UWSojourner
Contributor
"...the measure is a meaningless one, that is obviously invalid as used here, and the 30% is a junk number...."
eg... (40 ATA/mins) could be a 4 ATA for 10 minutes which is deadly, but could also be 0.1 ATA for 400 minutes that is harmless (weekend ski trip). Obviously the simplistic formula above, is not enough.
Your objection does not apply to the situation we're discussing -- decompression diving.. The diver is obviously exposed to far more than the minimal supersaturation pressures you reference.
And your objection tends to work against you (i.e. against VPM). If we modified the simple integral supersaturation calculation to reflect this threshold supersaturation idea, then we'd simply disregard compartments that never exceeded a certain threshold supersaturation. So, for example, if the supersaturation pressure never exceeded, let's say, 400mb in a compartment we'd not allow that compartment to contribute to the integral supersaturation since, as you point out, there is some level that is harmless.
I've gone through that exercise and it invariably hurts VPM-B and tends to put more distance between VPM and GF which is why I never decided to actually show it. It's just one more thing you could object to and, as you note, VPM-B is already shown to have higher supersaturation exposure due to the deep stops.
The reason it hurts VPM-B is because the deeper stops allow more on-gassing in the intermediate and slow compartments. That additional on-gassing causes more compartments to exceed the threshold sooner. So you might find yourself using, for example 12 compartments in the integral supersaturation calculation for VPM-B, but only 10 for GF. Not for every profile obviously, since at some point all the compartments exceed the threshold just due to the dive being more substantial.