Process variable
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. 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. 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.