What went wrong on your dive today/recently? And what did you learn?

[Rollin Bonz]

Exactly. I vary rarely dive without some level of exertion. I can count on one hand the number of times I've had an air starved feeling. Only need one finger for when using EAN. If the tiny change would be enough to trigger it on a dive with some level of exertion, I'd expect it to happen to me a lot more frequently.

Yeah, and at recreational depths, rebreather divers diving an SP of 1.2 will ALWAYS be breathing from loops with an fO2 above 21%.

It defies logic, common sense, AND the asymptomatic 99.99999999% of dives being done every weekend.

 

[Belzelbub]

The work of breathing is greater on exhalation than on inhalation. Here is a flow loop from a breathing simulator. The work of inhalation is 0.33 J/l, the work of exhaling is 0.85 J/l, for a total of 1.18 J/l. The work of breathing goes up at depth and higher flow rates.

And not all regulators are created equal in this regard. The testing done on the model I use came out at a bit lower than the example you posted when tested under very difficult conditions. Deep, low tank pressure, and high RMV. Those factors and the tuning of the regulator will have a much greater impact than the gas density of air vs. EAN32.

 

[L13]

And not all regulators are created equal in this regard. The testing done on the model I use came out at a bit lower than the example you posted when tested under very difficult conditions. Deep, low tank pressure, and high RMV. Those factors and the tuning of the regulator will have a much greater impact than the gas density of air vs. EAN32.

Exactly!

The density work of breathing increase from air to Nitrox32 is < 1%. There are a hundreds of things that will increase WOB by >1%.

 

[scubadada]

And not all regulators are created equal in this regard. The testing done on the model I use came out at a bit lower than the example you posted when tested under very difficult conditions. Deep, low tank pressure, and high RMV. Those factors and the tuning of the regulator will have a much greater impact than the gas density of air vs. EAN32.

Of course regulators are going to have different performances, see any series of testing. The sample loop I posted was at 40 m but a very high RMV of 75 l/min, 2.65 cu ft/min (standard conditions #2). The cracking pressure on inhalation is particularly amenable to adjustment, A to B on the loop.

I posted this to answer the question of WOB inhalation vs. exhalation, nothing at all to do with relative gas density of air and nitrox. I believe all testing is done with air and at a standard supply pressure (750 +/- 25 psi).

 

[L13]

The points Emphasized:

1. A little (insufficient) knowledge can be a dangerous thing.

Ironic how this post proves its primary point of emphasis true. It is an example of insufficient knowledge, or at least understanding.

 

[VikingDives]

Then there's this:

https://journals.physiology.org/doi/full/10.1152/japplphysiol.00885.2007?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org

(PaCO2 is arterial CO2, P02 is arterial O2)


" We cannot exclude hyperoxia as a factor contributing to changes in PaCO2 at depth. However, increasing Po2 from 0.7 to 1.3 ATA had no effect. This is a reassuring result for divers using enriched O2 breathing mixtures, at least to the depth of this study."

"The results of this study, the first of its kind with direct measurement of PaCO2, indicate that PaCO2 was increased during moderate and heavy short-term immersed exercise at 4.7 ATA. The hypercapnia was not extreme enough to affect consciousness or exercise performance in any of the subjects. The rise in PaCO2 is primarily attributed to the decrease in V̇e and slight increase in dead space. The two main factors contributing to the hypercapnia were the increased gas density/pressure at depth and the presence of external respiratory resistance. Minor contributors included HCVR and V̇o2max values, whereas inspired Po2, Ptr, and small variations in V̇o2 did not have a significant effect. A predictive model based on inputs of depth, external breathing resistance, HCVR, and V̇o2max shows generally good agreement between the predicted and the measured values."

 

[L13]

...
The two main factors contributing to the hypercapnia were the increased gas density/pressure at depth and the presence of external respiratory resistance. Minor contributors included HCVR and V̇o2max values, whereas inspired Po2, Ptr, and small variations in V̇o2 did not have a significant effect. A predictive model based on inputs of depth, external breathing resistance, HCVR, and V̇o2max shows generally good agreement between the predicted and the measured values."

Density increase due to depth dominates followed by Regulator or CCR WOB performance.

density increase due to Nitrox is insignificant (on the order of 1 ft per 100 ft of depth).

 

[Belzelbub]

I posted this to answer the question of WOB inhalation vs. exhalation, nothing at all to do with relative gas density of air and nitrox.

I get that. Your post just helped with the point. When trying to understand the causes of CO2 retention, there are many possibilities. The minor difference in gas densities is way down that list.

I believe all testing is done with air and at a standard supply pressure (750 +/- 25 psi).

I believe that is true for one of the tests. I think there are a few other standard pressures, depths, and rates used.

 

[scubadada]

I believe that is true for one of the tests. I think there are a few other standard pressures, depths, and rates used.

The 4 standard testing conditions are listed in my previous post. Apparently, the Navy uses a higher supply pressure in their qualification, condition #4. The rest are pretty standard. If a regulator performs excellently through all the standard tests, more strenuous testing is sometimes done, above the usual range.

 

[VikingDives]

I'd like to propose an alternate hypothesis:

@UofMI_Divegeek and his student had a fill with some CO or CO2 present on their first dive. This was improperly attributed to the study that we can't find.

Later, @UofMI_Divegeek went on a second dive with air, and did not have a contaminated fill, thus causing him to commit a Type-2 error.

Discuss.