Instrumentation

Instrumentation are the measuring devices used in automatic control; sensors and transmitters. It is critical to design, specify and install a measuring device that will obtain the correct measurement of the controlled variable. As these measuring devices not only control the input variable, but they actually change it. To obtain good control it is required to have instrumentation that is reliable, accurate and easy to maintain.

Instrumentation consists of a sensor and a transmitter. The transmitter is not always necessary, as it only provides convenience of the measured variable such as 4 to 20 mA signal, or a field bus. It is possible to connect a sensor directly to a controller, e.g. temperature measurement RTD (Resistive Temperature Detector) may be connected directly to a controller.

When selecting instrumentation the designer should consider the following in instrument selection:

Meets necessary code requirement, e.g. hazardous area instrumentation.
Meets process requirement: accuracy and response time.
Meets costs requirements, easy to maintain and acceptable mean time to failure.

Instrumentation Selection Concepts

Concept	Notes
Safety	Correct materials must be used to prevent chemical leaks, as well correct electrical installation as per appropriate Code requirements.
Accuracy	It is up to the user to determine the necessary accuracy level. Accuracy is the ratio of the error to the full-scale output, generally expressed as percentage of span.
Repeatability

Instrumentation References:

Endress + Hauser

Omega's: Pressure Book