substantially delayed and then finish rigging up and backflip in the water, you could very well now be...back at 21% and unless you get back to the surface, you will not be able to change it on the fly and will be stuck with the 21% NDL even though you are carrying 32%. This could also easily happen if in your haste to hit the water...you simply forget to set the darn thing up based on the mixture you are carrying.

Other thing with Suunto...By doing the simple calculations, we all know that MOD for 32% @ 1.4 is 111.375 ft ...if my memory serves me right. Suunto parameters set at P0 and A0 will give you an MOD of 107 ft. While it may not be much, you could now be safely diving at 110 ft with a computer giving you a big lip on and chirping like there is no tomorrow. Not being a big fan of lip on...I will set the PO2 at 1.5 instead and remain fully aware of my 111.375 MOD

 

[gcbryan]

To me the answer is as simple as you need to know what you are breathing. It's also a check to make sure the numbers on your computer make sense. If they look odd and you can see that you mistakenly set the % at 21 or 50 or whatever you can figure out what is wrong.

I'm sure many people have their computers set on 32% when they are now using air. They can easily catch their mistake while underwater if that percent is always displayed. For many/most of the recreational divers using Nitrox it's always going to be 32%/111 fsw MOD anyway.

 

[scubadiver888]

Nice. Several people have given good opinions on this, thanks for that. I agree with the importance of the O2 clock stated above, but I'd point out that Suunto didn't debate between a static MOD and a dynamic O2 clock, they debated between a static MOD (37m) and a static O2 percentage (29%). As it stands, one of the largest numbers on the display is a giant 29% throughout the dive. With which I can do...? (easily)

I totally misread your original post. In the famous words of Emily Litella, "Never mind."

 

[Kern]

I also have been diving a Vyper for two years. Not only is there an alarm that can be set but I am pretty sure that if you approach the MOD of your mixture, the PO2 will start flashing (not sure if you are going to get the beep as well as I am away from home and cannot consult the documentation).

-snip-

If it shows ppO2 the OP could just use the pressure shown in lieu of the depth. I'd recommend monitoring gauges rather than relying on alarms