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Someone once asked a colleague what his occupation was. He replied without hesitation, "I am an instrumentation and control engineer." "And what is that?" asked his interlocutor. "...Oh. Oh ... I'm in trouble," thought the engineer.
To explain what a mechanical, electrical, chemical, or electrical engineer does is relatively easy, but it is another story to concisely describe the work performed by an engineer who specializes in instrumentation and control.
Instrumentation and control are interdisciplinary fields. They require knowledge of chemistry, mechanics, electricity and magnetism, electronics, microcontrollers and microprocessors, software languages, process control, and even more such as the principles of pneumatics and hydraulics and communications.
This is what makes instrumentation and control so interesting and instructive.
In this article and the next, I will give a complete overview of the basic principles of instrumentation and control (I & C) used for the functioning and operation of industrial plants such as those involving oil and gas, pulp and paper, sugar, pharmaceutical products, food, and chemicals.
First, we'll need to cover how to measure, and to measure we need a measurement instrument.
What is a measurement instrument?
A measurement instrument is a device capable of detecting change, physical or otherwise, in a particular process. It then converts these physical changes into some form of information understandable by the user.
When the switch is closed, the resistor generates heat, increasing the temperature of the liquid in the tank. This increase is detected by the measurement instrument and shown on the scale of that instrument.
We can get the information on the physical changes in a process using direct indication or a recorder.
Indication
This is the simplest form of measurement; it allows us to know the current state of the variable.
Recorder
A device that can store data allows us to observe the current state of the variable and how it behaved in the past. A recorder provides us with the history of the variable.
Elements of a Measurement Instrument
Measurement instruments consist primarily of the following parts:
Sensor: This element is a device that experiences changes in its physical properties as a result of changes in the process it's measuring.
Amplifier / Conditioner: Changes detected by the sensor may be very small, so they must be amplified and then conditioned such that they can be properly displayed.
Display: The measured data should be presented in an understandable way. This can be done using a graduated instrument or an electronic display. Sometimes the display additionally acts as a recorder in order to convey the measurement's history or trends.
Usually, the measurement information generated by an instrument must be sent to a control center (or control room) that is physically distant from the instrument. In general, this information must conform to established specifications.
When an instrument has the ability to send information, we call it a transmitter (XMTR).
Classification of Instruments
There are different classifications for measurement instruments. We can classify them, for example, as in-field instruments or panel instruments. The in-field instrument is installed close to the process or measuring point. It must be physically robust if it will be exposed to harsh environmental conditions. Panel instruments are in a controlled-environment room (often a clean space with air conditioning and controlled humidity).
Another classification is pneumatic instruments vs. electrical/electronic instruments.
Pneumatic Instruments
As the name suggests, these are devices that are powered by air.
One of the advantages of these instruments is that they do not consume electricity, so they can be used in areas where it would be dangerous or inconvenient to use electrical power. They work with a single variable, are imprecise instruments, are affected by vibrations and temperature changes, and have high maintenance requirements. The output signal of the transmitters is between 3 and 15 psi, and the maximum transmission distance is approximately 200 meters.
Electrical / Electronic Instruments
Electronic instruments can be divided into three general categories: analog, smart analog, and digital.
Analog:
Output signal: 4 - 20 mA
Transmission distance: 1200 m (typical)
Data for one variable is transmitted
Good accuracy
Easy maintenance
Smart Analog:
Characterization of the sensor as measuring temperature, static pressure, etc.
Excellent accuracy
Self-diagnosis (i.e., the sensor can analyze problems in its own functionality)
One variable
Digital:
Multiple instruments can use a single cable
Transmission of multiple values for each instrument (process variables, calibration, diagnostics, range)
Distance: approximately 1900 m without a repeater
Data capacity is influenced by the mode of transmission (cable, fiber optic, wireless)