jonnythan:
'Charge' the bottle by turning it on then off, and recharge as necessary. Don't leave it on.
IMHO this is really the only safe way to go if the primary purpose of your sling tank is redundant gas.
Anyone who does otherwise makes several major assumptions:
First, they assume that they'll have a free left hand to turn off the valve should a free-flow occur. This might not always be the case. Their hands might be full of a toxing diver, an obstruction, an entrapment, an anchor line AND another diver, a reel, a fixed line in a cave, or god knows what.
Second, they assume that nothing else is going south simultaneously. I watched a guy get enmeshed in a gill net one time. Not only were his regs yanked from his mouth and bottle, but both began freeflowing as the net yanked him down. Uh, at the moment he didn't have a hand free to tend to any of his valves...see item one.
Third, they assume that they're the only one with sudden issues. Don't forget that your immediate source of frustration and sudden chaos might be another diver who is in far worse shape than you are.
Fourth, they assume they'll be able to see the valve. Ever been in zero vis and heard bubbles? Quick, which reg are the bubbles coming from?
You plan to carry a sling bottle because you're doing something where you've calculated you might need that much gas to respond to a situation. Not that much gas minus whatever free-flowed out while you were too busy doing something else.
Assuming you have issues, it only takes a minute to turn the valve on. But once that gas is gone, you better switch to plan B.
Charge your hoses, then turn the valve off. Check it periodically throughout the dive. It may lose pressure. No worries, charge it again and then turn it off again.
If you dive with your sling tank valves open, the time may come when you're losing gas and unable to do much about it except wish you'd turned the darn thing off.
FWIW. YMMV.
