Introduction to measurements systems

classification of measuring instruments,generalised system ,errors,types of errors

1. MECHANICAL MEASUREMENTS AND INSTRUMENTATION UNIT-1

2. INTRODUCTION Measurement :- It is an act or result of the quantitative comparison between a predefined standard and an unknown quantity. The physical quantity which is to be measured is termed as Measurand.

3. Significance or importance of Measurement: 1)To determine the true value of parameter measurement is important. 2) To convert any physical parameter into numerical value. 3) It is the fundamental basis for research and development. 4) Measurement is the fundamental element of any operation. 5) To evaluate and find out the performance of any system measurement is important. 6) Measurement is an essential part of the development of technology.

4. Instrument: A device or mechanism used to determine the present value or magnitude of a quantity or variable. The technology of using instruments to measure and control physical and chemical properties of materials is called Instrumentation. Methods of Measurement: There are 2 basic methods of measurements available 1.Direct method 2. Indirect method 1.Direct method: If the unknown quantity is directly compared against a standard then the method is called direct method. The physical standards like mass, length and time are measured by direct measurement. 2. Indirect method: The value of the physical parameter is more generally determined by indirect comparison with the secondary standards through calibration. The measurement is converted into an analogous signal which subsequently process and fed to the end device at present the result of measurement.

5. Classification of measurements: The complexity of an instrument system depending upon measurement being made and upon the accuracy level to which the measurement is needed. Based upon the complexity of the measurement systems, the measurement are generally grouped into three categories. 1.Primary measurement 2.Secondary measurement 3.Tertiary measurement 1.Primary measurement : In the primary mode, the measurement of an unknown quantity is made by direct observation i.e. a physical parameter is determined by comparing it directly with reference standard. Ex: Measuring length of an object by comparing with standard scale. 2.Secondary measurement: In this method of measurement the output is obtained by one conversion of the measurand.

6. Ex: The conversion of pressure and force into displacement. 3.Tertiary measurement: In this method of measurement the output is obtained by two conversions of the measurand. Ex: The measurement of static pressure by boundary tube pressure gauge is a typical example of tertiary measurement. CLASSIFICATION OF MEASURING INSTRUMENTS:- 1.Manual and Automatic instruments 2. Self generated and power operated instruments 3. Self contained and remote indicating instruments 4. Null and Deflection type instruments 5. Analog and digital type instruments 6. Contact and non-contact type instruments 1.Manual and Automatic instruments: The instruments which require the services of an human operator is Referred as Manual instruments. Ex: Ordinary balancing method, Resistance thermometer used for measurement of temperature.

7. The instruments which do not require the services of human operator is referred as Automatic instruments, the manual operation is replaced by an auxiliary device incorporated in the instrument. Ex: Digital weighing machine, Liquid in glass thermometer to measure temperature. 2. Self generated and power operated instruments: The instruments which do not require any external energy source for their operation are referred as self generated or active Instruments. In this the output is supplied entirely by the input signal. Ex: Mercury in glass thermometer, bourdon pressure gauge. The instruments which require an auxiliary power source(electrical supply, compressed air, hydraulic supply etc) for their operation are referred as power operated or passive instruments. Ex: L.V.D.T(Linear Variable Differential Transducer),Strain gauge load cell, Resistance thermometer and the mister.

8. 3.Self contained and remote indicating instruments: The different elements of a self-contained instrument are contained in one physical assembly. In a remote indicating instrument, the primary sensing element may be located at a sufficiently long distance from the secondary indicating element. In the modern instrumentation technology, there is a trend to install remote indicating instruments where the important indications can be displayed in the central control rooms. 4. Null and Deflection type instruments: In null-type instruments, the physical effect caused by the quantity being measured is nullified (deflection maintained at zero) by generating an equivalent opposing effect. The equivalent null causing effect then provides a measure of the unknown quantity. Ex: Measurement of weight by ordinary balance A deflection type instrument is that in which the physical effect generated by the measured quantity (measurand) is noted and correlated to the measurand.

9. Ex: Speedometer of a vehicle 5. Analog and digital type instruments: The instruments which provides their output in analog form which is a continuous fashion over a given range are called analog instruments. Ex: Wrist watch, speedometer of an automobile, fuel gauge, ammeters and voltmeters are examples of analog instruments. The instruments which provides their output in digital form are called as digital instruments. Ex: Digital voltmeter, digital revolution counter. 6. Contact and non-contact type instruments: If the instrument is kept in direct contact with measuring medium to be measured is known as contact type instrument. Ex: Tachometer, Thermometer. If the instrument is located at a distance from measuring medium to be measured is known as Non contact type instrument. Ex: Optical pyrometer, inductive pick up speedometer.

10. Generalized measurement system and its functional elements:- Most of the measurement systems will have the following 3 stages A Detector-Transducer stage (or) Sensor-Transducer stage. An Intermediate stage (or) Signal conditioning stage. Terminating stage (or) Read out-Recording stage.

11. 1.A Detector-Transducer stage (or) Sensor-Transducer stage: In this stage the physical variable is detected and the signal is converted into more usable form either by means of mechanical or electrical transducer. In this stage there are 2 elements: a)Primary sensing element b)Variable conversion element a)Primary sensing element: It receives energy from the measurand and produces an output depending on the measured quantity. primary sensing element is some physical variable such as displacement or voltage. b)Variable conversion(or)Transducer element: An element that converts the signal from one physical form to Another more suitable form without changing the information content of the signal. 2. An Intermediate stage (or) Signal conditioning stage: In this stage the transduced information is modified so that it is acceptable to the terminating stage. In addition, this stage also performs filtering to avoid noise, integration.

12. In this stage there are 3 elements: a) Variable Manipulation element b) Data Transmission element c) Data Processing element Variable Manipulation element: It modifies the direct signal by amplification, filtering ,Arithmetic conversion of data etc., so that a desired output is produced . Amplification: It means increasing the amplitude of signal without affecting its waveform. Signal Filtration: It means the removal of unwanted noise signals that obscure the transducer signal. Data Transmission element: It is necessary to transmit the data from one location to another location when functional elements of an instrument are physically separated. c) Data Processing element: An element that modifies the data before it is displayed or recorded is called Data processing element. It also performs repeated calculations that involve addition, Subtraction, integration etc.

13. 3. Terminating stage (or) Read out-Recording stage: This stage acts to indicate, record or control the variable being Measured. The output may be represented either in analogous form or digital form. This stage contains Data Presentation element. Data Presentation element: It provides record or indication of output from the signal processing element. Auxiliary elements: In addition to the basic elements a measurement system may also contain auxiliary elements some are: 1. Calibration element: This provides a built in calibration facility. 2. External power element: This element facilitates working of some basic elements which require external power. 3. Feedback element: This element facilitates the experimenter to control the variation of the physical quantity that is being measured. 4. Micro processor element: To facilitate the manipulation of data.

14. Bourdon Tube pressure gauge:

15. Working: Bourdon Tube represents the detector transducer stage as it senses the pressure and converts into displacement of tube end Mechanical linkage represents the data transmission elements as it connects the tube end to the gearing arrangement where the tube displacement gets amplified Gearing element acts as variable manipulation element since it amplifies the displacement of the tube Pointer and dial represents the data presentation stage .when the dial is calibrated the pointer movement represents the pressure. Dial Indicator: Dial indicators  are instruments used for making and checking linear measurements. Most of the  dial indicators  take the form of a circular or semicircular scale upon which a pointer gives a direct indication of the movements of a contact arm or spindle.

16. Working: Initially power is given to spindle and there the spindle senses the motion as primary sensing element. Then the Wheel train which is connected to spindle will rotate which acts as transmission element And there by pointer shows the value on the scale that is presentation element

17. Proving ring: The proving ring is a device used to measure force. It consists of an elastic ring of known diameter with a measuring device located in the center of the ring. Spring Balance: The  spring balance  is simply a spring  fixed at one end with a hook to attach an object at the other. It works by Hooke's Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position.

18. Precision: Precision is the repeatability of measuring process. It refers to the group of measurements for the same characteristics taken under identical conditions. Accuracy: Accuracy is the degree to which the measured value of the quality characteristic agrees with the true value. Error: The difference between the true value and measured value is known as Error of measurement. ⸫ Error = True value – Measured value. The error in measurement is expressed or evaluated as Absolute error (or) Relative error Absolute error: It can be expressed by 2 ways True absolute error: It is the algebraic difference between the result of measurement and the conventional true value of the quantity measured. Apparent absolute error: If the series of measurement are made then the algebraic difference between one of the results of measurement and the arithmetical mean is known as apparent absolute error.

19. Relative error: The  relative error  is the  absolute error  divided by the magnitude of the exact value. Sources of Error: 1) Calibration of Instrument 2) Instrument reproducibility 3) Measuring arrangement 4) Work piece 5) Environmental condition 6) Observer skill 1) Calibration of Instrument: For any instrument calibration` is necessary before starting the process of measurement. When the instrument is loaded frequently for long time, the calibration of instrument may get disturbed. The instrument which is gone out of calibration cannot give actual value of the measured. Therefore the output produced by such an instrument have error. Therefore this error can be eliminated by, properly calibrating the instrument at frequent intervals.

20. 2) Instrument reproducibility: Though an instrument is calibrated perfectly under group of conditions, the output produced by that instrument contains error. This occurs if the instrument is used under those set of conditions which are not identical to the conditions existing during calibration. i.e., the instrument should be used under those set of conditions at which the instrument is calibrated. This type of error may occur systematically or accidentally. 3) Measuring arrangement: The process of measurement itself acts as a source of error if the arrangement of different components of a measuring instrument is not proper. Ex: Actual value of length is obtained when measuring instrument and scale axes are collinear, and any misalignment of these will give error value. Hence this type of error can be eliminated by having proper arrangement of measuring instrument.

21. 4) Work piece: The physical nature of object (work piece) i.e., roughness, softness and hardness of the object acts as a source of error. Many optomechanical and mechanical type of instruments contact the Object under certain fixed pressure conditions. Since the response of soft and hard objects under these fixed conditions is different the output of measurement will be in error. 5) Environmental condition: Changes in the environmental conditions is also a major source of error. The environmental conditions such as temperature, humidity, pressure, magnetic or electrostatic field surrounding the instrument may affect the instrumental characteristics. Due to this the result produced by the measurement may contain error. There errors are undesirable and can be reduced by the following ways, i) Arrangement must be made to keep the conditions approximately constant. ii) Employing hermetically sealing to certain components in the instrument, which eliminate the effects of the humidity, dust etc

22. iii)Magnetic and electrostatic shields must be provided. 6) Observes skill: It is a well-known fact that the output of measurement of a physical quantity is different from operator to operator and sometimes even for the same operator the result may vary with sentimental and physical states. One of the examples of error produced by the operator is parallax error in reading a meter scale. To minimize parallax errors modern electrical instruments have digital display of output . Classification of errors : During measurement several types of error may arise these are: Static error which includes: a) Reading error b) Characteristic error c) Environmental error 2. Instrument loading error 3. Dynamic error

23. Static error: This errors result from the physical nature of the various components of measuring system. It remains constant during the operation of the system. There are 3 basic sources of static error: a) Reading error b) Characteristic error c) Environmental error Reading error: This type of error is related with the display devices of measurement system. It includes parallax error, interpolation error. Characteristic error: This type of error is related to characteristics of instrument such as accuracy, precision, linearity etc Environmental error: The environmental conditions such as temperature, humidity, pressure, magnetic or electrostatic field surrounding the instrument may affect the instrumental characteristics. Instrument loading error: It is the difference between the value of measurand before and after the measuring system is connected or contacted for measurement.

24. Dynamic error: It is the error caused by time variations in the measurand. It is caused by inertia, damping, friction or other physical parameters in the sensing or processing or display system. For statistical study these errors are classified into 2 types: a) Systematic error b)Random error a) Systematic error(Controllable error): These are regularly repetitive in nature. They are constant and they result from improper conditions or procedures. This type of error can be controlled easily hence this is also known as controllable error. b) Random error: These are non consistent. They occur randomly and are accidental in nature. It is difficult to eliminate such errors. Ex: a) operator error in scale reading b) Small variations in the position of setting standard and workpiece .

25. ⸫ True Absolute error =True value ~ Measured value (or) | Measured value – True value | ⸫ Apparent absolute error = (X - x̅  ), where x̅ =∑ x i  / n = (x 1  + x 2  + …. + x n ) / n ⸫ Relative error = absolute error / Real value