In the 649 Controller, the Baratron® Pressure Transducer measures absolute pressure. Full Scale ranges of 10, 100, or 1000 Torr are available. Each 649 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).
The flow through any orifice depends on the size of the orifice, the inlet and outlet pressures, and gas density. To simplify 649 orifice selection, use the following procedure:
You want to control your process pressure at 5 psia, with a flow rate of 750 sccm of N2. Your inlet pressure is 15 psig (30 psia), giving a differential pressure (delta P) of 25 psi. Your delta P of 25 psi gives an Index Number value of 175. Drawing a vertical line on the Orifice Selection Chart at 175 indicates that Orifice #2 would be the best choice.
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
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.
This information subject to change without notice.