Chapter Three: Overview How The 649 Pressure Controller Works
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How The 649 Pressure Controller Works
The 649 controller compares the pressure reading to the set point, and positions the valve to
maintain, or achieve, the set point pressure. The controller functions as a PI (Proportional-
Integral) controller. Both the Proportional (P) term, and the Integral (I) term have adjustable dials
on the top of the 649 controller.
Downstream pressure control is defined as having the process chamber located downstream of
the 649 controller. Therefore, the 649 controller controls the pressure of the process chamber
located downstream.
Example
Assume that your 649 controller is positioned for downstream control. The 649 controller is
positioned before the controlled pressure volume so it will regulate the pressure of the gas
entering the pressure system. Figure 5, page 17, shows the correct location for the 649 controller.
When the actual pressure reading is less than the set point value, the 649 controller opens the
valve to increase the amount of gas entering the system. As the valve opens, gas enters the
pressure system, so the pressure rises to meet the set point value.
When the actual pressure reading is greater than the set point value, the 649 controller closes the
valve to decrease the amount of gas entering the system. As the valve closes, less gas enters the
pressure system, so the pressure drops to meet the set point value.
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