10 temperature measurement

7/27/2019 10 Temperature Measurement

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Electronic InstrumentationLecturer Touseef Yaqoob 1

PART TWO

Measurement of Temperature


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Learning objectives To understand basic principles of temperature

measurement

To learn different temperature measurementtechniques

To be able to determine the operating rangeof different temperature sensors.


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Glass thermometers


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Pressure Thermometers


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Bimetallic sensors


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Bimetallic sensors

Bimetallic sensors can be directly used intemperature control applications.


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Bimetallic sensors

Helical Coiled Bimetal element


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Bimetallic sensors


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ThermocoupleThermocouples are the most popular temperature sensors.

They are cheap, interchangeable, have standard

connectors and can measure a wide range oftemperatures. The main limitation is accuracy, system

errors of less than 1C can be difficult to achieve.
http://www.picotech.com/thermocouples.htmlhttp://www.picotech.com/thermocouples.html


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Thermocouple: Principle

Temperature dependent voltagesource


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Thermocouple: Types

Type K (Chromel / Alumel) Range: -200 C to +1200 C. Type E (Chromel / Constantan) High output (68 V/C)

more suited for low temperature Type J (Iron / Constantan) Range: -40 to +750 C. Type N (Nicrosil / Nisil) High stability and resistance to high

temperature oxidation. Type B (Platinum / Rhodium) Suited for high temperature

measurements up to 1800 C. Low sensitivity (10 V/C)

Type R (Platinum / Rhodium) Suited for high temperaturemeasurements up to 1600 C. Low sensitivity (10 V/C) Type S (Platinum / Rhodium) Suited for high temperature

measurements up to 1600 C. Low sensitivity (10 V/C)


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Thermocouple: Characteristics


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Thermocouple: Connection

Care should be taken to compensate the voltagesgenerated at junctions

Extension cord

Thermocouple

Meter leads

Volt meter


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Resistive thermometers

This type of sensors is based on theobservation that different materials can

have different resistive profiles atdifferent temperatures. Theseproperties are mainly electrical in

nature.


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Resistive thermometers: RTD


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Resistive and temperature profile


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Mathematical relationships

For RTD made of

platinum, thetemperature profileis quite linear andcan be representedby

)( 000TT

R

R

For RTD made ofcopper, thecharacteristics canbe approximated by

20100

0

)()( TTTTR

R


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Characteristics and issues of RTD

Industrial RTDs are very accurate: the accuracycan be as high as 0.1C. The ultra high

accurate version of RTD is known as StandardPlatinum Resistance Thermometers (SPRTs)having accuracy at 0.0001C.

Special attention should be given on the wiring

of RTD bridge connection as well as self-heating when a current is sent through theRTD.


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Thermistors

Thermistor, a word formed by combining

thermal with resistor, is a temperature-sensitive resistor fabricated fromsemiconducting materials. The resistance ofthermistors decreases proportionally withincreases in temperature. (Note: opposite from

RTD)

The operating range can be as wide as -200Cto + 1000C


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Thermistors


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Measurement of temperature with some

examples

To understand the principle of bridge circuit and be able toperform simple calculations to use bridge to measure lowlevel voltages

To be able to determine the temperature from an RTDsensor reading

To be able to calculate the temperature from an thermistorreading

To be able to calculate the temperature from an

thermocouple reading

To be able to understand the basic concept of D/A and A/Dconverters and calculate quantization errors.


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Bridge circuits

Bridge circuit is usedquite often to measurelow level voltages, such

as the outputs fromRTD, thermister, orthermocouples.

In the case of a

balanced bridge, there isno voltage dropbetween, Band C,hence, Ig = 0.We have

0RIRI 3311

0RIRI 4422


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Bridge circuits (Cont)

Furthermore43II

21II

Therefore, the following condition is established for abalanced bridge:

3

4

1

2

R

R

R

R

Any change in one arm of the bridge willdestroy this balance condition. However, onecan use the measured voltage across the

bridge to calculate changes in one arm.


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Under this condition,suppose that there is achange in R1, such that:

111'

RRR

The corresponding change in voltage across Band Cis

)

RR

R

RR

R(EE

43

3

21

'

1'

i0

To simplify the analysis, if we assume that

RRRRR4321


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iE

)R/R(24

)R/R(E

RE2E

E4R

i


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Example

An RTD is connected in a Wheatstone bridge as

shown:


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Under a balancedcondition, the

parameters aregiven as follows:

500RR 32

100R1

C/00395.0o

The temperature constant of the RTD:


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Questions:

(a) What is the value ofR RTDunder the balancedcondition ?

(b) As temperature changes, it is found that themaintain a new balance, the new value for R1hasto be:

95.103R1

Determine the change intemperature.


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Solution:

(a) Using the relation:

3

2

RTD

1

R

R

R

R

100RR1RTD


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Solution:

(b) Since the relationship between the resistance of the RTDand the temperature change can be approximated by:

95.103R

We can conclude that:

]T1[RR 0

At the newly established balance condition with

C10)100

10095.103(

00395.0

1)

R

RR(

1T

0

0

0


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Temperature measurement with RTD

Typical characteristics of RTD


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Electronic Instrumentation

Lecturer Touseef Yaqoob 35


For RTD made of platinum, the temperature profile is

quite linear and can be represented by

2

0100

0

)()( TTTTR

R

)( 000

TTR

R

For RTD made of copper, the characteristics canbe approximated by


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A platinum RTD is connected as one arm of a Wheatstone bridgeas shown

The fixed resistors, R2 and R3 are 25. The RTD has aresistance of 25 at 0oC, and the coefficient of resistance ofthe RTD is

C/00395.0o


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2

31

R

RRR

RTD

A temperature measurement is made by placing the RTD inthe measuring environment and balancing the bridge byadjusting R1 to a new value of 37.36 .

Determine the temperature of the measuring environment

Solution

At balanced condition, we have

)1(2536.37 TFrom

We can find that the temperature is 126o C.


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Temperature measurement withthermistors

The mathematical relationshipdescribing a thermistor can oftenbe expressed as

)/1/1(0

0TTeRR

As temperature increases, theresistance decreases.

R0is the resistance at T0, and isa parameter ranging from 3500 to4600.


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Example

A thermistor is placed in a 100oC environment, and its

resistance is measured at 20,000. The material constant,, for this thermistor is 4000. If the thermistor is then usedto measure a particular temperature, and its resistance ismeasured as 500 , determine the environmental

temperature being measured.


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00

1)ln(11TR

RT

Solution:

From the basic equation of thermistor, we can have

C

TR

RT

0

00

159.1101

)ln(1

1

Further


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Temperature measurement withthermocouples


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Thermocouples: Example

Example

A type-J thermocouple circuit below is used to measure the

temperature T1 . The thermocouple junction # 2 is maintainedby 32o F. The voltage output is measured to be 15 mV.

Determine the temperature T1.

Solution

The temperature T can be read off from the graph fortype-J thermocouple to be 530o F.


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Quartz Thermometers

Quartz thermometer works on a principle thatthe resonant frequency of a material/quartz isa function of time.

The instrument has a very linear outputcharacteristic over the range between-40 and +230 degree centigrade.

The characteristics of the instrument are

generally very stable over long periods oftime and therefore less frequent calibration isrequired.


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Example of quartz thermometer


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Radiation Thermometers

Radiation thermometer deals with principlethat all bodies emit electromagnetic radiationas a function of their temperature.

The power spectral density of this emissionvaries with the temperature.

Measurement of radiation emitted from thebody allows the temperature of the body to

be calculated. Radiation thermometers have one major

advantage that they do not require to be incontact with the body.


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Radiation Thermometers


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Optical pyrometer

Optical pyrometers are non-

contact temperature measurementdevices. They work by using thehuman eye to match the brightnessof a hot object to the brightness of

a calibrated lamp inside the device.


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Practical pyrometer


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Thermography

Thermography , or thermal imaging,involves scanning an infrared radiationdetector across an object.


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How Thermal-Imaging Works

The basic operation of a thermal imagingdevice is a five-step process:

A special lens focuses the incoming infrared

radiation (heat given off from all objects) ofthe objects in the view.

The focused radiation is scanned by a phasedarray of infrared detectors. Thousands ofpoints and heat readings for the field of vieware collected in only one thirtieth of a second.The detector elements create a very detailedtemperature map called a thermogram.


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Continued

The thermogram created by the infrareddetector elements is translated into electricimpulses.

The electric impulses are sent to a circuitboard, called a signal-processing unit, whichhas a dedicated chip for translating the electricimpulses into data for the display.

The signal-processing unit sends the data tothe display, where it appears as various colorsor shades depending on the temperature of theinfrared emission. The image is created fromthe combination of all the impulses from all ofthe elements.


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Thermography device


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Thermography device result


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Acoustic Thermometers

Acoustic thermometer relies on the principle that thesound velocity in substances depends on temperature.In gases, the velocity is proportional to the square rootof the absolute temperature. In solids and liquids, thevelocity decreases as the temperature increases. Twocategories of acoustic sensing systems are used intemperature measurements:

a) The system measures the acoustic characteristics ofthe medium whose temperature is measured.

b) The system measures the acoustic characteristics ofthe object that is in thermal equilibrium with themeasured medium.


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Fiber optic temperature sensors

There are a wide number of devices thatutilize fiber optics to aid in measuringtemperature.

Most are actually slight variation of radiationthermometers, but not all.

Most all depend upon a temperature sensingcomponent being placed on the tip of the

fiber optic's "free end". The other end isattached to a measuring system that collectsthe desired radiation and processes it into atemperature value.


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Intelligent instrument

Intelligent temperature transmitters bring allthe usual benefits associated with intelligentinstruments, like the transmitters nowavailable include adjustable damping, noise

rejection, self adjustment for zero sensitivitydrifts and expanded measurement range.

All the intelligent transmitters presentlyavailable have no-volatile memories.

Such transmitters are separate boxes designed

for use with transducers with have either a d.c.voltage output in the millivolt range or anoutput in the form of a resistance change.

10 temperature measurement

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