English: Classification of sensors based on measurand, energy/power, physical contact, signal conversion, output signal, comparability, sensor material, specification, and applications (reproduced from White et al.):

(a) Active and passive sensors: Active sensors need an external energy source to operate, for example, microphones, thermistors, strain gauges, and capacitive and inductive sensors. These types of sensors are called parametric sensors (output is a function of the parameter). Passive sensors generate their signals but do not require external energy, for example, thermocouples, piezoelectric sensors, photodiodes. These types of sensors are called self-generating sensors. (b) Contact and noncontact sensors: Contact sensors require physical contact with a stimulus, for example, temperature sensors, while non-contact sensors require no physical contact, such as optical and magnetic sensors and IR thermometers. (c) Absolute and relative sensors: Absolute sensors, such as thermistor and strain gauge, react to a stimulus on an absolute scale. Relative sensors sense the stimulus relative to a fixed or variable reference, like a thermocouple that measures the temperature difference and the pressure that is measured relative to atmospheric pressure. (d) Analog and digital sensor: An analog sensor transforms a measured physical quantity to an analog form (continuous in time). Thermocouples, resistance temperature detectors (RTD), and strain gauge belong to this class of analog sensors. A digital sensor generates output in the form of a pulse. Encoders belong to the digital sensor category. (e) Signal detection: Based on the form of signal detection, sensors can be divided as (i) physical, (ii) chemical, (iii) thermal, and (iv) biological sensors.

 (i) 

Physical sensors: Physical sensors measure a physical quantity and convert it into a signal, which can be identified by the user. These sensors can detect environmental changes, such as force, acceleration, rate of flow, mass, volume, density, and pressure. Physical sensors have been largely employed in the biomedical field, particularly with the advancement of microelectromechanical system (MEMS) technology for developing more precise and compact sensors, along with the development of novel measuring technology.

 (ii) 

Chemical sensors: According to the international union of pure and applied chemistry (IUPAC), a chemical sensor is defined as, “a device that converts chemical information into an analytically useful signal ranging from the concentration of a particular sample component to total composition analysis.” Chemical sensor is employed to monitor the activity or concentration of the respective chemical species in the gas or liquid phase. They are also used for environmental pollution monitoring, food and drug analysis, and assay monitoring of organophosphorus compounds. They can also be used for clinical diagnostic purposes.

 (iii) 

Thermal sensors: A thermal sensor is a device that is used to measure the temperature of an environment and transforms the input data into electronic data to record or monitor signal of temperature changes. Examples of temperature sensors include thermocouples, thermistors, and RTDs.

 (iv) 

Biological sensors: Biological sensors monitor biomolecular processes, such as antibody/antigen interactions, DNA interactions, enzymatic interactions, or cellular communication processes. Biological sensors can be referred to as biosensors in short form.