Instruments are used to monitor and control a process to keep the process within a correct limit. The parameters or quantities that we wish to control at the correct limit are called Process Variables.A variable is something that can vary or change. Because process variables can and do change, instrumentation systems measure the variable then control the variable to keep the variable within the given limits.
Pressure, Temperature, Flow are the main three process variables in industries. The level is an another process variable in industries.
Pressure variable:
Pressure is a term used to describe the amount of force applied to a specific unit area. Below shows a flow channel of fluid from a compressor.
If you open the block valve, air is released to the atmosphere, so the pressure inside the receiver decreases. If you close the control valve, you keep the air inside the receiver and the pressure increases.The process variable that we are controlling is called PRESSURE.
Temperature variable:
Temperature is a measurement of how hot or how cold an object is. Below figure shows a temperature control unit.
Water in the vessel is heated by the heat exchanger. Measuring the temperature the steam flow to the exchanger is controlled. Temperature changes when the amount of steam is changed. The process variable is called Temperature.
Flow variable:
Flow is the movement of fluid inside a pipe in a given direction. Figure below shows the flow control loop.
A valve in the flow channel controls the flow through the pipeline. The greater the valve opening the greater the water flow inside the pipe. Here we are controlling the flow the fluid passing through the pipeline, so the process variable is flow.
Level variable:
The level in the tank change up and down depending up the fluid in the tank. A control valve is installed in the output line to control the level of the tank.
The process variable is level. You can change the level by closing or opening the control valve. If you close the valve, the level increases. If you open the valve, the level decreases.