Instrumentation is the basis for process control in industry. However, it comes in many forms from domestic water heaters and HVAC, where the variable temperature is measured and used to control gas, oil, or electricity flow to the water heater, or heating system, or electricity to the compressor for refrigeration, to complex industrial process control applications such as used in the petroleum or chemical industry. In industrial control a wide number of variables, from temperature, flow, and pressure to time and distance, can be sensed simultaneously. All of these can be interdependent variables in a single process requiring complex microprocessor systems for total control.
