nitrox MOD/TOD

[BarryNL]

I am begining to wonder if NITROX is worth the trouble?
As the PPO2 becomes more conservitive the advantages of NITROX seem to diminish.
Even if I am not diving NITROX, if I intent to go below 33m (100ft) I need a gas analayser to insure that it is 21% that I have in my tank.
Most dive operations will not allow me to utilise an extended bottom time as they want to get all the divers back onboard the boat at about the same time.
The only real advantage that I can see thats left is to have 40% available at the 5m (15 ft) safety stop.

Better to have 100%

 

[jonnythan]

Go plan a pair of 100 foot dives on EAN32 with about an hour of surface interval. You will see a dramatic difference, especially on the second dive.

 

[fire_diver]

I am of the more recent vintage of nitrox trained divers. I have to admit that when I see "MOD" I think it SHOULD mean maximum, as in 1.6. I think that many of the agencies teaching today are diluting the training because they know that "x" percentage of students will be idiots and violate a MOD anyway. So they reduce the MOD to 1.4 and then make up new term for what was originally MOD. Then they say that the (1.6 PP) MIGHT KILL YOU DEAD IN A HEARTBEAT SO STAY AWAY!

As a side note, I see a HUGE similarity in the scuba training today. Many, many shops have weekend courses, and then there's a ton of resort cert courses. The agencies don't seem to mind cranking out vast numbers of poorly trained, ill-equiped (informationally) divers. Then they tell the new divers, just stay shallow.

Then when a diver wants to expand his/her training and knowledge, and move into nitrox, they again tell them don't use 1.6, use 1.4. By the way, don't touch 1.4 either, just stay shallow.

I think when my son is ready to be certified they will be teaching "...just lay motionless on the platform at 21fsw, then surface with 1000psi in your tank..."

FD

 

[victor]

Exactly, why not just have 100% for the safety stop and everyone else on 21%.
The only situation I can see any theoretical advantage is diving at 33m on doubles (for example a wreck) where you have a near square profile.
As soon as you need to go much deeper then the risk of an O2 hit will disqualify the dive.
The conservative dive tables that are being used by the dive computers will send you back to the surface before you gain much by using NITROX.
Might as well stick to 21% and my trusty sherwood source. :light:

 

[wreckchick]

I've never heard of an accelerated safety stop.... but, whatever.

I think the point that a few people are missing is that MOD is the result of a formula that has 2 variables. Notice I said variables, not constants. The variables are PPO2 and PO2. Either one of those can change and the MOD will change along with it. Nowhere is it written in stone that the PPO2 has to be anything specific. Once you've decided what to use for your team on that dive, you plug in the actual mix and you get a number that is your MOD. That's it, that's all it is, nothing more complicated than doing some quick math.

Rachel

 

[Rick Inman]

My IANTD Adv Nitrox info says:...
Not sure if there have been updates since 2002 in their materials. I use 1.4 as my MOD on my doubles.

Nope. IANTD still teaches 1.6 MOD and 1.4 or 1.5 TOD (and suggests MOD for deco). The 1.6 keeps the term true to NOAA's original intent, and the 1.4-1.5 TOD variance is an attempt to create thinking divers who understand that the risks of toxing very depending on the diver, the conditions and (seemingly) the alignment of the planets.

I also took the PADI Nitrox course some time ago and in that class they define MOD as 1.4, so you don't have to worry about the thinking part.

 

[Damselfish]

Exactly, why not just have 100% for the safety stop and everyone else on 21%.
The only situation I can see any theoretical advantage is diving at 33m on doubles (for example a wreck) where you have a near square profile.
As soon as you need to go much deeper then the risk of an O2 hit will disqualify the dive.
The conservative dive tables that are being used by the dive computers will send you back to the surface before you gain much by using NITROX.
Might as well stick to 21% and my trusty sherwood source. :light:

I dive single tank, and a computer, hardly ever a square profile, and diving Nitrox makes a huge difference on NDLs.

Nitrox allows you more time whether it's a square or multilevel profile. And if you don't use the extra time, you are at least soaking up less N2.

A computer gives you a big time advantage on a multilevel dive whatever you're diving. And the Nitrox allows you to take advantage of more time so you're running low on gas before time.

 

[H2Andy]

I think the point that a few people are missing is that MOD is the result of a formula
that has 2 variables.

yah, try telling that to Thas



now, as to NOAA's definition, i believe after a night's sleep that he is correct.
NOAA does seem to define MOD (a term of art to be marked on the cylinder) as 1.6,
even though they allow maximum operating detphs of less than that.
makes no sense to me, but there you have it.

however, as pointed out by a bunch of people, NOAA does not set the standards
for recreational diving.

what NOAA says is basically irrelevant in this discussion. bottom line is, MOD
in recreational and technical diving has shifted from 1.6 to 1.4 over the years,
probably in search of safer margins.

1.6 is fine for a MOD for decompression gas, where minimum work is done

1.4 is MOD for the active portion of the dive, were more work is done

 

[jonnythan]

I just picked 70 feet out of thin air.

Using EAN32, you can easily do a 40 minute dive to 70 feet. After an hour on the surface, you're ready to do another 40 minute dive to 70 feet.

Using air, you can do a 30 minute dive to 70 feet. After an hour on the surface, you're ready to do a 25 minute dive to 70 feet.

80 minutes bottom time versus 55 minutes. Pretty convincing if you ask me. And that's at only ~1.0 ppO2.

 

[H2Andy]

oh, no doubt... Nitrox wins hands down in the 60 to 100 feet range over air

i thought this was settled 20 years ago?