Functional Elements of an Instrument

Functional Elements of Instrumentation System.

The Measurement and Control of Various Process Variables that Change at Different Stages of a Process in a Process Plant Is Required for Safe and Correct Operation of a Plant. For This Accurate and Efficient Instruments are Required.

An Instrument Is a Device That Is Used to Measure and Control a Process Variable. An Instrument Converts the Parameter That We Need to Measure into A Readable Format.

 With The Construction of More And More Large Scale Process Plants, the Automatic Operation and Control through Efficient Instrumentation and Automation has become even more necessary. Automatic Control Instruments are Employed to Measure and Control the Temperature, Pressure, Flow and Level of these Utilities. The Aim of the Instrumentation or Automation Is to Improve the Product Quality and Increase the Plant Efficiency for better Safety and better Economic Returns.     

A Quantity that Increases or Decreases Over Time, Or Takes Different Values in Different Situations is Known as a Variable. Variable is Something that Is Changed or Altered in an Experiment.

A Dependent Variable Is any Measurable Factor Whose Behavior is Controlled by another Variable.

An Independent Variable Is any Measurable Factor that Produces Change or Reaction in Another Variable.

When We Decide to Study a Variable We Need to Have Some Sort of a Devise to Measure It. Some Variables Are Easy to Measure and some are Very Difficult. The Values of Variables are Made Meaningful by Quantifying them into Specific Units.

 For Example, Instead of Saying that a Particular Fluid Is Hot, We Can Specify a Measurement and Specify that the Fluid Is Having a Temperature of 100C.

Function of Instruments and Measurement Systems

The Measurement Systems and the Instruments May Be Classified Based Upon the Functions they Perform.

There are Four Main Functions Performed by Them. They are
Indicating, Signal Processing, Recording and Controlling.

   I.  Indicating Function: This Function Includes indicating Information Concerning the Variable Quantity Under Measurement. Several types of Methods Could Be Employed in the Instruments and Systems for this Purpose

II.    Recording Function: In Many Cases the Instrument Makes a Record Of the Value of the Quantity Under Measurement against Time or against Some Other Variable.

III.  Signal Processing: This Function Is Performed to Process and Modify the Measured Signal to Facilitate Recording / Control.

IV.  Controlling Function: This Is One of the Most Important Functions, especially where the Processing Operations are Required to Be Precisely Controlled. In This Case, The Information is Used by the Instrument or the Systems to Control the Original Measured Variable or Quantity.

Thus, Based On the Above Functions, There are Three Main Groups of Instruments. The Largest Group has the Indicating Function.

Next Is the Group of Instruments Which Have Both Indicating and or Recording Functions.

The Last Group Falls into a Special Category and Perform All the Three Functions, ie, Indicating, Recording and Controlling.

2. Functional Elements of a Measurement System

The Operation of a Measuring Instrument or a System Could Be Described in a Generalized Manner in Terms of Functional Elements. Functional Element Indicates Only the Function to be Performed. Physical Elements are the Actual Components or Parts of the System. One Physical Element Can Perform More Than One Function. Similarly, One Function Could Be Performed by More than One Physical Element.

The Main Functional Elements of a Measurement System Are:

I)           Primary Sensing Element

II)         Variable Conversion Element

III)        Variable Manipulation Element

IV)           Data Transmission Element

V)         Data Presentation Element.

Primary Sensing Element

The Quantity or The Variable Which Is Being Measured Makes Its First Contact with The Primary Sensing Element of a Measurement System. The Measurement Is thus First Detected by Primary Sensor or Detector. The Primary Sensing Element Receives Energy from the Measured Medium and Produces an Output That Depends On the Measured Quantity. The Output Signal of the Primary Sensing Element Is a Physical Variable Such as Displacement, Voltage or Current.

Variable Conversion Element

For The Instrument to Perform the Desired Function, It May be Necessary to Convert this Output Signal from the Sensor to Some Other Suitable Form While Preserving the Information Content of the Original Signal. Sometimes, The Output from The Sensor Is Not Suited to The Measurement System.

The Output Signal of the Variable Sensing Element May Be any Kind. It Could Be a Mechanical or Electrical Signal. It May Be a Deflection of Elastic Member or Some Electrical Parameter, Such as, Voltage, Frequency Etc.

In Some Cases, The Primary Sensor and Variable Conversion Element Are Combined, And The Combination Is Known as A Transducer.

In Many Instruments Variable Conversion Element Is Not Required. Some Instruments/Measuring Systems May Require More Than One Element.

Variable Manipulation Element

 Variable Manipulation Means a Change in Numerical Value of the Signal. The Function of a Variable Manipulation Element Is to Manipulate the Signal Presented to This Element While Preserving the Original Nature of the Signal.

 For Example, A Voltage Amplifier Acts as A Variable Manipulation Element. The Amplifier Accepts a Small Voltage Signal as Input and Produces an Output Signal Which Is Also Voltage but of Greater Magnitude.

 The Variable Manipulation Element Could Be Either Placed After the Variable Conversion Element or It May Precede the Variable Conversion Element.

Data Transmission Element

There Are Several Situations Where the Elements of an Instrument Are Actually Physically Separated. In Such Situations It Becomes Necessary to Transmit Data from One Element to Another. The Element That Performs This Function Is Called a Data Transmission Element.

Data Presentation Element

The Function of Data Presentation Element Is to Convey the Information About the Quantity Under Measurement to The Person Handling the Instrument or The System for Monitoring, Control, Or Analysis Purposes. The Information Conveyed Must Be in A Convenient Form.

In Case Data Is to Be Monitored, Visual Display Devices Are Needed. These Devices May Be Analogue or Digital Indicating Instruments Like Ammeters, Voltmeters, Etc.

 In Case the Data Is to Be Recorded, Recorders Like Magnetic Tapes, High Speed Camera and T.V. Equipment; Storage Type C.R.T., Printers, Analogue and Digital Computers May Be Used.

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