Flow Control Assemblies For O Components; Critical Flow Orifices - Teledyne T750 User Manual

5.00 VDC. A separate 0 to 5 VDC command voltage is also generated that is proportional
to the target flow rate requested by the calibrator's CPU. The 0-5VDC command signal is
electronically subtracted from the 0-5VDC flow signal. The amount and direction of the
movement is dependent upon the value and the sign of the differential signal.
The MFC valve is an automatic metering solenoid type; its height off the seat is controlled
by the voltage in its coil. The controller's circuitry amplifies the differential signal obtained
by comparing the control voltage to the flow sensor output and uses it to drive the solenoid
valve.
The entire control loop is set up so that as solenoid valve opens and closes to vary the flow
of gas through the shunt, valve and sensor in an attempt to minimize the differential
between the control voltage for the target flow rate and the flow sensor output voltage
generated by the actual flow rate of gas through the controller.
This process is heavily dependent on the capacity of the gas to heat and cool. Since the heat
capacity of many gases is relatively constant over wide ranges of temperature and pressure,
the flow meter is calibrated directly in molar mass units for known gases (molar mass was
discussed in Section 2.5.3). Changes in gas composition usually only require application
of a simple multiplier to the air calibration to account for the difference in heat capacity
and thus the flow meter is capable of measuring a wide variety of gases.
Flow Control Assemblies for O
Whereas the gas flow rates for the final mixing of gases is controlled directly by the
calibrator's MFCs, under direction of the CPU, other gas flow rates in the calibrator are set
by various flow control assemblies located in the gas stream(s). These orifices are not
adjusted but maintain precise volumetric control as long as a critical pressure ratio is
maintained between the upstream and the downstream orifice. The most important
component of the flow control assemblies is the critical flow orifice.

Critical Flow Orifices

Critical flow orifices regulate stable gas flow rates. They operate without moving parts by
taking advantage of the laws of fluid dynamics: restrict the flow of gas though the orifice
to create a pressure differential. This pressure differential combined with the action of the
calibrator's pump draws the gas through the orifice.
As the pressure on the downstream side of the orifice (the pump side) continues to drop,
the speed that the gas flows though the orifice continues to rise. Once the ratio of upstream
pressure to downstream pressure is greater than 2:1, the velocity of the gas through the
orifice reaches the speed of sound. As long as that ratio stays at least 2:1 the gas flow rate
is unaffected by any fluctuations, surges, or changes in downstream pressure because such
variations only travel at the speed of sound themselves and are therefore cancelled out by
the sonic shockwave at the downstream exit of the critical flow orifice.
The actual flow rate of gas through the orifice (volume of gas per unit of time), depends on
the size and shape of the aperture in the orifice. The larger the hole, the more gas molecules,
moving at the speed of sound, pass through the orifice.
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083730750B DCN8606