Quote: (Originally Posted by
mempilot)
Cederic's work is a might cry from what 90% of Rebreather divers out there are doing. We often site these types of dives to justify enhancements to our units, but very few ever do them. For those that do, I wouldn't hesitate to agree that the radial is needed. The BOV is a different story. The BOV serves a huge purpose, even in 10 feet of water.
As for WOB:
CIS rad: 2.57
Meg axial: 2.5
Ext Air: 2.28
Meg rad: 2.22
Prism rad: 2.07
(note: I owned one of the ExtendAir adapters for my Meg. It sucked, big time. I don't trust these numbers, and for CO2 retention, it would have been the worst due to the design. MicroPore confirmed this with me at DEMA.)
The test data shows the Meg Axial and the Meg Radial only .27 apart. EN14143 states that 2.75 is the limit. The Meg Rad is .52 below the limit. The Meg Axial is .25 below the limit. The difference between the axial and rad is only .27, and that is hardly perceptable to the diver.
The radial in question wasn't really the ISC unit anyway. It was comparing the Golem radial to the Meg axial, as the Meg radial has not been available until just recently. If I recall correctly, the Golem radial did not fair any different than the axial scrubber in WOB. In my mind, this eliminated WOB from the decision tree and left duration. Since the duration of the radial goes beyond what most divers (I admit there are dives that require it), even those of us that do technical dives on our RBs, need. I'm not saying don't get one if you want it, but I personally don't see $1k+ enhancement out the unit unless you are pushing 5+ hour dives. If the WOB on the Golem were significantly better than the axial, then I'd be next in line to buy one or the ISC radial at that.
The order of the test pages posted on the ISC site is a bit confusing and seems to include some mistakes regarding the gases used in some of the tests. Initially I was looking at the radial results for 10/90 at 94m, which made the comparison btw the axial and radial more dramatic. So yes, you're right: According to ISC test the air dil comp of the 2 scrubbers produces only slightly more than a .27 difference. Still, the radial has a lower baseline WOB and my main point is that any little bit of help with WOB is crucial when your RMV is rapidly shooting up during a crisis or in periods of high workload.
As for using an extreme dive to justify the everyday use of a radial scrubber, this was not my intent but does bring up another relevant point. If you consider the difference in gas density for air dil and 10/90 heliox or other he mixes, the 10/90 is so much less dense a gas that it actually makes WOB on the radial easier at 314 ft than with air dil at 164. This makes baseline WOB performance even more important for anyone diving with air dil and who may ever find themselves in strong current, as I do frequently. And I'm sure I'm not the only CCR diver who does these kinds of dives.
As for perception of WOB, here's a recent anecdote. On the trip I returned from yesterday, I'm pretty sure I noticed a slight increase in the WOB of my unit just from using 8-12 over the WOB using the 6-12 I normally use. This could be due to the fact that we were doing even more high workload dives than usual in Alor than on past trips. But if this is true and the increase of WOB merely from using 8-12 vs. 6-12 was noticeable, my guess is that the .27 btw the Meg axial and radial would definitely be noticeable in high workload situations, which is not exactly the time you want to notice. And I'm not sure why this difference btw the Meg axial and radial is only .27 as the grain boundary on the radial is shorter by less than half. It may have something to do with the material used on the axial in/out scrim being only a fine screen material as opposed to the perf metal and screen double layer that the Meg radial appears to have.
And if you're bringing up CE 14143, given the limit of 2.75, the Meg axial appears to be above this figure on the CCRB chart, which supposedly reflects a test done to the full CE standards requiring calibrating the breathing machine to reflect the additional WOB load of a human respiratory system.