"It has though been pointed out to me that the drawback of 275N units is that they are very difficult to swim in "
In my limited experience, all the PFDs come with an oral inflation tube which is also a deflation tube. Even the standard "150" vests are almost impossible to swim with, a backstroke or sidestroke being recommended. But if you want more mobility, the answer to to release some gas from the vest so it is less bulky. Then blow more in as needed later.
IIRC the Practical Sailor tests that were done some time ago also pointed out that several of the 150n vests will turn the user face-up in the water
correctly--IF the vest has been correctly adjusted. No, you can't just throw them on and expect much of anything, these are tools that require operator training in order to function correctly, and that was the original argument against NOT certifying them for public use. Even though our our Secret Service
was using them on presidential escort duty for years before that.
I've a hunch that the Hammar, which does not show you any illustration but refers vaguely to "When the inflator is immersed in water
by 10 cm, the hydrostatic valve opens and water will have access to a water sensitive element. This in turn releases a stainless steel
spring mechanism, " is the one that is just a flap valve protecting an aspirin tablet. You could translate their statement this way:
"When the inflator is immersed in water by 10 cm, the hydrostatic valve opens and water will soak an aspirin tablet. This in turn crumbles and releases a conventional stainless steel
spring mechanism, "
What I would call a true hydrostatic release? Would be something like a glass tube, crushed and shattered by water pressure, releasing parts
that had been joined. Or, a hyrdaulic piston, activated by the excess water pressure now building on one side of it, moving the piston and tripping something at the other end. Or, you could take the pull tag on a manually inflated vest, tie a beach ball onto it, and THAT would be a hydrostatically activated vest. You fall in the water, plunge down, the beach ball stays up and pulls the string...voila, hydrostatic release. <VBG>
Calling these things a hydrostatic release...That's mistaking the sizzle for a good steak. Clever advertising (oh, our product is SO higher tech) but all it really means is that they've found a way to keep the tablet drier than it is in other configurations, making it more reliable against accidental deployment. (The tablets are infamous for sogging apart and causing false inflation, the vests should be stored with them out.)
A pressure sensor that detected six(?) feet of submersion, then sent power to a small explosive charge that activated the inflator, now THAT would be a true hydrostatic inflator. The same explosive charges that are used in car air bags would work perfectly well, they're quite conventional technology these days.
Try explaining THAT to a TSA baggage thief. Ergh, inspector. (Shh, they catch and prosecute hundreds of employees every year for that problem.)