by
Larry "Harris" Taylor, Ph.D.
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I know there are many who suggest it is unnecessary to dive on an oxygen enriched gas mix to be certified as either a diver or an instructor for mixes other than air. They say, rightly so, that the diving is pretty much the same, ie. "Breathe in and then breathe out." However, the purpose of diving with the gas in a basic "Nitrox" class is NOT to breathe the gas on a dive, but to establish a ritual (a defined order in doing tasks) of gas analysis while breathing a gas other than air. Basically, the ritual is that all cylinders used by a diver should be analyzed three times:
1. When the gas is initially mixed
2. When the gas is obtained from the vendor
3. Immediately prior to use.
To confirm my belief in on-site analysis, especially when acting in an instructional mode, I cite the following incident from one of my basic oxygen enriched diving classes.
My class involves about 12 hours of lecture (lots of in-class problem solving) and three dives (one each on NOAA I, NOAA II and air. I use three different mixes 'cause this forces students to use three different tables for deco planning, establishes the desired behavior of gas analysis and gives each student 2 in-the-field experiences in checking their cylinder composition.)
On the dive site and prior to the first dive, I gather all of the oxygen enriched gas cylinders and scramble them such that there is no clear distinction between the different compositions in the pile. Then, even though we are diving in a quarry with a max depth of 40 ffw, I tell the students to select the appropriate mix for a 110 fsw dive. They, of course, select the NOAA I (32 % oxygen) cylinder for this dive.
The cylinder oxygen composition is clearly marked on each cylinder.
On one occasion, a student conducted an analysis on his breathing gas and found the cylinder he had selected contained 37.2 % oxygen. I had him re-calibrate the analyzer and redo the analysis. Once-again, the observed analysis was higher than anticipated. I asked him what gas was supposed to be in the cylinder. His answer was 32 %. He told me he could have sworn that he grabbed a 32 % cylinder from the pile and the 36% cylinder for his next dive was over by his car. We checked and it was clear he had mixed up the cylinders during the process of preparing for the dive.
I asked him if there was a problem with using his chosen cylinder to a depth of 110 fsw. His face turned white and he said, "I could have died!"
I said, "not necessarily, but you would be definitely increasing your risk of in-water seizure and, as you know, such an incident could be fatal"
I then had the student calculate the MOD (Maximum Operating Depth) of his chosen cylinder. It turned out to be 91.2 fsw (for a 1.4 ata oxygen partial pressure).
I next asked the student what his partial pressure of oxygen would be at a depth of 110 fsw. It turned out to be 1.61 ata. (The recommended maximum for recreational divers varies a bit with the nature of the dive, but limits typically are in the 1.4 to 1.6 ata range. I choose the more conservative 1.4 ata for my diving.)
The points are:
The only way you know for certain what gas you are breathing is with an on-site, immediately prior to the dive analysis. While improper mixes are rare, they do happen.
Humans are not perfect creatures and, as such, we cope with our potential errors in performing complex tasks with redundancy and check-lists. When diving with mixes other than air, the appropriate ritual is three separate analysis procedures: one by the vendor at time of initial mix, one by the diver when cylinder is obtained and a final one by the diver on the site immediately before using the cylinder.
The "ritual" acts as your safeguard against human error in the preparation of your breathing gas and to skip a step may have severe consequences.
and finally:
The purpose of diving with oxygen enriched gas in training is NOT related to breathing a different mix, but rather an introduction to established safety procedures and protocols that are different from previous experiences diving on air.
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About The
Author:
Larry
"Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the
University of Michigan. He has authored more than 200 scuba related articles.
His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered
one of the best recreational sources of information In North
America.
All rights reserved.
Use of these articles for personal or organizational profit is specifically denied.
These articles may be used for not-for-profit diving education