In the world of process control, Transmitter is a device that converts sensor-generated signal into a standard instrumentation signal representing a process variable being measured and controlled.
Pneumatic vs electrical signal
In the early days of process control, the standard instrumentation signal was pneumatic signal where today it is likely to be an electric signal.
- The typical pneumatic signal is 3 to 15 psi.
- Typical electrical signals are 1 to 5 volts or 4 to 20 mA.
In process control, it is understood and goes without saying that the transmitter output range represents the 0 to 100% of the sensed physical variable. For example, the transmitter would produce an output current range of 4 to 20 mA for a measured temperature range of 0 to 300 degrees Fahrenheit (0 to 100%).
Let’s check where the transmitter fits into a process control loop. As already stated, the Transmitter converts the signal from the sensor to the Process Variable (PV) signal which represents the physical measured variable. The Controller is the device that looks at the difference between the Process Variable (PV) and the Set-point (SP). The Controller then determines what action to take place and generates an output signal that is a function of the result of this comparison. Controllers are either a DCS or a PLC in process control today. The Final Actuator is the device such as a valve that exerts a direct influence on the process as directed by the controller.
Types of transmitter
The four major process variables measured and represented by a transmitter are Pressure, Level, Temperature, and Flow. Transmitters are also used in industry to measure other variables such as Position and Speed and chemical properties such as pH and Conductivity.
4-wire and 2-wire transmitters
A 4-wire transmitter has 2 wires connected to a power supply and 2 signal wires connected to the PLC. The power supply can be AC or DC depending on the vendor and model. A 2-wire transmitter has only 2 wires. These 2 wires provide power for the transmitter and are also the signal lines!
Smart Transmitters
Smart Transmitters not only produce the 4 to 20 mA process variable signal, but also transmit and receive digital information such as Instrument Tag Names, Calibration Data, and Sensor Diagnostics. Protocols such as HART are commonly used on Smart Transmitters.
Summary
- In the world of Telecommunications, Transmitter is a device that produces radio waves from an antenna.
- Transmitter in process control is a machine that converts a signal generated by a sensor into a standard instrumentation signal representing process flexibility measured and controlled.
- The typical pneumatic transmission signal is 3 to 15 psi.
- Four major variables measured and represented by the sender pressure, level, temperature and flow.
- Instrumentation Transmitters can be connected to 4-wire or 2-wire configuration.
