ei2251 / industrial instrumentationchettinadtech.ac.in/storage/15-06-30/15-06-30-12-27-38... ·...

.

EI2251 / INDUSTRIAL INSTRUMENTATION – I

5/2/2014 1 EI2251/INDUSTRIAL INSTRUMENTATION – I

C.MAGESH KUMAR/AIHT

YEAR / Semester: II A / IV COURSE CO-ORDINATOR: C.MAGESH KUMAR https://sites.google.com/site/mrcmageshkumar

UNIT IV TEMPERATURE MEASUREMENT

5/2/2014 EI2251/INDUSTRIAL INSTRUMENTATION – I

C.MAGESH KUMAR/AIHT 2

UNIT IV TEMPERATURE MEASUREMENT

Definitions and standards Primary and secondary fixed points Calibration of thermometers Different types of filled in system thermometer Sources of errors in filled in systems and their Compensation Bimetallic thermometers Electrical methods of temperature measurement Signal conditioning of industrial RTDs and their characteristics-3 lead and 4 lead RTDs Thermistors. 5/2/2014

EI2251/INDUSTRIAL INSTRUMENTATION – I C.MAGESH KUMAR/AIHT

3

TEMPERATURE - Definitions

• “Thermal state of a body which distinguishes a hot body from cold body”

• Temperature of a body α stored molecular energy

• “Thermal state of a body which determine whether it will give heat to or receive heat from other bodies”

• 2nd law of thermodynamics states that “heat flows of its own accord from a body at higher temperature to the body at lower temperature”



TEMPERATURE standards / scales, fixed points

• Lower fixed point – ice point (0ᴼC)

The temperature of distilled water, melting under a pressure of 760mm of mercury

• Higher fixed point – steam point (100ᴼC)

The temperature of pure water, boiling under a pressure of 760mm of mercury

Note:

The boiling point of water differs with applied pressure



ABSOLUTE TEMPERATURE SCALE • This scale assigns zero temperature to a material which

has no thermal energy (no molecular vibration)

• Temperature of a body α stored molecular energy

• Scales:

– Kelvin scale (K)

• ice point 273.15 K

• steam point 373.15 K

– Rankine scale (ᴼ R)

• ice point 491.7 ᴼRs

• steam point 671.7 ᴼR

Relation between K & ᴼR T(K) = (9/5)T (ᴼR) = 1.8T (ᴼR) 5/2/2014


6

RELATIVE TEMPERATURE SCALES • ASSUMPTION: melting of ice and boiling of water

occur at fixed temperature at standard atmospheric pressure (14.7 psi)

• Fahrenheit scale: – Melting point of ice = 32 ᴼ F

– Boiling point of water = 212 ᴼF

• Celsius scale: – Melting point of ice = 0 ᴼC

– Boiling point of water = 100 ᴼC

• Relationship: T(ᴼF) = 1.8 T (ᴼC) + 32

• Differs from absolute scale only in a shift from zero axis.



TEMPERATURE scales, fixed points

T(K) = 1.8T (ᴼR)

T(ᴼF) = 1.8 T (ᴼC) + 32

T (ᴼC) = T (K) – 273.15

T(ᴼF) = T (ᴼR) - 459.7

5/2/2014


8

TEMPERATURE SCALES



International practical temperature scale (IPTS)

• Based on number of basic and secondary fixed points that can be used for calibrating temperature measuring instruments

• The fixed points are developed by normal atmospheric pressure of 760 mm of mercury

– Boiling point

– Freezing point

– Triple point



International practical temperature scale (IPTS)

• Boiling point:

–The temperature at which the substance changes its physical state and becomes gas.

• Freezing point

–The temperature at which the substance changes its physical state and becomes solid.

• Triple point

–For a particular temperature & pressure at which three different phases of one subatnce can exist in equilibrium is known s triple point. (solid, liquid, gas)

5/2/2014


11

Different types of filled in system thermometer

• Liquid filled system

• Vapor filled system

– Case1: bulb temp > ambient temp (atmospheric temp)

– Case 2: bulb temp < ambient temp

– Case 3: case 1&2 bulb temp < / > ambient temp

– Case 4 : for all temperature

• Gas filled system

• Mercury filled system

5/2/2014


12

Liquid filled system – other than mercury

• When temperature increase, liquid will expand and expansion indicates corresponding temperature if calibrated

• Fluid: inert hydrocarbon, Xylene (six times more co-effecient of expansion than mercury)

• Range: -100’F to 600’F



Vapor filled system

• Filling medium in both liquid & vapor form

• “pressure within the system is a function of vapor pressure of filling fluid at operating temperature”

• Fluids: propene, methyl chloride, hexane, ethyl alcohol, ethane, ethyl chloride

• Lower temperature: -73 to 29 ‘C (ethane)

• Higher temperature: 38 to 177 ‘C (ethyl chloride)



Case1: Bulb Temp > Ambient Temp (Atmospheric Temp)

Bulb: vapor Capillary: liquid



Case 2: bulb temp < ambient temp

Bulb: liquid Capillary: vapor



Case 3: case 1&2 bulb temp > / < ambient temp

When temprature is > / < ambient temp, but cannot be used when it cross ambient temperature” • Larger bulb



Case 4 : for all temperature • Volatile liquid: filled in bulb • Non-volatile liquid: filled in capillary, bourdon tube • Non-volatile liquid purpose: Only to transmit vapor pressure at volatile liquid- vapor interface • Larger bulb than all methods



Gas filled system • Inert gas (helium), nitrogen

• Charles’s law – absolute pressure of a confined gas is proportional to its absolute temperature

• Range: -268 to 760 ‘C

Mercury filled system

• Mercury is the filling liquid • Range: -40 to 649 ’C • Speed of response is higher than liquid / gas/vapor

filled system



Sources of errors in filled in systems

• Ambient temperature effect

• Head or elevation effect

• Barometric effect

• Immersion effect

• Radiation effect



Ambient temperature effect • Change in ambient temperature causes

volume changes in capillary & pressure spring thereby causing error in measurement

TE= (Vc Tc + Vp Tp ) / Vb

– Vb bulb volume

– Vp pressure spring volume

– Vc capillary volume

– Tc capillary temperature

– Tp pressure spring temperature



Head or elevation effect Bulb at different height wrt pressure gauge



Barometric effect • Due to change in atmospheric pressure

• Affects bourdon tube

Immersion effect • If bulb is not fully immersed in process • If bulb head not insulated, conduction of heat

from external source will take place

Radiation effect • When temperature difference between bulb

& other solid bodies around it due to improper insulation



Bimetallic thermometers



Bimetallic sensor types



Helix Bimetallic thermometers



Sprial Bimetallic thermometers



Electrical methods of temperature measurement



RTD



Signal conditioning of industrial RTDs 2 lead wire RTD



3 lead RTD



4 lead RTDs



Thermistors



ei2251 / industrial instrumentationchettinadtech.ac.in/storage/15-06-30/15-06-30-12-27-38... ·...

Documents