In metrology, a sensor is a sensitive device placed in contact with the physical quantity. It represents the compatible element and able to collect information from the physical quantity in the measurement environment. Each sensor is generally sensitive to several physical quantities, but its change of state is definitely more significant in relation to a specific quantity for which it has been realized, the others, unfortunately, produce undesired influence variations of the quantity to be measured.
A measurement chain can also use more than one sensor, thus generating a multisensory system. The sensors can interact with the measurement system either directly or as a stimulus. In the direct mode, the sensor is sensitive to an intrinsic physical quantity of the measurement system: for example, a thermocouple is temperature-sensitive. In stimulus mode, the sensor is sensitive to the response of the measurement system when a suitable source stimulates it.