7. Measurement and Units

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MEASUREMENT AND UNITS

Measurement has the purpose of quantifying a physical quantity such as length,temperature, or time. First thing towards measurement is therefore to standardize theunit of a particular physical quantity. Once we standardized the unit of a particular

physical quantity, then we can measure what ever we want to measures as number ofthose standardized units.For example, to measure the distance we decide that one unit is one step (The longestdistance between our feet when we are normally walking). Then we can quantify anydistance by the number of steps, like the distance from the admin building to work shopis120 steps. And of course the instrument used to measure the distance in this case is ahuman being. But this way of measuring is primitive.By evolution the measuring units reached at this time to a system called SI. TheInternational System of Units (abbreviated SI from the French language name SystmeInternational d'Units) is the modern, evolved form of measurement system. It is theworld's most widely used system of units. But still we use some of the older forms ofmeasurements like number of feet to measure the distance, number of gallons to

measure liquid volumes, number of pounds to measure weight.

Units and systems of measurementMost of the measurement units we use in our Plant and daily life are discussed

below. The units may not necessarily be SI units.

LengthLength is the distance between two points. If it is vertical distance we call it as Height. Ifit is side to side distance we call it as width or breadth. If it is near to far of an object wecall it as depth. If it is a rectangle, we call the longest side as length and the shorter oneas width.

In the International System of Units (SI), the basic unit of length is the meter.The centimeter and the kilometre, derived from the metre, are also commonly usedunits.

SPEED , VELOCITYThe rate of change of displacement.Speed can be called as velocity if the movement of the body is moving in one directiononly in a straight lneUnits are meter / second

ACCELERATIONAccleration is defined as the rate of change of velocity.

Unit is Meters / square second

AREAThe word Area expresses physical quantity of a surface.The unit used in square meters or square centimeters.

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VOLUMEThe volume of a solid object is how much space it occupies.The units of volume is measured in cubic meters.

(Another word used is specific volume and it's unit is "cubic meter per kilo gram" )

Volume formulae

Common equations for volume:

Shape Equation Variables

A cube: s = length of a side

A rectangular prism: l = length, w = width, h = height

A cylinder (circular prism):r= radius of circular face, h = distancebetween faces

Any prism that has aconstant cross sectional

area along the height**:

A = area of the base, h = height

A sphere:r= radius of spherewhich is the first integral of the formula forSurface Area of a sphere

An ellipsoid: a, b, c= semi-axes of ellipsoid

A pyramid:A = area of base, h = height from base toapex

A cone (circular-based

pyramid):

r= radius of circle at base, h = distance from

base to tip

Any figure (calculusrequired)

h = any dimension of the figure, A(h) = areaof the cross-sections perpendicular to hdescribed as a function of the position alongh

this will work for any figure (no matter if theprism is slanted or the cross-sections changeshape).

WEIGHTWeight is the force an object exerts on the place it is rested. Unit is Kilo gram

DENSITYDensity is the weight of a unit volume of an object. Kilo gram/ cubic meter is the unit.

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PRESSUREPressure (symbol: p) is the force per unit area applied on a surface in a directionperpendicular to that surface. Unit is Bar.

F=PX Awhere:

P is the pressureF is the normal force

A is the area.

Generally when pressure is measured it is the pressure above atmospheric pressure.Some times it is also called Bar g to declare that it is as measured by a measurementdevice which measures the pressure above atmospheric pressure. When there is a needto specify the absolute pressure (Pressure above the absolute vacuum) it is denoted asBar a, a for absolute.

Absolute pressure of a fluid is referenced against a perfect vacuum. Ie: Absolute

pressure zero refers to absolute vacuum.Gauge pressure is referenced against ambient air pressure, so it is equal to absolutepressure minus atmospheric pressure.Gauge pressure = Absolute pressure minus Atmospheric pressureDifferential pressure is the difference in pressure between two points.

FLOWFlow have two units one by volume and the other by weight.

Cubic meters per second is by volume and kilo grams per second is by weight.The unit of volumetric flow is m3/s

The unit of flow by weight is kg/ s

TEMPERATURETemperature is described as the amount of warmness or coldness in an object.The unit is degree Celsius. As mentioned earlier regarding some old measurement unitsstill being used in some parts of the world, degree Fahrenheit is still some times used.

The relationship between Centigrade and Fahrenheit scales is as followsC =(F- 32)*5/9,Where C is the temperature in degree centigrade(or Celsius) and F is the temperaturein degree Fahrenheit.Some times a different scale based on absolute temperature concept is used. This scale

is also called Rankine scale.

The relationship between Centigrade and Rankine scales is as followsR = C+273Where R is the temperature in degree Rankine and C is the temperature in degreecentigrade(or Celsius)

HEAT

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Heat is the energy contained in a body that expresses the energy as temperature. Unit isKilo joule.

HEAT CHANGEHeat Energy (Q) is the heat energy put into an object by warming it from temperatureT1 to temperature T2. The increase in heat (Q) is proportional to the temperature

change (T2 - T1) and proportional to the mass (m) of the object being warmed. Theproportionality constant (c), called the specific heat, depends on the material the objectis made of. Heat changeQ = c * m * (T2 - T1)

HEAT FLOWHeat flow is measured as the amount of energy transferred per unit of time. In SI unitsthis would be measured in Joules per second.

TIMEThe unit is second

A TABLE OF UNITSQuantity Unit Symbol

Length, width, breadth, depth, height.. Meter M

Area Square meters M2

Speed or Velocity Meters / second M/sec

Acceleration Meters / square

second

M/ sec2

Volume Cubic meter M3

Volume Imperial gallons ig

Weight Kilo gram kg

Density Kilo gram/ cubicmeter

Kg/ M3

Pressure Bar Bar

Flow Cubic meters/second

M3 / sec

Flow Kilo gram /second

Kg/ sec

Time Second S

Temperature Degrees C 0C

The next step is understanding the instruments used to measure the quanities.

Measuring tools for LengthFoot rules, Measuring tapes, Lasers..For accurate measurements vernier calipers, micrometer, lasers are used

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Measuring tools for SpeedAs we know, the speed is unit distance per unit time.Speedometers is the name of the instrument. It can be mechanical or magnetic or lasersensed.

In some cases, after measuring the distance and time, the speed or velocity is calculated

Measuring tools for ACCELERATIONIt is a calculated while measuring the velocities constantly.Measuring tools for AREAThe measurement is by calculation. Area is not directly measured

Measuring tools for volumeLitre jars, marked tanks and vessels

Measuring tools for weightWeighing scales

Measuring tools for DensityThe measurement is by calculation. Density is not directly measured

Measuring tools for Pressure

Manometer

The difference in fluid height in a liquid column barometer is proportional to the pressuredifference.

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Aneroid gauges are based on a metallic pressure sensing element which flexeselastically under the effect of a pressure difference across the element. A Bourdongauge uses a coiled tube which as it expands due to pressure increase causes arotation of an arm connected to the tube.

Bourdon Gauge

Pressure Transducers

A pressure transducer is a device that converts pressure into an analog electrical signal.The most common is the strain-gage base transducer. The pressure causes adeformation of strain gauge which is converted into an electrical signal.

Measuring tools for Flow

Flow measurement can be done in a variety of ways

INTRODUCTION

Measuring the flow of liquids is a critical need in many industrial plants. Flow rate isusually determined by inference by measuring the liquid's velocity or the change inkinetic energy. The basic relationship for determining the liquid's flow rate is:Q = V x AwhereQ = liquid flow through the pipeV = average velocity of the flow

A = cross-sectional area of the pipe

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Other factors that affect liquid flow rate include the liquid's viscosity and density, and thefriction of the liquid in contact with the pipe.

FLOWMETER TYPESNumerous types of flow meters are available for closed-piping systems. In general, theequipment can be classified as differential pressure, positive displacement, velocity, and

mass meters.

Differential Pressure flow metersDifferential pressure devices (also known as head meters) include orifices, venturi tubes,flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-areameters, Fig. 2.Orifices are the most popular liquid flow meters in use today. An orifice is simply a flatpiece of metal with a specific-sized hole bored in it.Venturi tubes handle large flow volumes at low pressure drops. A venturi tube isessentially a section of pipe with a tapered entrance and a straight throat. As liquidpasses through the throat, its velocity increases, causing a pressure differential betweenthe inlet and outlet regions.

Flow Nozzles, used to measure flow at high velocities. Liquids with suspended solidscan also be metered. However, use of the units is not recommended for highly viscousliquids or those containing large amounts of sticky solids.A pitot tube is an L shaped tube which is also able to measure fluid flow. It works bymeasuring the difference between the static pressure and the dynamic pressure. drop.Elbow meters operate on the principle that when liquid travels in a circular path,centrifugal force is exerted along the outer edges. Any 90 deg. pipe elbow can serve asa liquid flowmeter.Target meters sense and measure forces caused by liquid impacting on a target ordrag-disk suspended in the liquid stream. A direct indication of the liquid flow rate isachieved by measuring the force exerted on the target

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Variable-area meters, often called rotameters, consist essentially of a tapered tube anda float, Fig. 3.

Turbine meters consist of a multiple- bladed rotor mounted with a pipe, perpendicular to the liquidflow. The rotor spins as the liquid passes

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through the blades. The rotational speed is a direct function of flow rate and can besensed by magnetic pick-up, photoelectric cell, or gears. Electrical pulses can becounted and totalized, Fig. 5.

Vortex meters make use of a natural phenomenon that occurs when a liquid flowsaround a bluff object. Eddies or vortices are shed alternately downstream of the object.The frequency of the vortex shedding is directly proportional to the velocity of the liquid

flowing through the meter.

.

Electromagnetic meterscan handle most liquidsand slurries, providingthat the material being

metered is electricallyconductive.They are used in our plant

to measure chemical flow in desalination units.

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Measuring tools for Temperature

In industrial use there are many devices which are used to measure temperature. Thedifferent types are:

thermocouples,

resistive temperature devices (RTDs and thermistors),

infrared radiators,

bimetallic devices,

liquid expansion devices,

and change-of-state devices

Thermocouples

When any conductor (such as a metal) is subjected to a thermal gradient, it will generatea voltage. This is known as the thermoelectric effect or Seebeck effect. The voltagedeveloped varies with the type of material used. Thus if two different metals aresubjected to the same thermal gradient ,they will develop different voltages. Thedifference in voltage is converted to temperature measurement. This principle is usedfor measuring temperature by thermocouples.Thermocouples consist essentially of two strips or wires made of different metals and

joined at one end (hot end). Changes in the temperature at that junction induce achange in electromotive force (emf) between the other ends. As temperature goes up,this output emf of the thermocouple also rises and is proportional to temperature.Therefore it can be used for temperature measurement.In our Plant the thermocouple used is Type K (Chromel (Ni-Cr alloy) / Alumel (Ni-Alalloy) . They are low cost and available in a wide variety of probes. They are available inthe 200 C to +1200 C range.

ApplicationsThermocouples are most suitable for measuring over a large temperature range, up to

1800 K. They are less suitable for applications where smaller temperaturedifferences need to be measured with high accuracy, for example the range 0100C with 0.1 C accuracy. For such applications, thermistors and RTDs are moresuitable.

RESISTANCE TEMPERATURE DETECTORS

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Resistance thermometers, also called resistance temperature detectors (RTDs), aretemperature sensors which work on the principle of change in electrical resistance ofsome materials with changing temperature. As they are generally made of platinum,they are often called platinum resistance thermometers (PRTs). They are suitable inmany industrial applications below 600 C.

THERMISTORA thermistor like RTD is a type of resistor used to measure temperature changes,based on the change in its resistance with changing temperature. The difference iswhile RTDs rely on resistance change in a metal, with the resistance rising more or lesslinearly with temperature, thermistors are based on resistance change in a ceramicsemiconductor and the change is non-linear.Thermistors can be classified into two types .If the resistance increases with increasing temperature, the device is called a positivetemperature coefficient (PTC) thermistor, Posistor.If the resistance decreases with increasing temperature, and the device is called anegative temperature coefficient (NTC) thermistor. They are made of a semiconductorsuch as a sintered metal oxide.Most PTC thermistors are of the "switching" type, which means that their resistance risessuddenly at a certain critical temperature.The devices are made of a doped polycrystalline ceramic containing barium titanate(BaTiO3) and other compounds.

An example of this type of thermistor is the one used to trip Air compressors whenbody temperature goes high.Infrared Temperature Measurement DevicesInfrared sensors are non contacting devices. They infer temperature by measuring thethermal radiation emitted by a material

Bimetallic Temperature Measurement DevicesBimetallic devices take advantage of the difference in rate of thermal expansion betweendifferent metals. Strips of two metals are bonded together. When heated, one side will

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expand more than the other, and the resulting bending is converted into a temperaturereading by mechanical linkage to a pointer. These devices are portable and they do notrequire a power supply, but they are usually not as accurate as thermocouples or RTDs .

Fluid-Expansion Temperature Measurement DevicesFluid-expansion devices make use of the principle that an increase of temperaturecauses fluids to expand . This property can be used for temperature measurement. Theygenerally come in two main classifications: the mercury type and the organic-liquid type.

Change-of-State Temperature Measurement Devices

Change-of-state temperature sensors consist of labels, pellets, crayons, or liquid crystalswhose appearance changes once a certain temperature is reached. They are used, forinstance, to measure HRSG Scanner fan body temperature. when a certain temperatureis exceeded , a white dot on a sensor label attached to the scanner fan body will turnblack.Response time typically takes minutes, so these devices often do not respond to

transient temperature changes. Also accuracy is lower than with other types of sensors.The change in state is usually irreversible, which lets you know the max temperaturereached. However it will not show the present temperature if it is less than the maximumtemperature. liquid- crystal type of sensors have reversible characteristics which willalways show the present temperature. used to measure displays.

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Measuring tools of Heat

Calorie meter is the tool used to measure the heat

Measuring tools for timeWatches , clocks.

QUESTIONS

1. What is the purpose of measurement? How do we do it?2. Name any unit of measurement of Length, Area, volume and speed.

3. Give some examples of tools used for measurement of length, speed, weight,

volume and pressure.

4. What is the difference between speed and velocity?5. Define pressure? Name any unit of pressure?

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6. What is the relation between Absolute pressure, gauge pressure and atmospheric

pressure?

7. Name the two most popular units of temperature measurement?8. Define Heat energy? and Heat flow?

9. Give some examples of Differential flow meters?

10. Which flow meters are used for Antiscale flow measurement?11. Explain working principle of Thermocouple?

12. What is the difference between Thermistor and RTD?

13. Name the two types of thermisters and their difference?14. Briefly explain about change of state temperature measurement devices? Where

are they used in our plant?

15. Give some examples of thermocouples used in our plant?

16. For measuring bearing temperatures among thermocouple, RTD and thermisterwhich temperature measurement device would you choose? Why?

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7. Measurement and Units

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