For a process to be adequately controlled and manipulated, the variable of interest in the process (e.g. Temperature, Pressure or Flow) often called the Process Variable (PV) needs to be measured by a sensor which converts the measurement into a suitable signal format (4 – 20mA or digital) and then transmit it to a controller which makes the control decision and finally acts on a final control element in the control loop. What does this signal transmission is referred to as a transmitter. The schematic below illustrates the interactions between all the elements in the control loop:
Elements of a Process Control Loop |
What is a Transmitter?
A Transmitter is a device that ...view middle of the document...
3psig corresponds to the lower range value (LRV) and 15psig corresponds to the upper range value (URV). It is still commonly used today especially in remote locations where electric power is not readily available.
The invention of electronic instruments in the later part of the twentieth century significantly brought down the costs involved in running electrical signal wire through a plant as opposed to running pressurized air tubes. This has made the pneumatic signal technology less popular.
As shown below, a pneumatic pressure transmitter is supplied with air pressure typically 20 – 30 psig depending on the application. Process pressure is applied to the High port of the transmitter. As the process pressure varies, the transmitter produces an output signal (3 -15psig) that is proportional to the process pressure.
Pneumatic Type Pressure Transmitter |
Analog transmitters are mostly electronic in nature. They output an electrical signal (current or voltage) whose magnitude represents a physical measurement or a control quantity. The transmitter is classified as being analog by virtue of the fact that it uses an analog signal standard to communicate information. The most common standard for transmitting an analog signal is the 4-20 mA current signal. With this signal, a transmitter sends a small current, proportional to the physical measurement, through a set of wires. In this signal standard, 4mA represents the lowest possible measurement or the LRV (Lower Range Value) while the 20mA represents the highest possible measurement or URV (Upper Range Value).
As shown below, the transmitter produces an output signal of 4 – 20mA when the process variable is applied to the transmitter:
Analog Transmitter |
Digital transmitters produce digital signals that are combined in a variety of ways to enhance communication with the devices; enhances diagnostic capabilities of the device and makes control of the devices and processes relatively easy and smooth. Digital signals are discrete levels or values that are combined in specific ways to represent process variables and also carry other important information, such as diagnostic information. Digital transmitters combine the digital signals in a variety of ways leading to various communication protocols such as Fieldbus, HART etc.
Most digital transmitters may be referred to as smart instruments. They have inbuilt microprocessors that helps in signal conditioning and processing and gives the devices some diagnostic capability.
Electronic transmitters can be grouped according to the number of wires necessary to provide transmitter power. Accordingly, there are 2-wire, 3-wire and 4-wire transmitters. Please see Electronic Transmitters Wire Configuration for more details on transmitter wire configuration.
Transmitters Used in Process Instrumentation
In the process industries, there four...