How close do you get to your gas MOD's?

[Frontpointer1000]

Maximum to me is absolute. MOD is the maximum safe operating depth of a given gas. You decide what safe is. But the MOD to me doesn’t change based upon what I’m doing with the bottle. I am comfortable at 1.6. My MODs are 1.6. That doesn’t mean I have to select a gas that is at my MOD for a given dive. I can always thin the mix. For example carrying a gas with a 220fsw MOD on a 160’dive.

I agree here. I think the important rule that I am seeing come out of this is a person has an absolute, or a rule that they generally apply and follow. Some people find a partial pressure of oxygen of 1.3 or 1.4 or 1.6 or even 2.0 to be their guiding rule and they don’t cross that. Whatever your safety margin is keep it there and follow your rules.

 

[PfcAJ]

~1.2 for the bottom, 1.6max for deco.

 

[lv2dive]

If you substitute “switching” with “changing” in the original post, it looks like an inquiry about Nitrox. And for our non-tech diving brothers and sisters, it’s not obvious that “switching” has this whole other meaning (that those of us with tech experience read into it.)

Just wanted to reinforce a POV I believe a few others have already pointed out; I also agree we need to hear from @Landlocked123 to know for sure.

The OP specifically asked about "switching gas." That implies either switching to a stage, switching to or from a travel gas, or switching deco gases. Since he didn't specifically mention switching from a high FO2 travel gas to a lower FO2 bottom gas, which would imply hypoxic trimix concerns, I would assume that he's talking about switching from a lower FO2 gas to a higher FO2 gas, which has some serious implications if one were to switch at a depth where the gas he was switching to was inappropriate.

Really, the question shouldn't be in the "Advanced" forum since one of the technical forums would be more appropriate for a discussion on using high FO2 mixes for accelerated decompression, and you wouldn't have MOD gas switch concerns if your intention wasn't to utilize them in such a manner.

To the idea of conservatism when diving higher FO2 mixes in a recreational nitrox setting, it's all a personal choice, and the trade-offs are well known. I have no problem diving at 1.2-1.4 in a recreational setting if I'm swimming. I regularly deco out at 1.6, but I'm not working. If I had to work during that time, I would lower the PO2 of my decompression gasses to hedge my bets against an oxtox hit. If I knew I was going to hover on top of a wall for an hour and not move, I'd have no problem pushing my PO2 to 1.6, but realistically, in the realm of recreational nitrox level mixes, I would be out of gas before I hit my single dive limit of 45 minutes exposure, or my PO2 would drop significantly as I made my ascent.

 

[tbone1004]

I'm with @PfcAJ on this one, roughly 1.2 on the bottom *depending on what I'm doing, 1.4-1.5 is OK in a lot of circumstances for me, though not in caves*, 1.6 for deco.

Now, to answer @tep and @Landlocked123
Deco gases are marked at 1.6, and breathed at 1.6. I switch to my 70ft bottle *50%* at 70ft, my 20ft bottle *100%* at 20ft. I'm not moving much since it's deco, and I'm trying to get rid of the inert gases in my body. We chose 1.6 for a reason, so I'll use it.

My "normal" bottles are EAN32 since that is the standard in cave country and in the depth ranges that most of my dives occur. If you ask the DIR crowd, the MOD of EAN32 is 100ft, but it's not necessarily because of ppO2, it's because of narcosis and gas density, both of which I am annoyingly susceptible to and have gotten narc'd at 90ft before in a cave. If you ask most of the rest of the world, that bottle will have 111 or 132 on it to coincide to 1.4 or 1.6. Since I am primarily a cave diver and prefer to stay within normal "standards" if at all possible for ease of diving with my buddies, my EAN32 stage bottles all have 100ft stickers on them.

Where it gets a bit hypocritical is what do I do if I'm doing say a nice clear/warm/easy ocean drift dive where the max depth is 120ft and I have EAN32 in my doubles because that's all that ever goes in that set of doubles. I'll dive the EAN32. Again, I'm not concerned about the 1.5ppO2, I'm more concerned about the narcosis and the gas density based on how my body tends to react. If I was say Brett Gilliam, Neal Watson, Sheck Exley etc. who all seem to have/had super human resistance to that, then I wouldn't care, but alas, I'm human.

As a diver, you have to choose what your MOD is going to be and what is going to dictate that MOD. Is it a function of ppO2? In that case, in the example that @tep gave, he's OK going to 1.1 *80ft*, or 1.2 *90ft* on EAN32, but not 1.4, that's perfectly fine and that's your choice to make based on how you have learned that your body reacts to higher ppO2's and what your risk tolerance is. I'd argue that in that case you should also be concerned about narcosis and when you need to start adding helium in when diving deeper than that, but that's for you to decide.

TLDR: your mod is your mod, and doesn't have to be what the books tell you of 1.4 or 1.6. You are perfectly capable of making that MOD shallower or deeper depending on your risk tolerance. MOD is MOD though and if you put a number on that tank, it says you are OK breathing that mix at that depth so adjust your variables until they match

 

[Landlocked123]

Thank you for all your replies. To clarify my post I was inquiring as to how close to MOD depth do most divers get on a given Nitrox mixture. I use 1.4 which I know is more conservative but being in my late 40's and not as fit as most 20 year olds I have been limiting my depths to at least 10 feet short of MOD even at 1.4. I guess being extra cautious cannot hurt but I was not sure if this is common practice or overkill given I am using 1.4 as is. Thanks again for the replies.

 

[tbone1004]

Thank you for all your replies. To clarify my post I was inquiring as to how close to MOD depth do most divers get on a given Nitrox mixture. I use 1.4 which I know is more conservative but being in my late 40's and not as fit as most 20 year olds I have been limiting my depths to at least 10 feet short of MOD even at 1.4. I guess extra being extra cautious cannot hurt but I was not sure if this is common practice or overkill given I am using 1.4 as is. Thanks again for the replies.

and if you're going to do that, when calculating YOUR maximum operating depth, use 1.3 instead of 1.4 if you aren't comfortable with 1.4, though I would argue that I'm not sure your risk of oxtox goes up with age and that you'd be far better off being more conservative in your decompression profile

 

[Landlocked123]

Thanks again for all the feedback.

 

[Frontpointer1000]

Great discussion, a lot of good points came out of this thread.

 

[KenGordon]

Thank you for all your replies. To clarify my post I was inquiring as to how close to MOD depth do most divers get on a given Nitrox mixture. I use 1.4 which I know is more conservative but being in my late 40's and not as fit as most 20 year olds I have been limiting my depths to at least 10 feet short of MOD even at 1.4. I guess being extra cautious cannot hurt but I was not sure if this is common practice or overkill given I am using 1.4 as is. Thanks again for the replies.

You must have been reading Scubaboard.
1.4 is the normal limit for the bottom phase of a dive. People on here talk about using 1.6 but they are the deviants busy normalising their behaviour for the people that take them seriously. 1.4 is not extra conservative.

Mostly, people who have accidents diving have not followed their training. By and large following your training and diving within the limits of training is very effective at keeping safe. Forums and FB are full of people prepared to second guess that training.

 

[scubadada]

It is good to know your computer. I dive an Oceanic. I set the pO2 at 1.6. The warning alarm goes off at 1.4, and I rarely exceed that. I much prefer having the alarm at 1.4, to have it going off frequently at 1.2. At that, I have audible alarms disabled and see only a flashing red light.