Question How to dive nitrox for the first time

[ttc008]

I just finished reading the textbook in SSI's app, and I was surprised by how short it was, just 55 pages on a phone screen.

I will summarize the whole booklet in 4 key points (so that I don't have to read the entire thing again):

1. Know your MOD. Essentially, once you've set your ppO2 safe limit and identified your gas mix, the MOD is automatically calculated. Or I suppose one should choose the preferable mix with a set ppO2 safe limit and a desirable max depth depending on the specific divesite?
2. Keep in mind that there is an exposure time limit as well, though it's usually not a major concern. As a general rule for recreational diving, total bottom time should not exceed 3 hours within a 24-hour period.
3. You will absorb less nitrogen, which affects your bottom time, NDL, and risk of DCS. However, once you've set your dive computer to nitrox mode with the correct mix, simply follow the numbers it provides.
4. Make sure to analyze the gas personally. Calibrate, analyze, mark the tank.

Regarding the above discussion on using air mode when diving nitrox, the textbook suggests that this method is valid for the purpose of "safety." However, one should still monitor their oxygen exposure. Diving nitrox involves managing both oxygen and nitrogen exposure, and any dive planning that does not address both aspects is insufficient.

 

[Blackcrusader]

As a general rule for recreational diving, total bottom time should not exceed 3 hours within a 24-hour period.

Who told you this?
In places where there are no dive time limits I will do 75 - 90 minute dives and do 3 or 4 dives a day.
I also get paired up with the same guide. There is not such rule for those not staying at MOD for their dives. Recreational divers are not working professionals staying at depth.

 

[ttc008]

Who told you this?
In places where there are no dive time limits I will do 75 - 90 minute dives and do 3 or 4 dives a day.
I also get paired up with the same guide.

It's an easy ballpark number to remember. Of course if you are diving far below ppO2 limit and you have the computer to monitor your oxygen exposure in real time, you can probably dive much longer.

 

[Blackcrusader]

It's an easy ballpark number to remember. Of course if you are diving far below ppO2 limit and you have the computer to monitor your oxygen exposure in real time, you can probably dive much longer.

Does your DC log show your CNS percentages? Are you a working diver or a recreational diver?

F

Thread 'Nitrox MOD and time'

Aug 29, 2013

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?

• Foxfish
• Replies: 76
• Forum: Advanced Scuba

So what rule applies to those on 21% nitrox?

 

[NothingClever]

SSI is just plain dumb for granting even a narrow allowance for divers to set a computer differently from the gas one’s breathing.

Unbelievable.

 

[arkstorm]

It's an easy ballpark number to remember. Of course if you are diving far below ppO2 limit and you have the computer to monitor your oxygen exposure in real time, you can probably dive much longer.

View attachment 895795

I think that table creates confusion and misunderstanding regarding the risk of O2 exposure...

You are not generally limited to 180 minutes of diving over 24 hours even when breathing EAN.

The time limit corresponds to how much time you can remain at your MOD, i.e., at PPO2 of 1.4 ata, for the entirety of the dives, only then are you limited to 180 mins of exposure over 24 hours.

For example, if you are diving EAN32, you would have to stay at 111 fsw for >180 minutes within a 24 hour period to reach that limit.

Consider a LOB where you are doing five 60' dives a day on EAN32 but you are nowhere near your MOD. you could easily do five 60-minute dives, 300 minute total, and not get anywhere near your O2 exposure limit.

A much better way of explaining this concept is that a diver is allowed a limit of 300 oxygen toxicity units (OTU) per day. One OTU is equivalent to breathing 1.0 ata of O2 for one minute.

Dive computers do an excellent job of calculating this exposure in real time based on your actual gas loading per your varying depth but you have to tell the computer what gas mix you are breathing...

That is yet another reason not to dive an air profile on nitrox when using a computer.

 

[uncfnp]

A timely discussion…

Info Thread 'Some History of the NOAA Oxygen Limits'

Jan 2, 2025

It is popular on ScubaBoard to denigrate the NOAA Oxygen Exposure Limits, which were originally (from the NOAA Diving Manual 2nd Edition, 1979) given as:

The source of this table is clearly stated as the USNavy Diving Manual (1973).


By the NOAA Diving Manual 3rd Edition (1991) the limits had become:

The source of this info is clearly stated as from Butler and Thalmann (1986) and other data.

The 4th Edition (2001) of the NOAA Diving Manual gives:

The source is cited as:

The U.S. Navy has recently performed research on pure oxygen diving in...​

• tursiops
• Replies: 32
• Forum: Technical Diving

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[Blackcrusader]

SSI is just plain dumb for granting even a narrow allowance for divers to set a computer differently from the gas one’s breathing.

Unbelievable.

If this was the case why have a nitrox cert in the first place. When I was at Savedra a non certified Nitrox diver asked could he use nitrox on his dives and was told NO. He thought he could just use 21% settings and dive on 32%. When asked what the MOD of 32% was the diver didnt know. When asked what the MOD of 21% was the diver didn't know.

 

[boulderjohn]

It's an easy ballpark number to remember.

Any random number is easy to remember. Being easy to remember does not make it right.

 

[halocline]

Any responsible operator would not mix air and nitrox divers together.

That’s an oversimplification, and the fact is that in the real world, recreational dive boats mix nitrox and air divers without any problems. There’s nothing wrong with that as long as the nitrox divers understand they may not get the full bottom time benefit of their gas. It just depends on the dive profile, the amount of independence divers on the boat have, and what the expectations of the nitrox divers are. Lots of recreational divers use nitrox simply to lower their N2 loading; they’re frequently limited by gas rather than no deco limits anyways.