|
Measurement devices perform a complete measuring function, from initial detection to
final indication. Two important aspects of a measurement system are the sensor and the
transmitter. A third is the transducer.
Here are definitions of these three terms:
-
Sensor: Primary sensing element
-
Transducer: Changes one instrument signal value to another instrument signal value
-
Transmitter: Contains the transducer and produces an amplified, standardized instrument
signal
|
|
You may wish to refer again to the figure in the
Introduction to clarify the role of sensors, transmitters and transducers. The sensor
is the primary sensing element and exists in close proximity to the process. The sensor
measures the controlled variable in the process and sends a non-standardized signal to the
transmitter.
The transmitter contains a transducer which converts the non-standardized signal of the
sensor into a standardized form that it amplifies. The most common standardized forms are
either 4-20 milliamps or 3-15 psi. If digital signals are being used, the transmitter
converts the signal to digital. If it is a smart device
(which will be covered later) the transmitter may convert the signal into two signals,
both an analog signal and a digital signal, before sending them out. Smart
devices also have the ability to superimpose a digital signal over an analog signal.
Transducers convert one instrument signal value to another, usually a pneumatic signal
to an electronic signal (pressure to current, abbreviated "P to I") or an
electronic signal to a pneumatic signal (I to P). Technically, sensors and transmitters
are transducers because their basic function is to translate a signal. However, to clarify
specifically how they function the various elements are referred to individually (sensor,
thermocouple, transmitter, etc.) and the term "transducer is reserved for a device
that converts I to P or P to I.
|
