Slam opening any first stage, no matter what second stage is attached, is a great first step in starting a kindling reaction. I have never had a properly set up Jetstream second stage in my hand that would continuously vent gas when the cylinder valve was opened like it should be; continuously and gradually, but slowly.
Jetstream gas flowing.
I believe the root issue is that some Poseidons do vent a bit of gas for a couple of seconds if the valve is opened properly (slow and gradual). The gas loss in that case is largely irrelevant unless one deliberately opens the valve at an absurdly snail-like speed, since opening a valve only takes a few seconds even when done slowly. This is simply how the second stage is set up: it takes a couple of seconds for the membrane to inflate. The through-hole is so tiny that it takes a little while.
Deflated membrane and tiny throughput hole.
I'm sure some over-zealous technical divers have misinterpreted this very small gas loss to mean that the valve must be slam opened. And yes, slam opening a Jetstream will often nearly instantly cause the membrane to inflate.
But the whole thing is a bit like divers blaming their rather high gas consumption on a recreational dive on the tiny stream of bubbles from their SPG spool or yoke O-ring. In both cases, the gas loss is real but is it relevant? Of course, a blown O-ring on an SPG spool or a grossly cut O-ring on a yoke connection will result in significant gas loss. But for me, the couple of seconds of venting on Jetstreams is comparable to the tiny stream of bubbles from an SPG spool, not to a blown O-ring. If I recall correctly, Poseidon even quantified this somewhere in their documentation. If memory serves, the gas loss amounts to roughly 3 litres, practically nothing.
Jetstream inflated membrane.
If a Poseidon truly keeps venting even with a fully opened cylinder valve just because the valve was opened slowly, something is amiss. That would also mean the membrane might not inflate again after taking a breath. A broken or leaking servo valve would be a likely culprit.
I do want to clear up one thing that has crept into the dive industry: the notion that venting a second stage while opening a cylinder valve is a good thing, due to a supposed cushioning effect (I know this is not what you meant, Robert). This has especially proliferated among some technical communities, where it is claimed that the rise in adiabatic temperature would be counteracted. In fact, it does the exact opposite: rather than reducing the risk of a fire, it increases it, and the severity of any fire is greatly exacerbated.
The exact mechanics and physics are complicated and differ from one first-stage design to the next, but they all have a few things in common. The pressure in front of the first-stage seat will rise to or near cylinder pressure regardless of whether you purge the second stage or not. The reason for this is straightforward: the flow rate of cylinder valves vastly exceeds that of first and second stages. In other words, the cylinder valve delivers far more gas than the first stage can pass through.
Depending on the first-stage design, the seat either starts within the high-pressure chamber (as in flow-through piston regulators) or experiences the pressure drop directly across its surface (as in flow-by piston designs). Adiabatic heating occurs whether gas is flowing or not, and the first-stage seat is affected either way. The delivery of gas from the cylinder valve far outpaces the throughput of the first stage, so gas compresses in the first stage HP side and results in adiabatic heating. Whether or not you purge the second stage makes almost no tangible difference. The supposed slower build-up of pressure inside the HP side of the first stage due to purging a second stage is imagined, not real.
This area will rise to cylinder pressure almost in the same time, regardless of whether you purge the second stage or not.
It is much more effective to slowly open the cylinder valve. This directly ensures that the speed of compression within the HP side of the first stage is reduced by reducing the amount of gas flowing. This, in turn, reduces the "piston effect" of the moving gas, lowering the final temperature considerably. Think about how a water hammer in a pipe works: just because you have a tiny outlet (an open second stage) at the end of the pipe doesn't mean that water hammer will not occur. While not a perfect analogy, the concept is similar.
However, purging a second stage or having gas flow during opening introduces two major risks:
- First, it ensures a fresh supply of oxygen to any ignition sites. Think of a campfire flaring up when you blow on it. The moving oxygen will fan any flames that might occur.
- Second, the flowing gas ensures that if an ignition occurs, it has a chance to propagate. The moving gas will carry hot fumes or burning material further down the mechanism, ensuring that the kindling reaction doesn’t stop.
For these reasons, major industries place large and prominent warnings on their oxygen regulators. The welding industry struggled with this problem around 1980. Welders are notoriously lazy, and once they found an output pressure with the set screw they wanted, they often left it alone. They would turn the cylinder valve on and off rather than adjusting the set screw again. Manufacturers had to (and still do) place prominent labels on the regulators explicitly instructing users to back out the set screw completely before opening the cylinder valve (which is equivalent to ensuring the second stage isn’t venting while opening the cylinder valve). In fact, it's so important that nearly all tags specify backing out the set screw again at the end, so that the next person doesn't accidentally open the cylinder valve with the set screw engaged (i.e. gas flowing from the outlet).
The medical industry states exactly the same procedure on their oxygen regulators if they come with a tag; at the very least, it is stated explicitly in the owner's manuals. The continuous flow output must be set to "zero" before the cylinder valve is opened.
Frank Gusky wrote a paper on this around 1988, published in the annual ASTM Flammability and Sensitivity of Materials in Oxygen-Enriched Atmospheres compilation. They attempted to ignite welding regulators with the set screw backed out (no gas flowing from the second stage). In 75 tests, they did not manage a single kindling reaction that progressed beyond the first-stage high-pressure chamber and thus was always contained. If you read through his other work, it becomes clear that an engaged set screw was a major contributor to catastrophic fires that breached the HP side in the first stage and consequently consumed the whole regulator.
All of that to say: I agree with Robert that the Jetstream design isn’t the most optimal from an oxygen fire prevention standpoint, though I arrive at that conclusion for slightly different reasons. The "automatic" venting is an issue in itself, and when combined with people slam opening the cylinder valve to counteract it, the risk is compounded.
That’s not to say the design isn’t safe for oxygen use, quite the opposite. It is an exceptionally good oxygen regulator, with the first stage being particularly well-engineered. However, if you strip it back to fundamentals and factor in the resulting human error, it’s not as rock-solid as it could be.
