System Design Elements
Chapter Three: Overview
94
Example 1: Assume you have two MFCs configured for ratio control and you change the
control set point to 100%. Each MFC has a set point of 50%. The PID control algorithm
determines that full scale flow is required to meet the new set point, so the equation becomes:
MFC control signal = ½ (10 Volts)(50%/100) = 2.5 Volts
Each MFC would receive a 2.5 Volt control signal.
Example 2: If the control algorithm determined that the change necessitates a 60% flow rate, the
equation becomes:
MFC control signal = ½ (6 Volts)(50%) = 1.5 Volts
Each MFC would receive a 1.5 Volt control signal.
Example 3: Assume that you have three MFCs with ratio set points of 10%, 30%, and 60%.
The PID algorithm determines that a 75% control signal is required to meet the control set point.
The 146 instrument calculates the control signal for each MFC:
MFC 1 = ½ (7.5 Volts)(10%) = 0.375 Volts
MFC 2 = ½ (7.5 Volts)(30%) = 1.125 Volts
MFC 1= ½ (7.5 Volts)(60%) = 2.25 Volts
The total control signal is 3.75 Volts (75% of 5 Volts).
Pressure Measuring Control
When using a pressure measuring control channel, up to three MFCs can be configured to
function in a pressure control loop. In this case, the total flow of the three MFCs determines the
pressure, however, the percent of gas through each MFC remains constant. For example, if the
ratio set points for the three MFCs are 30%, 30%, and 40% and pressure rises in the system, the
gas flow through each MFC decreases but the 30%, 30%, 40% relationship remains intact.
Flow Measuring Control
When using a flow measuring control channel, an MFC channel is selected as the control input
channel. The Ratio mode MFCs follow the controlling MFC with their respective proportions of
flow as set by their Ratio mode set points.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
