The 640B absolute pressure controllers is a self contained, compact, closed-loop electronic pressure control system containing a Baratron® capacitance manometer, normally closed proportioning control valve, and closed-loop control electronics. Electrically similar to a standard MFC, the 640B is powered by ±15 VDC at only 200 mA, permitting use with standard power supply and display electronics. The pressure output and input control signals are linear 0-5 or 0-10 VDC. Two trip points with LED status indicators are for use as simple on/off process limits.
The flow through any orifice depends on the size of the orifice, the inlet and outlet pressures, and gas density. To simplify 640/641 orifice selection, use the following procedure:
In the 640 Controller, the Baratron pressure transducer measures absolute pressure; in the 641 model, it measures gauge pressure (pressure relative to ambient barometric pressure). In both models, Full Scale ranges of 10, 100, or 1000 Torr, or 60 or 100 psi are available. Each 640/641 can control pressure from Full Scale to less than 2% of Full Scale. Prudent design suggests choosing the lowest possible Full Scale for the application, taking into consideration the overpressure to which the sensor may be exposed (both normal and accidental).
You want to control a vacuum process at a pressure of 0.5 psia, with a flow rate of 2000 sccm of He. Your inlet pressure is 15 psia, giving a differential pressure (delta P) of 15 psi, resulting in an uncorrected Index Number value of 90. The gas density correction for He is calculated as shown above. Multiplying 2.6 by 90 gives a density-corrected Index Number of 234. Drawing a vertical line on the Orifice Selection Chart at 234 indicates Orifice #3 would be the best choice.