30 / N.S.D. Basic Mechanical Engineering 2 MEASUREMENT Measurement is the determination of magnitude or size of a physical quantity. It is a comparision of the given physical quantity with standard or reference quantity called unit. Measurement is carried out with the help of measuring instruments like vernier calliper, micrometer, thermometer etc. Methods of Measurement : 1. Contact Method : In this type of measurement the measuring instrument make physical contact with the object being measured. Example : Vernier calliper, Screw gauge etc. 2. Non-Contact Method : In this method measuring instrument does not make contact with the object to be measured. Example : Sensors, optical and non-optical instruments. 3. Direct Method : In this method the given quantity is measured by directly comparing it with a standard. Example : Measuring length with graduated scale. 4. Indirect method : In this method the given quantity is measured in some other units and then it is used to calculate the required dimensions. Example : By measuring external dimensions of object, we can find Mass, Volume, Density etc. Measuring Instruments : These are the devices which transform the measured quantity or related quantity into an indication or information. They are used to measure quantities like Length, Flatness, Angle, Surface finish etc. Characteristics of measuring instruments : 1. Accuracy : Accuracy of an instrument means closeness to the true value. 2. Least Count : Least count of an instrument is the smallest value that can be measured by it. Example : Normal scale has least count of 1mm 3. Stability : It is capability of instrument to retain calibration. 4. Sensitivity : It is capacity of instruments to identify smallest variation of the quantity of interest. 5. Range : It is the capacity or total range of values which can be measured by the instrument. Example : micrometer of 50-75 mm range. Temperature : Temperature is the degree of hotness or coldness that can be measured using a thermometer. It is also a measure of how fast the atoms and molecules of a substance
Measurement / 31 are moving. Temperature is measured in degrees on the Fahrenheit, Celsius and Kelvin scales. Thermometric Property : It is the measurable property that changes with respect to temperature change. Example : In Mercury thermometer, in this thermometer the thermometric property is length (Expansion or contraction). With rise in temperature the mercury level rises in the glass, i.e. change in length. Measurement of Temperature : The temperature measurement means measurement of any one of thermometric property of the sensing device. The thermometric property may be pressure, volume, resistance, length, brightness, etc. Following are the temperature measurement devices having different thermometric property. S. No. Thermometer Thermometric Property 1. 2. 3. 4. 5. 6. 7. Mercury thermometer Constant volume gas thermometer Constant pressure gas thermometer Thermocouple Radiation pyrometer Optical pyrometer Electrical resistance thermometer Length Pressure Volume Electrical potential Radiation energy Brightness Resistance Pressure : It is the force exerted by the fluid on unit area perpendicular to it. Pressure = Force Area It is usually expressed in Pascal (Pa = N/m2) Measurement of pressure : Many techniques have been developed for the measurement of pressure. Instruments used to measure pressure are called pressure gauges. Pressure is also measured by instruments like Piezometer, Manometers, Bourdon Pressure gauge, Pressure transducers etc. 1. Manometers : Manometers are the devices which are used for measuring the pressure at a point in a fluid by balancing the column of liquid by another column of liquid. They are classified as : (i) Simple Manometer : (a) Piezometer (b) U-tube manometer (c) Single column manometer Piezometer P. h Pipe Pipe h Liquid .P Liquid or water Fig. 2.1. Manometer
32 / N.S.D. Basic Mechanical Engineering (ii) Differential manometers : P = ρ gh ρ = density of liquid h = height of column P = pressure of liquid 2. Mechanical gauges : In this type of devices the pressure is measured by balancing the fluid column by spring (elastic element) or dead weight. They are generally used to measure high pressure. Following are the commonly used mechanical gauges : (i) Bourdon pressure gauge. (ii) Dead-Weight pressure gauge. (iii) Diaphragm pressure gauge. Where, Velocity measurement : Velocity is defined as the rate of change of displacement of the object with time. Velocity = Displacement Time Velocity is a vector quantity as it has both magnitude (speed) and direction. The velocity can be measured by following devices :– 1. Milometer : Velocity of a moving automobile is measured by milometer. It consists of non-twisting wire whose one end is connected to a small gear and other end to milometer. The small gear meshes with worm gear mounted on axle of moving wheel. As small gear rotates, wire also spins and it gives movement to needle of milometer. 2. Tachometer : Angular velocity may be measured with the help of a tachometer. A tachometer is an electro-mechanical device in which output voltage is proportional to velocity of the shaft. It is a transducer which converts velocity into an electric signal. The tachometers are classified as follows : (i) Mechanical Tachometers : (a) Fly-ball tachometer (b) Hand Speed tachometer (c) Chronometeric tachometer. (ii) Electrical Tachometers : (a) A.C. tachometer (b) D.C. tachometer (c) Electromagnetic tachometer (d) Capacitive tachometer 3. Electro-magnetic transducers : This type of transducer utilizes the voltage produced in a coil on account of change in flux linkages resulting from change in reluctance. It is the most commonly used transducer for measurement of linear velocities. Flow Measurement : The basic principle of flow measurement is to measure the rate of flow of a fluid flowing through a pipe. The rate of flow (i.e. volume or mass that flows during a fixed time interval) is measured by following instruments : 1. Venturimeter 2. Orifice meter or orifice plate 3. Pitot–Tube 4. Rota meter etc. Venturimeter : It is a device used for measuring the rate of flow of fluid flowing through a pipe. It consists of following three parts. (i) A short converging part (ii) Throat (iii) Diverging part.
Measurement / 33 Inlet Pipe line Flow Outlet pipe line Divergent part Convergent Throat Part 1 2 d1 d1 Flow U-tube monometer h Mercury (Hg) Fig. 2.2 Venturimeter. Working Principle : Its working principle is based on Bernoulli's equation (energy balance principle) and conservation of mass principle (Continuity equation) The discharge 'Q' is given by, Q= a1a2 × a12 – a22 Q = Cd 2 gh a1a2 × a12 – a22 under ideal conditions 2 gh Actual discharge Where, cd = Co-efficient of venturimeter or co-efficient of discharge and its value is less than unity (varies between 0.96 and 0.98) d1 = Diameter of pipe ( at inlet & outlet) a1 = Area at section (1) = π 2 d 4 1 a2 = Area at section (2) h = difference of pressure head at section (1) & (2) Orifice meter or Orifice plate : Orifice meter is a device used for measuring the rate of flow of a fluid through a pipe. The working principle of orifice meter is same as that of venturimeter (i.e. it is based on Bernoulli's theorem). 1 Pipe 2 Orifice plate Flow Flow U-tube monometer h Mercury (Hg) Fig. 2.3 Orifice meter.