Essay about Evaluation

1409 Words Jan 11th, 2014 6 Pages
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 converts the signal produced by a sensor into a
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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
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

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