Solution - Physical Assembly

 
F or the purpose of prototyping, PVC tubing was selected for the cylinder and all plumbing, because it was simple and cost-effective to implement.  It did, however, add the additional complexity of installing a sight glass apparatus to enable the user to visually verify the interface level, and to enable the design team to perform calibration of the system.  This was accomplished by mounting a flexible vinyl tube on the side of the apparatus.

The PVC assembly was suitable for prototyping, but will not be sufficient for practical use: When the fluid levels are changed within the cylinder/sight glass assembly, they are drawn out of or forced in to the two chambers at different rates.  This slows down the process considerably, the equalization of the tow chambers takes some time.  There is a significant effect on measurement accuracy as well, because a thin coating of oil remains on the capacitive rod assembly for a long period of time after equalization.

This problem will be significantly reduced to within acceptable levels by replacing the PVC cylinder with a transparent glass one, and eliminating the sight glass entirely.

T o force water into the system, a submersible centrifugal pump was selected for simplicity of plumbing and appropriateness of performance specifications.  The Little Giant 1-Y pump, model #501025, is epoxy-encapsulated and selaed with Viton to withstand caustic petroleum solvents.  As it requires a 120V 60Hz AC source, it is controlled by a relay which is under the control of the system software.
Little Giant 1-Y
The pump's performance curve is shown to the right.  The pump must overcome that the height of the cylinder plus the heights of the reservoirs, since it must sit on the bottom of the water reservoir.  At approximately 4 feet, the pump will drive nearly 150 gallons per hour.  With only one cylinder and a small number of Flo.Point devices, this is more than adequate for the time insensitive task of filling the system.  This fact may change as the system is scaled to handle several devices at once, and the reservoir depths and desired flow rates are scaled accordingly.
F or downward Control, the ASCO Red Hat II 8210-B54 solenoid valve was chosen.  The 8210-B54 has a brass body and piston, and is available with Viton sealing, making it impervious to the caustics which it may encounter.
The valve is normally closed, which means that in its de-energized state, fluid is not allowed to travel through it in the forward direction.  Because it is a pilot-operated, or indirect-drive valve, it does allow a slightly arrested flow in the reverse direction, because the pilot valve is only held shut by positive pressure. This fact is taken advantage of to allow the pump to fill the cylinder without having to open the valve.  Control of the system is reduced to a measurement, a single signal for filling, and a single signal for emptying.

While the pump may require an upgrade when scaling the system upward, the valve should be sufficient as long as 1/2 inch pipe is appropriate.  Flow rates through the valve are high enough to support a large number of Flo.Points, and response time is fast enough to ensure precision for a small number.

ASCO 8210-B54