There are many classification methods for detection and process control instruments (usually called automation instruments), which can be classified according to different principles. For example, according to the classification of energy used by instruments, they can be divided into pneumatic instruments, electric instruments and hydraulic instruments (rare); According to the instrument combination form, it can be divided into base instrument, unit combination instrument and comprehensive control device; According to the instrument installation form, it can be divided into field instrument, tray instrument and rack instrument; With the development of microprocessor, tuxedo can be divided into intelligent instrument and non-intelligent instrument according to whether microprocessor (device) is introduced into the instrument. According to the form of instrument signal, it can be divided into analog instrument and digital instrument.

According to the functions of recording and indicating, analog and digital, display instruments can be divided into recording instruments and indicating instruments, analog instruments and digital display instruments, among which recording instruments can be divided into single-point recording and multi-point recording (indicating can also be single-point and multi-point), including paper recording or paperless recording, and recording and printing if there is paper recording. The regulating instruments can be divided into base regulating instruments and unit combined regulating instruments. Due to the introduction of microprocessor, there are programmable regulators and fixed program regulators.

The actuator consists of two parts: the actuator and the regulating valve. Actuators can be divided into pneumatic actuators, electric actuator and hydraulic actuator according to energy source, and can be divided into diaphragm actuator, piston actuator (cylinder actuator) and long-stroke actuator according to structural form. Regulating valves are classified according to their structural characteristics and flow characteristics. According to structural characteristics, they are usually divided into straight-through single seat, straight-through double seat, three-way, angular, diaphragm, butterfly, ball valve, eccentric rotation, sleeve (cage), valve body separation, etc. According to flow characteristics, they are divided into straight line, logarithm (equal surface ratio), parabola and quick opening.

This kind of classification method is relatively reasonable, and the coverage of instruments is relatively wide, but any classification method can not classify all instruments into orderly wells, and there is mutual penetration and communication among them. For example, transmitters have many functions, such as temperature transmitter can be classified as temperature detector, differential pressure transmitter as flow detector, pressure transmitter as pressure detector, and level detector if liquid level is measured by pressure method, so it is difficult to exactly classify it, and it is also difficult to merge the calculation and auxiliary units in Chinese and foreign unit combination meters.