Deco stop depth when breathing nitrox

[Mitus]

Hi all,

I was taking the Nitrox speciality last week and I still have one question remaining in my mind :S

My instructor (and also the book) says that the deco stops depth is the same as when using air (i.e., 3m, 6m, 9m and so on). On the other hand, the deco times are calculated by using the equivalent air depth (EAD). I understand that I carry out the plannification of the dives, no problem.

However, I don't understand the reason for that. I can understand that my tissues are saturating as if I were in the EAD (which is less than the real depth). However, once I start ascending I stop at, lets say, 3m. The EAD for this stop would be around 2m and, therefore, I would have overgone the depth where I should perform my stop. How is it possible? Is it possible to reach a critic oversaturation because of it? Why do we not correct also the deco depths as we do when planning a dive in altitude?

 

[Dive-aholic]

First, it's not a deco stop, it's a safety stop. One is mandatory to help prevent DCS, the other is recommended. With nitrox, the higher the percentage of O2 used, the shallower your equivalent air depth. With altitude diving, the higher the altitude, the deeper the theoretical depth. So with altitude diving you have to adjust so you are doing your safety stop at a theoretical depth equivalent to 5-6 meters. This will put less pressure on your tissues and allow the nitrogen to off gas at the rate prescribed by dive tables. If, as you suggest, a safety stop was done at a shallower depth when diving nitrox, you would be decreasing the pressure on your tissues and allowing the nitrogen to off gas faster than prescribed by the tables. Apparently, your nitrox class didn't really cover this properly. Nitrox is not about adding oxygen to your breathing gas. It's about decreasing nitrogen in your breathing gas. Nitrogen is what drive no decompression tables and is what needs to be considered, not the oxygen. That being said, do keep in mind you do need to know the oxygen content for the purposes of keeping your ppO2 less than 1.4.

 

[DuboisP]

Hi all,

I was taking the Nitrox speciality last week and I still have one question remaining in my mind :S

My instructor (and also the book) says that the deco stops depth is the same as when using air (i.e., 3m, 6m, 9m and so on). On the other hand, the deco times are calculated by using the equivalent air depth (EAD). I understand that I carry out the plannification of the dives, no problem.

However, I don't understand the reason for that. I can understand that my tissues are saturating as if I were in the EAD (which is less than the real depth). However, once I start ascending I stop at, lets say, 3m. The EAD for this stop would be around 2m and, therefore, I would have overgone the depth where I should perform my stop. How is it possible? Is it possible to reach a critic oversaturation because of it? Why do we not correct also the deco depths as we do when planning a dive in altitude?

you can do your stops at the depth you want.

we have 3, 6, 9 m because Haldane used imperial measures when he calculated the deco tables, so 10, 20 , 30 feet. so we stay with by usual.
the Suunto computers don't give stops with a fixed depth, but with a ceiling to not overleap.
if you are at/near this ceiling, your stops are shorter than you are deeper.

that is relation with the difference of pressure between the nitrogen in your cells and the nitrogen in the breathed gaz
more is this difference, shorter is the stop.
but other rules exist, as not doing bubbles in the blood, so we have to go up slowly and make stops deeper if needed.

 

[DazedAndConfuzed]

I think the OP is saying why is the safety stop not done at the same EAD as air. So a safety stop at 5m would give you around 1.19 N2 PP, but with 32% EAN, 5m would give you 1.02 N2 PP, thus, he it is as if he is doing a safety stop at 4m with air. He thinks he should go down to 6m, where he would give him something close to 1.22 N2 PP.

The issue for N2 is not the partial pressure that you inhale like O2, but the water pressure exerted on your body. At 5m, saturated nitrogen would slowly off-gas. Having less N2 in the air you breath would in fact allow faster off-gassing without increasing the rate of bubbles forming since you are also intaking less nitrogen into your bloodstream as the saturated ones are being exhaled out.

 

[DiveNav]

you can do your stops at the depth you want......

.... as long as you are below the deco ceiling

 

[DuboisP]

.... as long as you are below the deco ceiling

re-read my post, please

 

[Mitus]

I think the OP is saying why is the safety stop not done at the same EAD as air. So a safety stop at 5m would give you around 1.19 N2 PP, but with 32% EAN, 5m would give you 1.02 N2 PP, thus, he it is as if he is doing a safety stop at 4m with air. He thinks he should go down to 6m, where he would give him something close to 1.22 N2 PP.

The issue for N2 is not the partial pressure that you inhale like O2, but the water pressure exerted on your body. At 5m, saturated nitrogen would slowly off-gas. Having less N2 in the air you breath would in fact allow faster off-gassing without increasing the rate of bubbles forming since you are also intaking less nitrogen into your bloodstream as the saturated ones are being exhaled out.

Thank you, Lwang that explanations enables me to match all the parts of the puzzle. My problem was that, as you suggested, I thought that the nitrogen off-gassing was due to difference between the tissue tension of nitrogen in my body and the partial pressure of nitrogen I was breathing (which does depend on the EAN mix). I see now that it depends on the gradient between my tissue tension of nitrogen and the outer pressure (which does not depend on the EAN mix).

 

[Searcaigh]

I do a 6M safety stop for 3 mins whether on Air or EAN.

 

[knotical]

Thank you, Lwang that explanations enables me to match all the parts of the puzzle. My problem was that, as you suggested, I thought that the nitrogen off-gassing was due to difference between the tissue tension of nitrogen in my body and the partial pressure of nitrogen I was breathing (which does depend on the EAN mix). I see now that it depends on the gradient between my tissue tension of nitrogen and the outer pressure (which does not depend on the EAN mix).

I'm not so sure that's correct. Consider that one of the reasons we use oxygen on the surface as first aid for suspected DCS is that it increases the gradient between the nitrogen in our body and in our lungs.

Adjusting safety stop depth for gas mix seems a reasonable proposal.

 

[racerx_]

Yeah as stated basically the issue is the EAD is equivalent AIR depth giving you the no deco limit for that depth on air. Emphasis is on the gas you are breathing and how that affects your limitation.

To put it simply (correct me if I'm wrong) Your safety stop is not about air or nitrox, but the depth that you're at and the pressure on your body.. AFAIK..

Also, plannification is an awesome word ha