temperature measurement - wordpress.com… · · 2017-02-21temperature measurement prepared by:...

Temperature Measurement

Prepared by:

Mr. Hiren Prajapati

Asst. Professor,

Institute of technology,

Nirma university,

[email protected]. Hiren Prajapati, ME, IT, NU

Introduction

oDefinition: Degree of hotness or coldness of a body measured on a definite scale

oPhysical characteristic used to measure temperature

• Change in dimension

• Change in electrical resistance

• Thermo-electric emf

• Change in intensity and colour of radiation

• Fusion of material

Prof. Hiren Prajapati, ME, IT, NU

Introduction Cont…

oTemperature Scale:

oCentigrade and Fahrenheit

• Freezing point and boiling point of water as reference points

• In degree 0 C Freezing point & 100 C boiling point as, divided in 100 parts

• In Fahrenheit 32 F & 212 F, divided into 180 parts

oKelvin and Rankine absolute scale

• No molecular activity means no heat content in a body


oInternational Temperature Scale

oIt is in form of some fixed points

oOxygen point - -182.97 C

oFreezing point - 0 C

oSteam point - 100 C

oSulphur point - 444.6 C

oSilver point - 960.8 C

oGold point - 1063 C


Classification of Temp. Measuring Instruments Based on ASME

oGlass thermometer

oPressure-gauge thermometers

oPartially filled

oCompletely filled

oDifferential expansion thermometers

oElectrical resistance thermometers

oThermometer pyrometer

oOptical pyrometer

oRadiation pyrometer

oFusion pyrometers

oCalorimetric pyrometers

oColour-temperature charts


Liquid in glass Thermometer


oGenerally Mercury is used, because

oBroad Temperature span

oLinear Coefficient of thermal expansion

oNon wetting of glass

Liquid in glass Thermometer


Liquids Range (C)

Mercury -35 to 510

Alcohol -80 to 70

Toluene -80 to 100

Pentane -200 to 30

Creosote -5 to 200

Calibration


Calibration Cont…


Bimetallic thermometer


Invar = 0.009 × 10−4

Cobalt = 0.189 × 10−4

Helix Configuration


Filled system / Pressure thermometer


Classification based on working fluid

oGas filled thermal system

oLiquid filled thermal system

oVapor filled thermal system

oDuel filled thermal system


Filled system thermometer Cont…

oGas filled thermal system

oNitrogen• Inert gas• High coefficient of thermal expansion• Low Sp. Heat

oOperating Pressure, below 35 bar

oReference is triple point, P = 4.5 mm of Hg, T= 0.01 C

o -130 to 540 C

𝑃𝑉=𝑅𝑇

P = 𝑅𝑇

𝑉

V = R𝑇2−𝑇1

𝑃2−𝑃1Prof. Hiren Prajapati, ME, IT, NU


oLiquid filled thermal system

oMercury, Ethyl alcohol, Ether, Pentane, Toluene, Acetone

oOperating pressure up to 100 bar



oVapor Pressure thermal system



oDual filled thermal system

oArgon

oDiethyl ether

oEthyl alcohol

oMethyl alcohol

oToluene

oWater


Filled system thermometer -Errors

oAmbient temperature effect

oRemedy

• Large thermometer bulb

• Case compensation

• Full compensation

• Self compensation


Filled system thermometer –Errors Cont…

oHead or elevation effect

• Liquid-filled system (100 bar) –Negligible

• Gas-filled system (35 bar) –little effect

• Vapour pressure system ( 7 to 15 bar ) –susceptible to error


Filled system thermometer –Errors Cont…

oBarometric effect

• Liquid-filled system (100 bar) –Negligible

• Gas-filled system (35 bar) –little effect

• Vapour pressure system ( 7 to 15 bar ) –susceptible to error

oImmersion Effect:

• Liquid-filled system –susceptible to error

• Gas-filled system –susceptible to error

• Vapour pressure system –Negligible


oResponse time

oDepends on

oType of system (Liquid, gas or vapour filled)

oBulb size

oInstallation with or without thermal well


Thermocouple

oA thermocouple is a device used extensively for measuring temperature.

oWorking Principles of thermocouple is based on three effects:

oSeeback Effect: when two different metals are joined together at two junctions, both thejunctions are different temperature then an electromotive force (emf) is generated at thetwo junctions


Thermoelectric Effects

oPeltier Effect: When two dissimilar metals are joined and current is passed throughjunctions then heat is evolved at one junction and absorbed at another junction.(Potential difference always exists between contact with each other two dissimilar metals

oIt is converse of seeback effect.

oThomson Effect: A potential gradient exists even in the single conductor having atemperature gradient.

• When current is flowing from unequally heated conductor. Heat is absorbed orevolved through out the length of conductor.

• Positive Thomson

• Negative Thomson

• Zero Thomson


Laws of Thermocouple

oLaw of Homogeneous circuit

oLaw of Intermediate temperature

oLaw Of Intermediate Metals


Law of Homogeneous circuit


oAn electric current can not be sustain in a circuit of single homogeneous metal by theapplication of heat alone

Law of successive or intermediate Temperature

oThe emf generated in thermocouple with junctions at temperature T1 and T3 is equal to the sumof emf generated by similar thermocouples, one acting between temperature T1 and T2 andother acting between T2 to T3.

oHelpful to get correction when cold or reference point is other than calibrated cold point.Prof. Hiren Prajapati, ME, IT, NU

Law of intermediate Metals

oThe introduction of third metal into thermocouple circuit will have no change in emfgenerated as long as the junctions of third metal with thermocouple metals are at sametemperature.


Application of Law of Intermediate metals


Law makes possible to use extension wires of a metal different from the thermocouple metal

Allow to introduce instrument in the circuit to measure emf

Allows the use of joining materials such as soft or hard solder in fabricating the thermocouple junctions

Thermopile


Series Connection Parallel Connection

Limitation of Millivoltmeter


oChange in resistance change the currentflow due to emf . This may give error inmeasurement.

oKeep large internal resistance 600 to6oo ohm.

Instrumentation in thermocouple


oMove selector switch to SC (Standardcell) and adjust resistance R to obtainbalance between the standard cell.The galvanometer would read zero(null).

oNow change selector switch to TC(Thermocouple). The thermocoupleemf is now connected to ckt.Galvanometer will deflect from Null.

oAdjust position of slider on slid wireto bring the switch position zero.

oNow position of slider will read emfin ckt.

Thermometer Numerical


oThe hot junction of a chromel-alumel thermocouple is connected to a potentiometerwhose terminals are at 24 C. The potentiometer reads 25.76mV. What is the temperatureof the thermocouple hot junction?

oFollowing values based on reference junction at 0 C.

oE0/H = E0/24+E24/H

=0.95+25.76

=26.71

= 488 C


Temp. C 20 24 28 ….. 480 488 493

Voltage, mV 0.8 0.95 1.12 …... 26.25 26.72 27.04

RTD (Resistance Temp. Detector)

oPlatinum, Nickel, Copper

o(1000 C,600 C, 300 C)


oA platinum resistance thermometer has a resistance of 175 ohm at 120℃. When exposedto high temperature gas, the resistance increases to 325 ohm. Determine the temperatureof hot gas. At 0 ℃ resistance is 110 ohm and 𝛼=0.0039𝑝𝑒𝑟℃

oAns.: 469.65 ℃


Thermistors

oSemi-conductors -100 to 300 C

oChange in Resistant Inversely proportional to change in temperature.

oMore sensitivity but nonlinear relationship between resistant and temperature.

oSintered mixture of metallic oxides like Manganese, nickel, cobalt, copper, iron anduranium


oA thermistor is characterized by the relation 𝑅𝑡=𝑅0𝑎𝑒𝑏/𝑇. Determine temperature

indicated by the thermistor if 𝑅𝑡 =1000ohm. 𝑅0 =4500ohm, 𝑎=300×10−6and 𝑏=3000.

oAns.: 452.63 C


Total radiation pyrometer

oTemperature: 700 C -2000 C

oTarget area factor

oContinuous Use


Optical pyrometer

o540-650 cherry red

o700-820 orange

o870-1050 yellow

o>1100 white


temperature measurement - wordpress.com… · · 2017-02-21temperature measurement prepared by:...

Documents