Is ppO2 change too large?

[ivobj]

Usually I go from botton gas to 50% oxygen at 21 mts and then to 100% at 6 mts, but I did a couple of times from trimix (If I remember well 18/40) straigh to 100% at 6mts without any problem and I don't see why it would be dangerous, as someone mentioned above, the problem is PPO2 itself, not the magnitude of change.

I use 1.4 max in the botton and 1.6 for deco gases, when I am usually more relaxed.

 

[rjack321]

Some may tell you that going from 18/35 to Oxygen invites a risk of isobaric counter diffusion (which isn't a function of increasing the PO₂ but rather decreasing the fractional content of the light gas - helium). However, whatever theory may predict, switches like that are done time and time again without ill effect.

Even if this were plausible for the gases the OP mentions...
isobaric counter diffusion requires an inert to "counter diffuse"
its not a problem going onto O2 from anything

 

[dab]

I think liteheaded was refering to the probability that a dive requiring an 18/35 bottom mix would likely involve an intermediate deco gas prior to 100% 02 or a lot of 18/35 to get to 20ft.

 

[Gilldiver]

In the 70's, 80's, and most of the 90's deep diving was done on 21%, that is air for those who need to ask, right to 100% that was supplied from the boat on a hooka. It was just the gases that we had at the time. I know of no hit that was ever identified as being caused by the ppO2 switch. So there is a lot of empirical evidence that there will be no problem, and I trust empirical, real world, test results a lot more then theoretical with limited real world testing.

But as the MOD for a 21/35 mix, indicated above, is 200', I would rather use a deco gas I can use at 100' or a bit deeper (36% or 32%) so that I can get onto it earlier. Then I would get onto 100% at 20 if it was available, but would not need to use a 3rd gas.

 

[Doc Intrepid]

I think liteheaded was refering to the probability that a dive requiring an 18/35 bottom mix would likely involve an intermediate deco gas prior to 100% 02 or a lot of 18/35 to get to 20ft.

I agree that an intermediate gas (such as EAN 50 at 70 fsw) should 'likely be involved', but I didn't get the feeling that the OP was asking that question.

Tomeck clearly stated he was going from his bottom gas (18/35) to O2.

 

[wedivebc]

Tomeck clearly stated he was going from his bottom gas (18/35) to O2.

No actually he clearly stated he was going from air to O2 which makes the possibility of IBCD impossible. At the time LiteHedded posted his reply helium had not been mentioned.

 

[DA Aquamaster]

No...he stated going from 21% Oxygen or 18/35 to 100% O2, so he mentioned air and helium mixes.

The concern was whether or not it was dangerous as the change in PPO2 was so large. In regard to 18/35 it might be, indirectly speaking, as he would be doing a larger portion of the deco on 18/35 rather than switching to a mix like 32% or 30/30 at 100-130 ft or so ad it is a lot less efficient - but a larger change in PPO2 is not the issue.

But that's all been covered - at this point it is just nit picking and arguing.

 

[wedivebc]

No...he stated going from 21% Oxygen or 18/35 to 100% O2, so he mentioned air and helium mixes.

Sorry missed that.

 

[Tomeck]

I know somebody had decompression accident at 15 feet during the change from trimix to oxygen. I think this method is not good, and it is better to use 50% at 70 feet.

 

[wedivebc]

how deep had they been?

I know somebody had decompression accident at 15 feet during the change from trimix to oxygen. I think this method is not good, and it is better to use 50% at 70 feet.