Ok, here's the answer I got back. *** It comes with a .PDF *** I'll try to get it posted somewhere but in the meantime just pm me with a real email addy and I'll send it.
There is no simple way to explain this, but here is the short version. When you lower the supply pressure to a regulator, depending upon its construction, the secondary side pressure may increase. This is more common in smaller regulators where design limitations do not allow for "balanced" valve construction.
The attached pressure characteristics charts illustrate this with the AR1000. A supply pressure of 0.7 MPa with a set point of 0.2 MPa is used in the illustrations. First draw an imaginary line from the 0.7 MPa supply pressure on the bottom chart straight up to intersect the set point and then draw a second line to the left to the secondary pressure of 0.2 MPa. Now do the same thing with a lower secondary pressure of 0.5 MPa.
Now, why does this happen? Well, a regulator is trying to compensate for pressure changes on the inlet side, (or outlet side), by adjusting the size of an orifice to change the pressure drop across it. This process is a balancing act between multiple spring forces and the force of air acting on the diaphragm and surface areas of the valve.
Because of the nonlinear nature of the force in springs as they change in length and other forces such as friction, getting a perfectly linear relationship between a pressure change and the orifice size to compensate for them is impossible for standard regulators which utilize a simpler construction then precision types.
In larger standard regulators, the construction is also more sophisticated then the smaller ones The valve that controls the orifice size is designed with equal amounts of surface area on either side to eliminate any force bias. This is known as "P" compensation. This requires an additional seal that adds considerable friction. The friction is why this compensation cannot be applied in small size regulators. The forces available in smaller regulators to actuate the valve are too weak in comparison with the increased mechanical resistance and the performance is worse then before.
For regulators without "P" compensation there is more surface area on the bottom of the valve then on the top. This is factored into the choice of the springs used to return the valve and also the adjustment spring. When inlet pressure drops, there is slightly less proportional force from air pressure acting on the bottom of the valve to counteract the force of the adjustment spring that is forcing it open. A slightly larger orifice is the result which creates less pressure drop. The pressure on the secondary side will be higher when there is less pressure drop from the inlet side.
Observations and Recommendations
If you note the charts for the other regulators, the pressure drop characteristics generally improve the larger you go. There are some differences and also a hysteresis as illustrated by the "loop". The hysteresis is the difference caused by internal friction and minute play within the linkage between the handle and the valve inside the regulator when the valve is either moving up or down in response to pressure changes. A larger regulator may be an option to consider.
Regulators operate most accurately in the middle portion of their regulation range. Regulators with narrow a narrow regulation range are available that will improve their pressure characteristics. ** smilin note: pdf gets useful here **
Precision regulators offer the best pressure characteristics in regards to changes in inlet or outlet pressure, but also incorporate a constant bleed .
The SRH and SRP series regulators are rated for Nitrogen use. Standard regulators are not.
