Landlocked123
Contributor
What I am trying to find out is what safety margin most people use on MOD when switching gas. Meaning how close do you get to the MOD limits before switching?
How close do you get to your gas MOD's?
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I'm assuming a gas switch at a decompression stop? If so, you might want to consider asking a mod to move this to a tech forum and out of advanced.
Anyway, switch at 1.6. You're not going to be there for very long and PO2 will drop fairly rapidly until your 100% stops at 6m/20ft where you're usually there for longer at higher PO2. (If you notice, the depths don't line up in metric and imperial, so as you can imagine, a couple points off doesn't make an appreciable difference)
Easiest thing to do is dive standard gases and switch at the tank MOD. It puts everyone on the same page and removes any questions during the dive. Put stickers on your tanks and use them.
Anyway, switch at 1.6. You're not going to be there for very long and PO2 will drop fairly rapidly until your 100% stops at 6m/20ft where you're usually there for longer at higher PO2. (If you notice, the depths don't line up in metric and imperial, so as you can imagine, a couple points off doesn't make an appreciable difference)
Easiest thing to do is dive standard gases and switch at the tank MOD. It puts everyone on the same page and removes any questions during the dive. Put stickers on your tanks and use them.
I'm not the OP, but I since I'm not a deco diver, I read this as "how close do you go to the MOD for your Nitrox mix"?
Which is a question I was thinking of asking a few days ago.
With EAN 32 the MOD is 112fsw, how often do people dive that to 100. Personally, I'd use it to 80, maybe 90 in good conditions, but I'm hoping to be an old, not bold, diver, so I know I'm way more conservative than most.
What do "real divers" use for "typical planned max depth" compared to MOD?
Which is a question I was thinking of asking a few days ago.
With EAN 32 the MOD is 112fsw, how often do people dive that to 100. Personally, I'd use it to 80, maybe 90 in good conditions, but I'm hoping to be an old, not bold, diver, so I know I'm way more conservative than most.
What do "real divers" use for "typical planned max depth" compared to MOD?
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.
broncobowsher
Contributor
For me it also depends on the PPO2 chosen to calc the MOD. MOD is an arbitrary number you choose. Are you comfortable spending time at 1.4 PPO2? If so calculate your MOD for 1.4 PPO2 and know that is the max. Too much for you, calc at 1.3 PPO2 and you will get a shallower MOD with the same mix. Nothing changed in how anything works, physics are exactly the same.
This is part of the "plan your dive" If you are only going to 80% of your MOD, change the target PPO2 to a lighter number and you can now figure 100%.
Now you are talking gas switching, which would be a deep technical dive. To need a gas switch on decent you are planning on deep enough that the mix doesn't have enough oxygen on the surface. So a travel gas to get you from the surface to a deep enough point you can breath the bottom mix. For the most part you are looking at a minimum PPO2 to switch, not a max. On ascent it is usually getting into the richest (highest O2) mix you can as soon as possible to help with deco. Often you will find the target PPO2 will increase for the deco. 1.3 on the bottom but spike 1.6 for deco.
In short, you are not going to find a clear answer. In part the question is too vague. Also it is a moving target. User defined maximum as well, which is why 100% should be just fine.
This is part of the "plan your dive" If you are only going to 80% of your MOD, change the target PPO2 to a lighter number and you can now figure 100%.
Now you are talking gas switching, which would be a deep technical dive. To need a gas switch on decent you are planning on deep enough that the mix doesn't have enough oxygen on the surface. So a travel gas to get you from the surface to a deep enough point you can breath the bottom mix. For the most part you are looking at a minimum PPO2 to switch, not a max. On ascent it is usually getting into the richest (highest O2) mix you can as soon as possible to help with deco. Often you will find the target PPO2 will increase for the deco. 1.3 on the bottom but spike 1.6 for deco.
In short, you are not going to find a clear answer. In part the question is too vague. Also it is a moving target. User defined maximum as well, which is why 100% should be just fine.
I’ve always used 1.6 for MOD...1.4 is obviously more conservative for long dives...but 1.4 isn’t the Maximum Operating Depth by any means. IIRC 2.0 used to be common..1.6 for years was broadly acceptable for NDL dives...because NDL was never near 100% CNS even at 1.6...
Bottom line, Mod is mod...pick a max and stay at or above it...there’s no need for some random Mod minus 5ft mentality that i think I’m detecting here.
Switching gas? Are you deco diving? What did you learn in class about gas switches? I’d start there...take a deco class.
Bottom line, Mod is mod...pick a max and stay at or above it...there’s no need for some random Mod minus 5ft mentality that i think I’m detecting here.
Switching gas? Are you deco diving? What did you learn in class about gas switches? I’d start there...take a deco class.
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Diver-Drex
Contributor
Each cylinder should be marked with its MOD after analysis. Depth at PPO2 1.4 for bottom gases. Depth at PPO2 1.6 for deco gases. The MOD is the MOD.
I do round shallow to the next 10’ increment. MOD at PPO2 1.4 for EAN30 is 121, I will mark it as 120fsw.
I would push to 1.6 as the contingency limit for emergency situations. So EAN30 would be 140 as a contingency depth if say I had to assist another diver.
The common exception would be O2 for deco at 20fsw. It’s just over 1.6 but this is accepted as safe practice.
I do round shallow to the next 10’ increment. MOD at PPO2 1.4 for EAN30 is 121, I will mark it as 120fsw.
I would push to 1.6 as the contingency limit for emergency situations. So EAN30 would be 140 as a contingency depth if say I had to assist another diver.
The common exception would be O2 for deco at 20fsw. It’s just over 1.6 but this is accepted as safe practice.
Frontpointer1000
Contributor
The title makes it look like the OP is asking a nitrox question in regards to MOD. And by “gas switching” I believe he is really not thinking about Deco diving and gas switching in the traditional sense, I am assuming the OP is really talking about changing an EAN mix from richer to leaner gas mixes as one approaches the gas’s MOD (during dive planning). For instance: depending on what nitrox mod table you use, my 1.4 PPO to MOD is 111 feet on 32% . If I want to go to 126 feet, I’m going to get a special mix of 29% assuming I keep my PPO2 at 1.4. Or go with a standard leaner mix.
Perhaps this is the real question the OP was asking? So depending on what you use as your limit (1.4, 1.5, 1.6), you can then say if I’m on 32% nitrox (MOD 111 ft) perhaps plan on using a different mix if you are going to be as deep as 108 ft (with 3 feet safety margin) so I’m going to switch from 32% to some leaner mix.
I personally follow my MOD at 1.4 PPO2 and will not go beyond the MOD but will go to it. If the dive requires greater depth, then I select a leaner mix.
In terms of decompression diving and MOD planning, I plan more conservatively. (I’m not AN/DP trained, though I’m in prep phase for the course now). However, I believe it’s safe to deco out at 1.6 assuming you are at rest.
Perhaps this is the real question the OP was asking? So depending on what you use as your limit (1.4, 1.5, 1.6), you can then say if I’m on 32% nitrox (MOD 111 ft) perhaps plan on using a different mix if you are going to be as deep as 108 ft (with 3 feet safety margin) so I’m going to switch from 32% to some leaner mix.
I personally follow my MOD at 1.4 PPO2 and will not go beyond the MOD but will go to it. If the dive requires greater depth, then I select a leaner mix.
In terms of decompression diving and MOD planning, I plan more conservatively. (I’m not AN/DP trained, though I’m in prep phase for the course now). However, I believe it’s safe to deco out at 1.6 assuming you are at rest.
Man, if that's the case I was waaaaay off. In that case, 1.4.
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