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

[tomfcrist]

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

In my opinion, a rec diver that has no training in advanced nitrox or deco should not be “switching gasses”. Select a mix that is acceptable to you at the max depth of the dive....live with the consequences of shorter NDL....it’s still more NDL than air.

 

[Frontpointer1000]

In my opinion, a rec diver that has no training in advanced nitrox or deco should not be “switching gasses”. Select a mix that is acceptable to you at the max depth of the dive....live with the consequences of shorter NDL....it’s still more NDL than air.

Agreed!!!

I’m thinking (and hoping) the OP is using the term “gas switching” to mean gas selection PRE-DIVE. By utilizing the MOD, he’s asking others and looking at what others consider safe and appropriate. I.E.: how close to you approach the MOD before selecting a leaner mix FOR THAT ENTIRE DIVE. Not gas switching during the dive. The “gas switching” term throws us off bc traditionally that’s used when a diver switches from travel gas to back gas or during deco stops—which a diver should not be doing unless he/she is trained to do so.

 

[tomfcrist]

Agreed!!!

I’m thinking (and hoping) the OP is using the term “gas switching” to mean gas selection PRE-DIVE. By utilizing the MOD, he’s asking others and looking at what others consider safe and appropriate. I.E.: how close to you approach the MOD before selecting a leaner mix FOR THAT ENTIRE DIVE. Not gas switching during the dive. The “gas switching” term throws us off bc traditionally that’s used when a diver switches from travel gas to back gas or during deco stops—which a diver should not be doing unless he/she is trained to do so.

You must be the guy that sees the good side of everything. Lol.

I’m unfortunately no longer surprised about hairbrain schemes that people can come up with. I hope your assumption is right, but until it comes out in the wash, I’ll assume he’s trying to get his technical diving plan out of a Cracker Jack box.

 

[Frontpointer1000]

You must be the guy that sees the good side of everything. Lol.

I’m unfortunately no longer surprised about hairbrain schemes that people can come up with. I hope your assumption is right, but until it comes out in the wash, I’ll assume he’s trying to get his technical diving plan out of a Cracker Jack box.

Tom, That’s what happens when you work for the government (Navy), you get sour!

 

[tomfcrist]

Tom, That’s what happens when you work for the government (Navy), you get sour!

IM NOT SOUR!!!!

LMAO

 

[dberry]

We're all just spinning our wheels until the OP clarifies if he's talking about pre-dive choice of EAN / MOD, or tech diving switches at various deco stops.

In any case, it's worth pointing out that 1.4 ("working") and 1.6 ("resting") pO2 are both empirically determined values that aren't magic depths. Just like there are "undeserved hits" due to an individual's physiology, I'm sure there are people who are more or less sensitive to OxTox. There is a 20 fsw difference in MOD using EAN32 (1.4 vs. 1.6 pO2). Sure, pick a MOD that feels comfortably conservative, but don't freak out if you're a few feet deeper. "measured with a micrometer, cut with an axe..."

 

[eelnoraa]

Some agency teach 1.2 for working PPO2, so MOD is really a objective number in a grand scale.

 

[tomfcrist]

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.

 

[Blasto]

What I am trying to find out is what safety margin most people use on MOD when switching gas.

0% when diving in a team or with FFM.

You don't instantly convulse at PPO2=1.6 or 1.4 with exercise. These are already conservative values. Realistically, you could dive at PPO2 as high as 2, with a very small probability of any ox-tox effects.

The one exception is solo diving with the kind-of-mask-i-believe-should-not-be-used-in-scuba. I've only done that a few times, only when there was no other option, and ox-tox was never an issue, since it never occurred to me to go deep with unsafe equipment.

But if I had to, I'd limit my depth or gas choice to PPO2=1.5 at rock bottom or 1.3 (common CCR setpoint) at dive plan bottom. At most, that is, if it was my once-in-a-lifetime chance to see something incredible. Anything else, even less.

 

[Michael Guerrero]

1.4 ppO2 for bottom gas, 1.6 ppO2 for deco.