Nitrox

[Meng_Tze]

If I remember correctly, the NAUI Nitrox tables are based on one hour surface intervals, which may be where that idea came from.

and

Be aware that a minimum surface interval of one hour is recommended when breathing Nitrox. So getting right out from one dive, strapping on a fresh tank and going right back in is not wise.

Are two different things.

The first says:"if you dive these (nitrox) tables, be aware that these are based on 1 hr SIT." IOW, in order to be relatively safe, use 1 hrs sit. There is calculation (time, depth, gas, NDL etc) behind this to devise the tables and is based on 1 hr SIT.

The second says: 'If you breathe nitrox, it is recommended to have 1hr SIT between dives. If you do not, it is not a wise thing to do' This is a blanket statement, that may have its origin in the first, but as a single statement as presented is misleading at the minimum. Does this breathing of Nitrox have equal effects if I breathe 32% or 90%? Does it matter (and how much) if I stay down 20 mins or 30 mins? There is no calculation behind this statement to be able to justify why it is not wise, or what this 1hr SIT is based upon.

Maybe nitpicking, but that statement came across misleading at the best

 

[Bubble Junky]

PADI recommends the one hour surface interval to reduce the likelihood of oxygen toxicity, especially if the diver's oxygen exposure is creeping up. As a recreational diving course, it does not teach how to calculate surface interval credit for oxygen exposure, they just recommend the one hour SI.

 

[Joe-Diver]

Yep, Bubble Junky is right....found the reference to it:

PADI Enriched Air Diver Manual, pp. 75-76

" Although these NOAA limits don't specify minimum surface intervals, and there's no measurable surface interval credit, it's recommended that you allow a surface interval of at least an hour between enriched air dives, whenever possible, especially if you exceed more than 50% of allowable exposure. This is believed to further reduce the likelihood of oxygen toxicity."

 

[Meng_Tze]

PADI recommends the one hour surface interval to reduce the likelihood of oxygen toxicity, especially if the diver's oxygen exposure is creeping up. As a recreational diving course, it does not teach how to calculate surface interval credit for oxygen exposure, they just recommend the one hour SI.

Yep, Bubble Junky is right....found the reference to it:

PADI Enriched Air Diver Manual, pp. 75-76

" Although these NOAA limits don't specify minimum surface intervals, and there's no measurable surface interval credit, it's recommended that you allow a surface interval of at least an hour between enriched air dives, whenever possible, especially if you exceed more than 50% of allowable exposure. This is believed to further reduce the likelihood of oxygen toxicity."

Okay,
I find it strange that on one hand when we increase the O2 levels in a gas, the students are not taught to account for that. And on the other they are told that the 1hr SIT is especially important when exceeding 50% allowable exposure.

Nevertheless, the quote from the PADI pages does quantify it, even if it is lacking in reality. The original statement at hand did not....

I have no issue with the PADI quote (even though I do not agree), I have an issue with an unfounded/non-quantified statement. But I guess that is clarified now.

 

[crpntr133]

Without looking it up aren't RGBM tables set with a 1 hour SI?

 

[Bubble Junky]

Okay,
I find it strange that on one hand when we increase the O2 levels in a gas, the students are not taught to account for that. And on the other they are told that the 1hr SIT is especially important when exceeding 50% allowable exposure.

Not sure I understand what you mean here. It is early in the morning though.......

 

[Joe-Diver]

X2....PADI teaches how to account for increased O2 by limiting depth according to max pp02 in a given blend (up to 40%) to avoid CNS tox, and to account for greater O2 exposure over time as a percentage based on pp02 and time to avoid pulmonary tox. For recreational divers, and the key word here is Recreational, with max blends to 40%, what more should be taught? Is there something critical being left out of the PADI EANx course that recreational divers should know that results in lives being in imminent danger? I am assuming, of course, that you've looked over the PADI materials even though you list IANTD in your profle, which is a fine organization that takes training to levels beyond recreational.

I'm not trying to be confrontational with you; I'm just curious what your concern is.

Not sure I understand what you mean here. It is early in the morning though.......

 

[fppf]

Sorry for the late response, we just got back from Cape Ann.

I paid $59 for a sensor from Main line Medical. The sensor puts out a simple mV signal. I have mV panel meters that can be set with custom scaling so I can set them to read % of O2. The R17 sensor uses the 1/4" mono plug that can be had at radio shack. For the flow I went to lowes and bought a bunch of plumbing fittings to make a sampler that flows 2 L/min and hooks to the LP hose.

I don't know many people that have panel meters laying around the house though. But you can get them off ebay.

Oxyhacker is a good book, I did not go with his version because I had better stuff in the house. But his meter can be built on the cheap easy. But a strong warning, you need to know what your doing with electronics and calibration. You need to calibrate and check these things out before you trust your life with them.

 

[J.R.]

Without looking it up aren't RGBM tables set with a 1 hour SI?

YUP...

 

[bubba105]

Having a tester is a good idea. Although most of the places I have gone to dive have had testers, on occasion they do not. When I first bought mine I ordered nitrox 32 & 36 for boat dives in Cozumel. The operation gets nitrox from a supplier who puts the number on a piece of tape on the tank. Apparently there are people who trust someone else to test their tanks, I do not happen to be one of those people. When I tested the tanks they were showing 26 or 27%. Since it was the first time using the unit I thought I was doing something wrong so I calibrated again on an air tank and got the same results. Magically, the next day when I tested the tanks they tested 32 & 36%. Human error could fill a tank with too much O2 also. I bring my tester on the first day to compare results and if I'm satisfied I use their tester for the rest of the trip.

As for waiting an hour, let's all remember that if you push the envelope and lose, you can't dive for a while, not to mention hurting yourself, having to sit back analyzing stupidity instead of nitrox. I'll wait the hour.

Richie