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The sensor as the first link of a measuring chain serves for the conversion of the physical measured quantity (e.g. pressure, distance, temperature) into an electrically measurable quantity (e.g. voltage, resistance). This is done by taking advantage of various electrical and / or physical effects.

In the following, we want to explain which physical quantities can be measured and which methods are applied.
 

INDIRECT MEASUREMENTS

With measuring systems you can only measure few quantities directly (e.g. current, voltage). Therefore physical laws including a current or voltage relation are applied (indirect measurements). A resistance measurement for example is done by connecting a resistor to a constant current source and measuring the voltage drop. The resistance results from the division of the measured voltage through the applied current (This principle is also used by multimeters.).
 

SENSOR ADJUSTMENT

As you see sensors "translate“ physical into electrical quantities. To connect them to a measuring system, the output must correspond in unit and codomain with the input of the measuring device. This is done by electrically transforming and / or amplifying the signal of the passive sensor.
 

ACTIVE SENSORS

In many sensors this electronic function is included already and they need their own power supply. Those sensors are called active sensors, as they convert the measured quantity into a measurable electrical quantity themselves.