fweber:
OK ... earlier you wrote ...
On a recreational dive to 80 feet you've determined your rock bottom turn point to be 750 psi. On your next breath you feel an unussually high resistance to inhalation. On the one after that there's not much. Your buddy is ahead and slightly below you working into a current, absolutely insistant on catching that lobster and so maybe not quite as attentive as normal. The only mistake you've made, is not knowing that your gauge is broken.
First of all, unless you were diving large doubles then your first mistake was determining your rock bottom to be 750 psi. On even a large single cylinder such as a 130 it should be much higher than that for a depth of 80 feet.
My first question would be "what do you mean by rock bottom?" Are you calculating how much gas it would take to get both you and your buddy safely to the surface using the standard rules of rock bottom (e.g. 30 fpm with stops)? Did you start by asking yourself, if either myself or my buddy lost our gas RIGHT NOW, would the other buddy have enough to get us both to the surface safely? If the answer to either of those questions is no, then you are not calculating rock bottom. That's your first mistake ... you're too deep for the reserve you're allowing.
Second mistake ... if you cannot get your buddy's attention then you are effectively diving solo. Now, you can choose to do that ... but in that case you should be diving a redundant supply. You're bug hunting, so you knew in advance that there'd be times when you and your buddy would be swimming in a formation that would not allow for constant surveillance of each other. So plan your configuration accordingly.
Out here we also bug hunt ... but for dungeness crabs rather than lobsters. We have contingencies that account for depth, formation, "lost" buddy, etc. If we're going to 80 feet, we pretty much plan it as a solo dive and pack ponies.
Adherence to basic gas management rules would've prevented the need for a CESA in your situation ... even with the equipment failure that you experienced.
If this makes sense to you, and if you want to learn more, PM me your e-mail addy. I'll send you a paper I wrote on gas management that might help you understand better how to plan for such contingencies.
... Bob (Grateful Diver)