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Nowadays transducers are essential in both automation and control
systems. Transducers are needed when physical quantities are used for a process
control. The reason is that it is necessary to record a large number of
quantities. In addition to the most common quantities as temperature or
pressure, there are other parameters such as pressure, gas content or flow. For
electronics to receive a readable signal, transducers must convert the
physical quantity to an electrical signal. To ensure flexibility, normalized
signals have been determined to be read by many meters.
Here you can see the Transducers
functioning:

The use of an analog normalized signal allows transducers for any
quantity to be connected to the same analog input of a digital display. The
digital display should only be scaled, this means that digital display should
indicate the value from transducers as an electrical quantity corresponding
to a physical quantity value. The next image shows the assignment of a 4-20mA
output signal to a temperature range of 0 - 100 ºC:

Some transducers have a flexible
scale which allows the users to adjust the measurement range according to
their needs. Apart form its flexible use, the normalized signal of 4-20 mA
has other advantages such as: connection cables from one device to another
can be very long, it is possible to add diverse units of analysis to the same
circuit, so a current signal from the transducer can be used in different
places. As even with the lowest measurement value current flows, users can
use the 4 mA to feed sensors. Current below 4mA indicates that there is
something wrong with the analysis units. In order to read the measurement
value in situ, some transducers have a display. Other transducer models have
relays which make a contact when limit values are exceeded.
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