Introduction To Typical Applications Of Instrumentation Systems

Prepared By: Dr Sam Sung Ting

Introduction

• An ordinary watch- an instrument to measure time

• Automobile driver needs an instrument panel to facilitate him in driving the vehicle properly

• Automobile Sensor- knock detection, manifold pressure, coolant level, temperature, oil level, brake fluid and fuel level etc.

• Some vehicles are equipped with Micro-Electro-Mechanical System (MEMS) to operate the safety air bags for passenger; Global Positioning System (GPS) for geographical information etc

• Some typical Application

Typical Application of Instrument System

• Measuring of system parameters information

Determine the various parameter/ information of a process or system

Present the information about the condition of the system in the form of visual indication/registering/recording/monitoring/ suitable transmission according to the needs and requirement of the system

• Control of Certain Process or Operation

It is very important to have an accurate measurement system in measuring the performance of a control system

The accuracy of the control system cannot be better than the accuracy of measurement of the control variable

Example of instrument as control device:

Thermostat is fitted in refrigerator to maintain the temperature in certain range

• Simulation of System Condition

Analytical tools like dimentional analysis may be employed to translate the experimental results on the model to prototype

The lift, drag and other relavent parameters of aerodynamic bodies are usually obtained by testing the models in controlled air streams generated in wind tunnel that simulate the flow condition

• Experimental Design Study

The design and development of a new product generally involves trial-and-error procedure which generally involve the use of emprical relations, handbook data, the standard practices etc

We sometimes have to resort to experimental design studies to supplement design and development work.

• To Perform Various Manipulation

The instruments are employed to perform operations like signal addition, substraction, multiplication, division, differentiation, integration etc

Instruments are also use to determine the solution of complex differential equations or other mathematical manipulations.

• Testing of Materials

The material/product need to meet the specific requirements so that they function properly and enhance the reliability of a system.

Example: Air aircraft engine is subjected to extensive endurance tests by the civil aviation authorities as per their specification

• Verification of Physical Phenomena/Scientific Theories

Quite often experimental data is generated to verify certain physical phenomenon

Example: Coulumb postulated that the friction between two dry surfaces is proportional to the normal reaction and is independent of the area of contact. (Coulumb’s Law)

When scientist propose any hypothesis predicting the system’s behavior, it needs to be checked experimentally to put the same on the sound footing

• Quality Control in Industry

It is quite common to have continuous quality control tests of mass produced industrial product.

Enable to discover defective components at early stage of production

Example: X-ray examinationof the plate for defects like blow holes/crack

-- Using metallographic examination to detect the defect.

Fuctional Elements Of a Measurement System

• Measurement System consists of

Basic Functional Element

Auxiliary Functional Elements

• Basic Functional Elements They are basically integral part of all instrument. A) Transducer Element that senses and converts

the desired input to a more convenient and practicable form to be handled by the measurement system

B) Signal Conditioning or Intermediate Modifying Element for manipulating/ processing the output of the transducer in a suitable form

C) Data Presentation Element for giving the information about measurand or measured variable in the quantitative form

• Auxiliary Functional Elements May be incorporated in a particular system depending on

the type of requirement A) Calibration Element to provide in built-in calibration

facility B) External Power Element facilitate the working of one or

more of the elements like the transducer element, the signal conditioning element etc.

C) Feedback Element to control the variation of the physical quantity that is being measured. It is provided in the null-seeking potentiometric or Wheatstone devices to make them automatic or self balancing

D) Microprocessor Element to facilitate the manipulation of data for the purpose of simplifying or accelerating the data interpretation.

Figure 1.3

Example of Identification of Functional Elements

Bourdon tube gauge

Calibration and Standards

Two requirement must be met in order to ensure the results are meaningful

A) The standard that is used for comparison must be well-established, highly accurate and reproducible

B) The measurement devices and the calibration procedures adopted in the act of measurement must have proven reliability

Standard of Measurement

• A Standard measurement is defined as the physical representation of the unit of measurement

• A unit of measurement is generally chosen with reference to an arbitrary material standard or to a natural phenomenon that includes physical and atomic constant

International Standards

• International standards are designed and constructed to the specification of an international forum

• Represents the units of measurement of various physical quantities to the highest possible accuracy that is attainable by the use of advanced technique of production and measurement technology

Primary Standards

• Devices maintained by standard organizations in different part of the world

• Represent the fundamental and derived quantities and are calibrated independently by absolute measurement

Secondary Standards

• Basic reference standard employed by industrial measurement laboratory

• One of the important functions of an industrial laboratory is the maintenance and periodic calibration of secondary standards against primary standards of the national standards laboratory/organization

Working Standards

• There are high accuracy devices that are commercially available and duly checked and certified against either the primary or secondary standard

• Working standards are widely used for calibrating general laboratory instrument, for carrying out comparison measurement or for checking the quality of industrial product.

Calibration

• Calibration is the act or result of quantitative comparison between a known standard and the output of the measuring system measuring the same quantity

• Is in effect the procedure for determining the scale of the measuring system

• If the output-input response of the system is linear, then a single-point calibration is sufficient.

• If the system is non-linear, then a set of known standard inputs to the measuring system are employed for calibrating the corresponding outputs of the system.

Primary Calibration

• When a device/standard is calibrated against primary standard

• After primary calibration, the device is employed as a secondary calibration device.

• Example: Standard resistor/standard cell

Secondary Calibration

• When a secondary calibration device is used for further calibrating another device of lesser accuracy.

• Secondary calibration devices are widely used in general laboratory practice as well as in industry because they are practical calibration sources

• Example: standard cell may be used for calibrating a voltmeter or an ammeter with suitable circuitry

Direct Calibration with Known Input Source

• Is in general of the same order of accuracy as primary calibration

• Devices that are calibrated directly are also used as secondary calibration device

• Example: A flow meter such as a turbine flow meter may be directly calibrated by using the primary measurements such as weighing a certain amount of water in a tank and recording the time taken for this quantity of water to flow through the mater. Subsequently, this flow meter may be used for secondary calibration of other flow metering such as orificemeter or venturimeter

Indirect Calibration

• Indirect calibration is based on the equivalence of two different devices that can be employed to measure certain physical quantity.

• Example: turbine flow meter The requirement of dynamic similarity between two geometrically similar flow meter is obtained through the maintenance of equal Reynold’s number, i.e

𝐷1𝜌1𝑉1𝜇1

=𝐷2𝜌2𝑉2𝜇2

Where subscript 1 and 2 refer to the ‘standard’ and the meter to be calibrated, respectively. Therefore, the discharge coefficients of the two meters are directly comparable

Routine Calibration

• Routine calibration is the procedure of periodically checking the accuracy and proper functioning of an instrument with standard that are known to be accurately reproducible