Sensors and transmitters are concepts originally related to thermal instruments. A sensor is a general term for any device or apparatus that can measure a regulated quantity and convert it into a usable output signal according to certain rules; it typically consists of a sensitive element and a conversion element. When the output of a sensor is a regulated standard signal, it is called a transmitter.
A transmitter is an instrument that converts non-standard electrical signals into standard electrical signals. A sensor is a device that converts physical signals into electrical signals. Previously, we often referred to physical signals, but now other signals are also considered. Primary instruments refer to field measuring instruments or central control instruments, while secondary instruments refer to instruments that use primary instrument signals to perform other functions, such as control or display.
Sensors convert non-electrical physical quantities such as temperature, pressure, liquid level, material properties, and gas characteristics into electrical signals, or directly transmit physical quantities such as pressure and liquid level to transmitters. Transmitters, on the other hand, amplify the weak electrical signals collected by sensors for transmission or to control elements. Alternatively, they can convert non-electrical quantities input from sensors into electrical signals and amplify them for remote measurement and control. Analog quantities can also be converted into digital quantities as needed.
Sensors and transmitters together constitute the monitoring signal source for active control. Different physical quantities require different sensors and corresponding transmitters. There is also a type of transmitter that does not convert physical quantities into electrical signals, such as a "differential pressure transmitter" for boiler water level gauges. This transmitter sends water from the lower part of the level sensor and condensate from the upper part of the steam through an instrument tube to both ends of the transmitter's bellows. The differential pressure between the two ends of the bellows drives a mechanical expansion device to indicate the water level using a pointer—a remote instrument. Of course, there are also transmitters that convert electrical analog quantities into digital quantities.
