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In order to control a dynamic variable in a process, there must be information about the variable
itself. This information is obtained from a measurement of the variable. A measurement
system is any set of interconnected parts that include one or more measurement devices.
Measurement devices such as sensors, or primary elements, measure the variable.
Sensor and Transmitter
This figure highlights the role of the primary element and transmitter in the context
of the measurement system. The system "starts" with the process shown above. The
sensor produces a response representing the value of the process variable. A transducer,
within the transmitter, converts this response to a standard instrument signal (usually
either 3-15 pounds per square inch or 4-20 milliAmps). The transmitter amplifies this
standard signal and sends it to a controller and/or other instruments. Understanding the role of each piece of equipment is important.
This set of notes gives an overview of such measuring systems.
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