Many applications require precise pressure control to a process that is relative to a different pressure in that same process – in other words, differential pressure control. In this schematic, two Proportion-Air QPVs are working in tandem to dispense a combination of a sample and a solution in a differential dispensing application.
A counting sensor is gathering information at the point of dispensing and sending this information to the PLC, where the command signal is calculated and supplied to both QPVs. The QPV controlling pressure over the sample is calibrated 0-30 PSI. The QPV controlling pressure over the solution is calibrated -1 to +1 PSI with the intention of constantly maintaining a specific differential pressure of solution to sample.
The QPVs (when calibrated to gauge reference) come standard with an atmospheric reference port. This is the “zero” or atmosphere point from where pressure is referenced. The solution dispensing QPV in this application has its atmospheric reference port tied to the output of the sample dispensing QPV. This means that whatever pressure is being controlled by the sample dispensing QPV, the solution dispensing QPV will use this as its baseline for controlling pressure.
Differential dispensing and differential pressure control is both complicated and simple, with Proportion-Air! The solution dispensing pressure will always track at whatever offset is commanded based on the pressure it sees in the sample vessel.