Nitrox MOD and time

[Foxfish]

I understood that, and I used a Cobra for years, but in this case you are really talking about two different things.

1. Violating the MOD as you described earlier is risking CNS oxygen toxicity, which brings on a seizure and almost certain drowning.

2. When people are talking about the CNS clock that is being checked by a graph on your Cobra, they are talking about total oxygen exposure over time, what some people measure via OTUs (Oxygen Toxicity Units). The danger in this case is pulmonary oxygen toxicity, essentially an irritation of the lung. Back when I took the PADI nitrox course, that was a big part of the class, and we were given a special table to help us keep track of it. When you took the class, that was barely mentioned, which is why you are probably confused. The reason it is barely mentioned today is that it is pretty darn hard to get near those limits on a day of multiple recreational dives. It really isn't going to happen, and if it does, it isn't life-threatening. You can ride your Cobra this way if you want, but it won't be all that exciting--you won't get anywhere near the limits.

(Edit. Deleted my previous comments)

Quote from the Suunto Cobra manual:

3.4.3. OXYGEN LIMIT FRACTION (OLF)
In addition to tracking the diver’s exposure to nitrogen,
the instrument tracks the exposure to oxygen, if set to
Nitrox mode. These calculations are treated as entirely
separate functions.

The dive computer calculates separately for Central
Nervous System oxygen toxicity (CNS) and Pulmonary
Oxygen toxicity, the latter measured by the addition of
Oxygen Toxicity Units (OTU). Both fractions are scaled
so that the maximum tolerated exposure for each is
expressed as 100%.

The Oxygen Limit Fraction (OLF) has 11 segments, each
representing 10%. The OLF bar graph displays only the
value of the higher of the two calculations. When the
OTU value meets and exceeds the CNS value then in
addition to displaying its percentage the lowest segment
blinks to indicate that the value shown relates to OTU.
The oxygen toxicity calculations are based on the factors
listed in section 6.3. “Oxygen Exposure”.

End quote.

From http://www.aqualung.com/technical_library/cobra_manual_en_2e285.pdf

My take of this is that the computer display monitors both so providing you stay below the limits you are okay. Interested to know how many people dive in this way and if it is generally considered safe.

 

[PfcAJ]

The "CNS clock" and OTUs are not the same thing. Issues with OTUs are tough to come about in Rec diving. The CNS clock issues are not as difficult to come across (think live aboard with nitrox).

All that said, you can tox before hitting 100% and not tox after passing well past 100%. The entire concept is highly suspect. The only sensible approach is one of conservatism. Unlike DCS, having an oxygen enduced seizure is almost always a death sentence.

 

[boulderjohn]

My error--I was headed for bed and misread.

 

[Mr Carcharodon]

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?

If you have a copy of the NOAA diving manual (NOAA is the National Oceanic and Atmospheric Administration, a US Government agency) they have a table that indicates the percentage of a maximum allowable exposure as a function of ppO2 and time. As BoulderJohn already pointed out exposure time matters. So for example the 45 minute exposure to 1.6 ATA ppO2 you mentioned corresponds to 100% of the oxygen clock in the NOAA table. 33% is at 15 minutes, and 67% is at 30 minutes. 1.6 ATA ppO2 is considered aggressive by most agencies today with 1.4 ATA seeming to be the standard, and even 1.2 ATA used during the working portion of dives. NOAA published their table about 20 years ago and there have since been a few fatalities at lower ppO2s. So do be conservative with their table. That said the common apoplectic response that you will die if you even momentarily slip below MOD seems to be based on nothing at all. I’d get the table, make a plan, and keep it on the conservative side.

There is also pulmonary toxicity to plan for. In that case there is a similar table that gives OTUs as function of ppO2 and time and provides a maximum allowable number of OTU. In practice CNS limits will drive unless you are doing many dives.

 

[Simon-]

To the OP,

do you understand the difference between OTU's and CNS? Do you know your OTU limit for each day of diving, or the limit for repetitive days of diving?

Do you know how to calculate OTU and CNS, what the surface credit intervals are etc?

Sadly, you seem to be (and this is not an insult) typical of newish divers who are happy to fly the dive computer with not too much though of what is actually happening.

My suggestion, learn the theory behind it all before you start talking about pushing boundaries and if it is safe to do.

 

[Dr. Lecter]

As in using the computer indication of your oxygen exposure rather than the MOD. I tried to explain this previously when I said, "I should add that my Suunto Cobra computer has a bar graph that indicates your oxygen partial pressure ie. oxygen exposure. In theory you could 'ride the graph' like you can when approaching the no decompression limit ie stay at a depth that keeps you just under the limit."

You seem to have somehow managed to confuse pO2/MOD with cumulative O2 exposure; regrettable, as it might get you killed. CNS hits are not like deco; or more accurately, you probably don't want to think of them in the same way. While time of exposure to a given pO2 (as well as work effort, CO2, etc) can play a role in whether you seize or not... it's not really understood as a saturation effect like deco.

 

[Foxfish]

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.

Simon, thanks for your comments. The Nitrox course doesn't go into calculating exposure limits for the different modes of oxygen toxicity. I've had my Nitrox specialty for a while and have done a number of dives on Nitrox but generally in areas where MOD is not an issue until recently when it was. I guess the lesson here is that over time it is easy to forget some of the things you learn on the courses. If in doubt check. That was the point of this thread!

 

[tphelps]

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?

The MOD is just that, the MAXIMUM operating depth. Imagine you're playing a game of 1 on 1 "tug of war." In the middle marks each half of the "court." The closer you get to that line, the more at risk you are of going over. Which is why you usually want to be far away from that line. Because once you cross it, BOOM you plow face first into a pile of mud and the game is over.

And as you probably remember from your nitrox course, there is CNS oxtox and Pulmonary oxtox. I don't know exactly how you figured out you would get oxtox at 45 mins past the MOD?? Pulmonary toxicity is the result of long exposure to high FO2 gases. This can mean diving nitrox for hundreds of minutes in a single day for consecutive days before you are at risk of Pulmonary oxtox. My suggestion - always dive within the limits and follow your tables using a computer as back-up.

---------- Post added August 30th, 2013 at 03:50 AM ----------

If ever MOD is a concern for you when you are planning your dives, you can always resort to switching to a lower FO2 nitrox blend. For example, you have greater depth restrictions with a higher Nx36 blend than with an Nx32 blend.

 

[HIGHwing]

I don't know why everyone (particularly instructors) seem to propagate Nitrox as such a mysterious thing.

As long as you consider Nitrox as a drug/poison, it's very easy to understand. Given the correct dosage & duration it will very easily chemically burn your lungs or perhaps kill you.

PO2 = Current dosage.
MOD = Prescribed dosage limit of the gas.
CNS Clock = Dosage exposure over 24-hours.

Regarding the comment about riding the computer, I wouldn't really encourage it. Going over NDL the worst that happens is you go into deco. With O2, you might be getting O2 halftime credit which is not a recreational diver "thing". Unbeknownst to you, it's possible you could be diving against a paradigm for which you might not be fully informed, and perhaps you don't want to accept the associated risk.

As always, see a competent instructor and read the manual.

Safe dives.

 

[MrChen]

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.

Simon, thanks for your comments. The Nitrox course doesn't go into calculating exposure limits for the different modes of oxygen toxicity. I've had my Nitrox specialty for a while and have done a number of dives on Nitrox but generally in areas where MOD is not an issue until recently when it was. I guess the lesson here is that over time it is easy to forget some of the things you learn on the courses. If in doubt check. That was the point of this thread!

Why exceed it when its much safer to just pick a different mix that increases your mod?

Every dive I go on I ask Max depth on the dive and pick my mix with a mod greater than that depth.

I never ride any limits and I've only gotten close to my ndl once and used my computer to make sure it kept reading at least 15 min by decreasing my depth.

Sent from my Kindle Fire using Tapatalk 2