The only way I could see hissing being attributed to the pressure set at the regulator being too high is if there is a leak in the oxygen valve and oxygen is hissing through.
Where is the hissing coming from? If it is coming from between the valve needle and valve seat, then perhaps there is a leak there and higher pressure would create more of a hissing sound than lower pressure.
If the hissing is coming from where the flame comes out of the torch, then my earlier suspicion that she was compensating for a reduction flame (caused by a wonky candle) is probably right.
Or, in rare instances, there could be a large burr (left over from machining parts) in the valve body or a port that is causing the noise and it (the sound) can be carried up through the torch to the front, making a hum, whistle, or a hissing sound. It's a harmonic vibration thing that originates from one place and is tranferred to another. But, again, given the other clues in her post, I don't think that this is the cause of the hissing.
Back to pressures...
If the valves of a torch were marked like the dial on an infinite controller, with numbers ranging from 0-10, for instance, and say you have the valve set on "4" while your regulator pressure was set at 10 psi, you would have a certain flame and would probably not be getting a hiss out of it (and let's just say that you would have to turn it up to "6" to get a hiss). Now, if you were to leave the valve set to the same number, "4", but adjust the pressure at the regulator to 20 psi, then you will have a different flame and might be getting a hiss. You would be controlling the amount of oxygen getting to the torch jets (thereby controlling your flame) by adjusting the regulator pressure. Side note: This is what guys do with their Herbert Arnolds that have pre-set valves (HAs have very complex valves that are preset by the factory with one valve that the user can adjust to control the torch). They cannot change the settings of the pre-set valves on the torch (to change the flame characteristics pre-set by the factory), so they change the pressure at the regulator (in-line regulators make this easier).
Now, say your torch is still set at that 20 psi and you turn the dial down to "3" and you get that certain flame again (that you once had at "4" when the regulator pressure was set at 10 psi) and the hissing goes away. You have just controlled the amount of oxygen getting to the torch jets (thereby controlling your flame) by adjusting the needle valve setting.
So, the flame that once took turning the valve knob from 0-4 at 10 psi, now only takes turning the valve knob from 0-3 when you are running 20 psi. You can still get the same flame, but at the higher pressure, you do not have to turn the valve as far to get it. The valve control is more responsive at the higher pressure.
And to take the example further, with that torch set at 10 psi, you might turn the valve to "8" and reach a plateau where you cannot get a larger flame. And when you have the same torch set at 20 psi, you might get that same flame (that "8" would have given at 10 psi) at "6" and then be able to go on and achieve a larger flame at "8" and an even larger one at "10."
I still believe that the hissing in Barbara's case is due to the addition of oxygen needed to "clean up" a reduction flame caused by a center candle that is longer and more fuel-rich than the rest. At the lower pressure, she still got the hissing when she turned the valve up.
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