Thanks, so the difference is less than one foot of equivalent depth.
In other words, the nitrox at 60 feet is lighter than air at 61 feet, correct? So you would have us believe that you observed huge differences in performance with an equivalent depth differential of 8 inches?
Obviously this is silly, you could not replicate the actual depth profiles between two drift dives to that degree of precision. How can you do the math without throwing out your conclusion?
Note, I didn't check your calculations, just assumed you were correct.
Consider:
The point of my original post (a "war story" from my web site) here was to alert divers to the possibility of CO2 issues WHILE DOING WORK (activities that generate much increased metabolic CO2). Typical recreational diving does not involve (or shouldn't (g) intense physical labor.
The 0.0116 g/L value was at STP, i.e at one atmosphere
Using the ideal gas assumption that density is directly proportional to pressure, the density difference between NOAA I and dry compressed air at 60 ffw can be approximated:
(0.0116 g/L/atm) x (60ffw/34 ffw/atm + 1 atm) = 0.0205 g/L
This density difference is indeed small, but it is not zero.
The fact remains that 2 divers on 2 separate dives while doing intense aerobic exercise in extreme current exhibited symptoms consistent with a CO2 overload while using NOAA I as a breathing mix. The same divers doing a repetition of this dive (using same equipment and same dive plan) did not experience any CO2 issues while using a compressed air breathing mix.
There must be a physiological explanation for this difference in observed fatigue and perceived air starvation.
If the CO2 retention cannot be explained by increased respiratory workload while breathing a slightly more dense gas while doing an intense aerobic activity in extreme current, then what explanation is there?
I should add that this particular dive profile is the only time I have experienced such a profound air starvation event while breathing oxygen enriched air (values from 31 to 38% O2). This event was downright scary ... only time my sense of air starvation was so extreme that I felt a growing urge to spit out the regulator. A sense that went away during rest stops.
Oxygen enriched air (using proper MOD and O2 toxicity constraints) is a wonderful recreational breathing mix. But I believe, based on experience, that oxygen enriched air should not be used in dives with anticipated physical labor or intense aerobic activity.
BTW: river flow under the Blue Water Bridge is ~ 85 million gallons / min with current velocity often exceeding 10 knots (11.5 mph, 18.5 km /hr). So, this is not the typical recreational dive site.
A recent weather channel special on flooding stated that water flow of 10 mph exerts the same pressure as a wind speed of 270 mph.
As I have told my river diving students, "Things are different when you play in a world where exhaled bubbles do move straight up!