Process variable
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A process variable, process value or process parameter is the current measured value of a particular part of a process which is being monitored or controlled. An example of this would be the temperature of a furnace. The current temperature is called the process variable, while the desired temperature is known as the set-point. The set point is usually abbreviated to SP, and the process value is usually abbreviated to PV.
Control system use
[[File:Set-point control[1].png|thumb|Block diagram of a negative feedback control system used to maintain PV = SP]] Measurement of process variables are essential in control systems to controlling a process. The value of the process variable is continuously monitored so that control may be exerted.
Four commonly measured variables which affect chemical and physical processes are: pressure, temperature, level and flow.[citation needed] but there are in fact a large number of measurement quantities which for international purposes use the International System of Units (SI)
The SP-PV error is used to exert control on a process so that the value of PV equals the value of the SP. A classic use of this is in the PID controller.
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