Single Stage Regulator
In a single stage regulator pressure is regulated by the pressure of the gas that has already gone past the poppet and not by tank pressure.
The entire design of a pressure regulator is to maintain a specific pressure by the action that any increase or decrease in the pressure you are trying to maintain will cause the opposite to occur, keeping everything stable.
As your working pressure increases the diaphragm is lifted pulling the poppet up and restricting the orifice, preventing working pressure from increasing any more.
If the working pressure starts to decrease the diaphragm will have less force acting against it and the orifice will open, letting in more gas and keeping the pressure stable.
Any decrease in downstream pressure/resistance, such as by opening your needle valve more, will cause the orifice to open more, again keeping the pressure stable while increasing flow rate.
Dual Stage Regulator
The Dual Stage Regulator is really no different.
Pressure in the first stage is regulated by pressure in the first stage, if pressure decreases, the factory set spring(in some it is adjustable with a precision nut) will push the poppet and increase the orifice size, regulating pressure in this stage.
The main difference is that the pressure in the first stage is regulated by the pressure in the second stage, and not by what is downstream/restricting the outlet.
When something downstream increases the flowrate requested, the second stage pressure decreases which opens the orifice making first stage pressure drop... which opens the orifice.
Conclusion:
What this means is simply that there will be more fine control with a dual stage regulator, the pressure between the first and second stage is going to be pretty constant, so as flow rate requested changes the pressure will be more stable.
In a single stage regulator if the flow rate changes there will be greater fluctuations in the pressure.
Albeit they might not last for even an entire second but in a medical application it might be very important to have a constant 9.8 psi with a largely variable flow rate.
These kinds of variations are not a problem to us, as they only occur for a brief second after you adjust your needle valve.
When you open the needle valve the orifice opens briefly before it returns working pressure to its original value, causing a brief increase in bubble count before it balances out again.
Go ahead and try it.
Open your needle valve, this we can all conclude causes a brief decrease in working pressure as more gas is being let through the needle valve, diaphragm now has less pressure acting on it, orifice in the regulator opens wider, brief surge of pressure as everything balances out, temporary(.5-1.0 seconds) increase in bubble count, and then you see your stable bubble count.
So what can really cause EOTD?
My guess is that it is due to bad springs in cheap regulators.
The spring plays a large role in keeping things balanced and if it seizes or is old, possibly rusty, then there is not much regulating going on.
I thought it possible that if inlet pressure is less than working pressure, something odd could happen, but working pressure is whats regulating the orifice so it really shouldn't be possible for tank pressure to affect it in any way.
So how can we avoid EOTD?
I say other than buying quality regulators (I don't think we need dual stage regulators, although they are higher quality typically)
Occasionally check your regulator, see if the knob turns freely.
Also if your regulator has a pressure release valve, run the regulator as close to the pressure that opens said valve as you can - if pressure somehow increases the valve will open.
My regulator is set at 45 PSI working pressure, and my valve opens at 48.
Downside is that there may be leaks that you didn't know were there when running these pressures, but go ahead and check!
Critiques? Concerns? Corrections?:icon_surp
I wrote this, based on assumptions and observations, so if someone has info onto the more scientific aspects of it that might explain why we need a dual stage regulator then please, please chime in.