Sensors and transducers are devices that used to measure physical variables in a process industry. There is no hard rules to distinguish the transducer and sensor.
Definition:
As a definition for sensor and transducer for study purpose we can say:
Sensor: An electrical sensor is a device that converts a quantity or an energy form into an electrical output signal. The form of that output signal, whether it is an ac or a dc signal, whether the output is a change in resistance or a change in capacitance, whether the signal is digital or analog, to name just a few, is determined by the sensor’s electromechanical and/or chemical makeup.
Transducer: An electrical transducer is a device comprised of a sensor whose output signal is modified or conditioned to suit a particular application or need by its user. Furthermore, that transducer may have an on board power source for its proper operation.
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The transducer uses a sensor as one of its many possible operating elements, while a sensor does not have independent internal operating elements, only the sensing element itself.
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The transducer output is depended on the sensor value produce the quantity or energy conversion.
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The transducer is capable of modifying the sensor’s output signal to convert that signal into a more desirable form. To understand the working of the transducer, a circuit for the photoresistive transducer is shown:
The sensor element is shown inside the transducer assembly.
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Here the sensor’s output is in the form of a changing dc resistance, rest of the circuit conditions the signal for the transducer.
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This particular transducer uses amplifiers to convert the resistance to these quantities.
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The noise filters that are shown have the job of "stripping off’ or diminishing any electrical noise that might be present on the modified signal coming from the sensor.
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The transducer has a very predictable output based on the energy input or quantity it is converting. This product can be measured and quantified, usually in very precise quantities. In other words, the transducer can be calibrated
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The transducer can be precisely calibrated if desired and, for the most part, are easily transportable.