Nitrox MOD and time

[tracydr]

When I did a chamber ride with 100% O2 at 60 feet for 5 1/2 hours, they did 5 minute air breaks every 20 minutes, if I remember right.
I can tell you that my lungs and sinuses were not happy after the chamber dive. I coughed and felt like I had a bad cold for two days.

 

[gcarter]

If the NDL comes up first on a dive then the dive time will be constrained by that requirement. Rules about duration at the MOD are given by the various diving agencies. In my case it was SDI. There are also restrictions on the percent oxygen in the mix and the allowable cumulative time at the MOD over a 24 hour period. People who have done the Nitrox specialty course should be familiar with them. I've forgotten some of them already so being prompted to review them was a good thing. My computer also provides an indication of both CNS/P oxygen toxicity - which to date I've never monitored.

On a slightly different note, I've found personally and have observed in others that one of the big challenges we face as divers is remembering all the important things we learned during our dive courses. Like the things that we've just been discussing. They can be the difference between life and death. Maybe you don't have that problem Simon.

This was bought home recently while on a diving holiday. I spoke to a diver who was keen to go for a dive but felt appprehensive because although she'd done the course it was some time since her last dive. We talked a bit about the things you need to keep in mind on a dive. I said "You do remember the dive tables that tell you how long you can stay at a certain depth before ascending". She thought for a moment. Her face went blank. Then she said "No"! My guess is that this is probably a lot more common that you might expect.

I think you are still misusing and misunderstanding the term MOD. Maximum Operating Depth is based on depth and FO2, and the resulting PPO2. It is not a time based measure. Yes, it plays into / changes NDL (which is time based) and other oxygen toxicity measures that have time based considerations. But MOD has nothing to do with time.

 

[Nirvana]

Its not 5mins every 30.

12mins on o2, 6mins off (switching to the lowest ppo2 gas you have) is the current rotation for o2 times over about 25mins. The stop prior to the gas switch is done on back gas, too. I'd be long gone if this scheme didn't work.

I'm curious, I was taught - not too long ago - to do air breaks on the 20min back gas / 5 min O2 rotation scheme and to switch straight from, say, EAN50 to O2 without first going back to back gas. Why do you say the method you described is the current one (and not simply the one you feel most comfortable with)?

 

[lindenbruce]

To the OP,
MOD is there for a reason. Can you violate it? Who really knows. I wouldn't. Taking and Ox Tox hit is nothing like getting nitrogen narcosis. Narcosis builds gradually and gets worse the deeper you go. And it is something you can learn to recognize and correct by ascending to a shallower depth.

As far as an Ox Tox hit those hit like a light switch. One moment you are fine and the next you are convulsing, unconscious and drowning. If you are on open circuit you are most likely going to be another diving statistic. If you are on a rebreather or diving a helmet you may just live as long as someone recognizes what has happened and gets you to a shallower depth right now.

As others have stated it is a matter of time and exposure. But why push that limit. MOD means MOD. If you are still diving down to deeper water and running your PPO2 up after you have violated the MOD you are in for some deep dodo. But if you have an emergency and you are ascending when you have to go to a higher Oxygen level breathing gas you are prolly going to be just fine. Just get your lazy butt up the water column until you reach the MOD for that mix. Otherwise it's just not worth it. B.

 

[boulderjohn]

I'm curious, I was taught - not too long ago - to do air breaks on the 20min back gas / 5 min O2 rotation scheme and to switch straight from, say, EAN50 to O2 without first going back to back gas. Why do you say the method you described is the current one (and not simply the one you feel most comfortable with)?

Different agencies teach different protocols for this. There is no one universally accepted method.

 

[Superlyte27]

I'm curious, I was taught - not too long ago - to do air breaks on the 20min back gas / 5 min O2 rotation scheme and to switch straight from, say, EAN50 to O2 without first going back to back gas. Why do you say the method you described is the current one (and not simply the one you feel most comfortable with)?

I was taught in Commercial Dive School 5 minutes every 30. I think there are several Training Agencies teaching that also. I've been doing 5 every 20 for nearly 20 years.

 

[Dan G]

To the OP, if I am misunderstanding your question, forgive me. It sounds as if you are asking if other divers look at the O2 bar on their computers and ride it like some divers ride the NDL number. My computer, and I think most likely your Suunto, display an NDL number based on the least amount of time any particular calculation will allow you to stay at your given depth. This includes any O2 loading. So if you are riding your computer, you don't need to look at your NDL number and your O2 loading bar separately, because that NDL may in fact be based on O2 loading at that given moment.

 

[ianr33]

If you are on a rebreather or diving a helmet you may just live as long as someone recognizes what has happened and gets you to a shallower depth right now.

My understanding is that if someone is having an O2 seizure you DO NOT want to take them shallower until the seizure has passed. During the seizure their airway is most likely closed. Ascending will cause barotrauma.

 

[Foxfish]

I think you are still misusing and misunderstanding the term MOD. Maximum Operating Depth is based on depth and FO2, and the resulting PPO2. It is not a time based measure. Yes, it plays into / changes NDL (which is time based) and other oxygen toxicity measures that have time based considerations. But MOD has nothing to do with time.

Check your course notes if you have done the Nitrox specialty. The concept of a time limitation at the MOD is real.

 

[gcarter]

Check your course notes if you have done the Nitrox specialty. The concept of a time limitation at the MOD is real.

Yes, there is a time limitation at the MOD, just like there is at any depth. MOD is still not a time based measure, and you are using MOD incorrectly. Whether using EAN21 or EAN32, there is a MOD. Diving something other than EAN21 doesn't change any of the principles involved or change the meaning of the terms. MOD is a depth. It is not influnced BY time. It INFLUENCES time.

MOD (FSW) = 33' x [(PPO2/FO2) - 1]

There is no time element in that formula. Time is introduced when looking at other measures.

Going back to your OP:

If you stay at or below the maximum operating depth (MOD) for a nitrox mix more than 45 minutes then you can get oxygen toxicity. I'm interested to know how many divers venture below the MOD for shorter periods of time. If you do dive below MOD, how do you determine when it is time to ascend to avoid problems with oxygen toxicity?

This shows a fundamental misunderstanding of what MOD is.

In another post, you write

I checked the course book for the Nitrox specialty course and it was adamant, don't exceed your MOD. It is okay to dive at that depth for a period up to 45 minutes but should not exceed the depth. As a recreational diver I'm happy to dive within that limitation.

According to wikipedia (too lazy to check the NOAA reference directly)

"Acute oxygen toxicity is a time variable response to the partial pressure exposure history of the diver and is both compex and not fully understood.

The maximum single exposure limits recommended in the NOAA Diving Manual are 45 minutes at 1.6 bar, 120 minutes at 1.5 bar, 150 minutes at 1.4 bar, 180 minutes at 1.3 bar and 210 minutes at 1.2 bar."

​

The time limits referred to here are related to PPO2 NOT MOD, and you are incorrectly over simplifying with your 45 minute number. PPO2 DETERMINES your MOD, is is not determined BY your MOD.

And to be more inclusive / clear, NDL needs to be considered IN ADDITION TO the toxicity measures. 150 minutes at 1.4 on EAN32 with a MOD of 110fsw @ PPO2 of 1.4, staying at the 110FSW depth is for that length of time greatly exceeds the recreational no-deco NDL.

You may think I am arguing semantics, but the fact is that your posts are more difficult to parse than they need to be because you are using terms interchangeably that are not interchangeable.