Sensor is a device that detects and responds to some type of input from the physical environment. The specific input could be light, heat, motion, moisture, pressure, or any one of a great number of other environmental phenomena. The output is generally a signal that is converted to human-readable display at the sensor location or transmitted electronically over a network for reading or further processing. 

For the sensor, the input can be divided into static and dynamic quantities according to the input state.We can obtain the static characteristics of the sensor based on the relationship between the output quantity and the input quantity according to the stable state of each value.The main indexes of the sensors' static characteristics are linearity, hysteresis, repeatability, sensitivity and accuracy.The dynamic characteristics of the sensor refer to the response characteristics of the input quantity over time.Dynamic characteristics are usually described by automatic control models such as transfer functions.

Criteria to choose a Sensor

There are certain features which have to be considered when we choose a sensor. They are as given below:

1.  Accuracy

2.  Environmental condition - usually has limits for temperature/ humidity

3.  Range - Measurement limit of sensor

4.  Calibration - Essential for most of the measuring devices as the readings changes with time

5.  Resolution - Smallest increment detected by the sensor

6.  Cost

7.  Repeatability - The reading that varies is repeatedly measured under the same environment