Instrumentation is an electrical or pneumatic device placed in the field to provide measurement and/or control capabilities for the system.
The simplest measurement instrumentation device is a thermistor. A thermistor is very similar to a typical resistor, except that it greatly varies its resistance depending on its temperature.
Therefore this device can easily be used for measurement of temperature in the field. Other temperature-sensitive devices include RTDs, which also change resistance depending on temperature, and thermocouples, which produce a varying voltage when subjected to heat.Control instrumentation includes devices such as solenoids, Electrically Operated Valves, breakers, relays, etc. These devices are able to change a field parameter, and provide remote control capabilities.
Transmitters are devices which produce an analog signal, usually in the form of a 4-20 mA electrical current signal, although many other options are possible using voltage, frequency, or pressure. This signal can be used to directly control other instruments, or sent to a PLC, DCS, SCADA system or other type of computerized controller, where it can be interpreted into readable values, or used to control other devices and processes in the system.
Instrumentation plays a significant role in both gathering information from the field and changing the field parameters, and as such are a key part of control loops.
Instrumentation engineering is the engineering specialization focused on the principle and operation of measuring instruments which are used in design and configuration of automated systems. They typically work for industries with automated processes, such as chemical or manufacturing plants, with the goal of improving system productivity, reliability, safety, optimization and stability.
Instrumentation technologists, technicians and mechanics specialize in troubleshooting and repairing instruments and instrumentation systems. Installation, on the other hand, is normally performed by industrial electricians. This trade is so intertwined with electricians, pipe fitters, power engineers, and engineering companies, that one can find him/herself in extremely diverse working situations.