Transducers, Transmitters, and Sensors
Pressure transducer, pressure transmitter, and pressure sensor are often used interchangeably. This is not entirely correct. First, let’s look at the differences between pressure transducers and transmitters. In its most basic form, a transducer is a device used to measure pressure, load, force, or other factors, and convert the reading into an electronic signal. A transmitter also converts a reading into an electrical signal, but subsequently amplifies, modifies, and sends the signal to another receiver.
In a pressure transducer, a thin-film or piezo resistive pressure sensor is mounted on a process connection. Piezo resistance refers to a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. The transducer converts pressure into an analog output signal, usually in the form of a millivolt (mV) per volt output. Transducer signals are not linearized or temperature compensated. A pressure transmitter, on the other hand, has additional circuitry that linearizes, compensates, and amplifies the signals from a transducer. The varying signal types are usually voltage signals (0-5 or 0-10 volts), milliamp (4-20 milliamp) or digital. The transmitter can then communicate the signals to a remote receiver.
While the two devices have distinct differences, it isn’t necessarily critical that people use one or the other. The most important factor to consider is which device better suits the intended application and provides the necessary characteristics for optimal operation. When deciding which instrument to use, factors such as accuracy, range, working temperature, and medium are all important to consider. In terms of the output signal, there are also important factors to take into account:
- Typical mV outputs do not have temperature characterization.
- A current signal has higher resistance to interference and noise than a voltage signal.
- Current signals can travel farther than voltage signals.
- Analog signals are just pressure readings.
- A digital signal allows users to collect more information than just pressure.
- The input card of many control systems accepts only amplified signals.
Much like the comparison of transmitters and transducers, sensors and transducers are also often mistaken for one another. Again, both ‘sensor’ and ‘transducer’ can refer to the same product, such as in the case of linear sensors and linear transducers. However, in most cases, sensors and transducers have different meanings. The primary difference is related to the output signal. Sensors and transducers are both used to sense a change within an environment, but a sensor will give an output in the same format, while a transducer will convert the measurement into an electrical signal.
A sensor is defined as a device used to measure a physical quality such as light, sound, or temperature and give the output in an easy-to-read format. A mercury thermometer is an example of a sensor, in that the mercury simply expands when the temperature rises, providing a reading for the user. There is no electrical interference or change. While a transducer can similarly carry out the task of measuring physical qualities, it will also convert the signal from a physical form to another, meaning the input and output differ from each other. Examples of sensors include pressure switches, thermistors, mercury thermometers, and motion sensors. Examples of transducers include pressure transducers, cable extension transducers, linear transducers, and microphones.