Abbo

My primary battery died last weekend. They do seem to keep going till a little South of 5V, but the recommendation is to change them at 5.7V. It's a major league pain where the Sun don't shine to get ISC's special batteries shipped out to Japan, especially since half of what you're paying for is transportation costs. Fortunately I landed on my feet instead of my rump, for a welcome change: Tanaka-san, under whose tutelage I was, not only had a couple of off-the-shelf lithium cells but some connectors that he had made up himself. That way, I am freed from the need to call ISC for a rush shipment every time my batteries run down. The voltage from a lithium battery drops off sharply at the end of its life, so you should change them around 6.5V, rather than 5.7V.

The primary, which needs to do the mechanical job of opening the solenoid, lasts about half as long as the secondary. When my secondary's battery pops its clogs I'll switch the primary into that slot and put a new one in the primary slot. I had been taught to switch off the head as soon as I leave the water, and to turn on at the last minute before entering: none of the people around me were doing that - they were turning on at the dive shop, before even leaving for the beach. They explained that being cameramen they often had a lot to think about before getting in the water, so they didn't want to risk being rushed into the water and forgetting to switch the handsets on. I can't see what they'd be doing during the pre-breathe to forget to switch on, but I now appreciate how being able to use off-the-shelf batteries frees you from having to be quite so miserly in their use.