As you can tell if you have been reading these recent posts I'm working on gas valve design. We are going to need good control of very low flow rates. Very low. On the order of 1E-3 to 1E-6 cc/second at STP.
A gas valve with a major orifice .200" in diameter will not give us much control. It goes from fully on to fully off (at the very low flow rates required) with a travel of .1 thousandth of an inch. What can be done to increase the travel and also increase the flow control?
Angle the sealing plate (assuming a soft seal such as an O-ring). This will give more control as the valve comes closer to full off. It will also make the control algorithms trickier as the gain goes up with increased opening. An orifice plate in the vale would be a help in limiting maximum flow (gain) and it would help increase control in the proportional range by adding a flow resistance to the valve resistance.
It also looks like we will need a multi stage regulator. 20 psia (5 psig) to 5 torr. 5 torr to 1E-3 torr. And the final control stage of 1E-3 torr to 1E-7 torr. Individual stages for the experimental reactor should be able to vary pressures over at least a 10 to 1 range (100 to 1 is better) to account for required gas flow variations depending on operating pressure. Once we understand operation better we can optimize for actual flow requirements.
It looks like piezo electric benders are not going to work since the travel is much too small. Something on the order of 1 to 2 micro meters. What we need is something better. A voice coil motor. That can easily give +/- 30 thousandths of an inch travel. It has a drawback in that it is magnetic in nature and thus can't be placed inside the the magnetically shielded grids. For small reactors a 2 ft length tube will add about 2 mS delay to the control system. Not good. Not insurmountable.
As the reactors increase in size delay will go up linearly with size. This is bad. Volume will go up as the cube of size. This is good. More volume reduces the need for fast control.
In fact the whole gas system may need a number of reservoirs to make the control problems tractable. It is pretty much a truism in business these days that the faster the manufacturing system can respond to demand the smaller the required inventory. So it is with any control system. We will adjust our inventories accordingly.