chapter 7 temperature measurement

7/24/2019 Chapter 7 Temperature Measurement

1/11

CHAPTER 7: TEMPERATURE

MEASUREMENT

Cool

Bath

HotBath

Chapter 5: Measure the temperature

Associate Professor Thai Thi Thu Ha 10/15/20151

OBJECTIVES

At the end of this chapter, the students should be able to:

1. Understand the concept of temperature, scales and units by air.

2. Understand and use the instruments follow the ground rules.

3. Know and use the tools on the principles of electricity.

4. Understand and use the tools under the optical principle.



CONTENTS

1.Introduction

2.Definition of temperature

3.Temperature scales

4. Classification temperature measurements5. Temperature measurement by mechanical effects

6. Temperature measurement by Electrical Effects

7. Temperature measurement by changes in Emitted thermal radiation

8. Conclusion



Temperature Measurement

and ControlApplications for physicistsNecessary for some other process of interest

Purification by vacuum sublimationDevice fabricationCrystal growthCold traps for numerous applicationsProcess needs to be done under predetermined thermal conditions

Inherent to an experimentMeasurement of temperature dependence of some propertyDetermination of temperature at which some physical phenomenon occursTemperature dependence of experiment needs to be controlled with high

precision



1.INTRODUCTION

CHAPTER 7: TEMPERATURE MEASUREMENT


2/11

Temperature sensors appear in building, chemicalprocess plants, engines, appliances, computers, and

many other devices that require temperaturemonitoring

Many physical phenomena depend on temperature, sowe can often measure temperature indirectly by

measuring pressure, volume, electrical resistance, and

strain



1.INTRODUCTION


Devices and Techniques for

Temperature Measurement

Requires material parameter proportional to temperature Uncertainty principle applies! How much does the act of

measuring the temperature and getting the result out change the

system temperature?

Under what circumstances is the act of taking the measurementinsignificant?

1.INTRODUCTION




To measure temperature accuratelyis far more complicated than it seems

Design of sensors

Bimetal, RTD, thermocouple, thermistor, infrared

Pocket, handheld, hard-wired

Design of equipment

Batch or continuous

Location of the coldest and hottest pieces

Parameters affecting temperature measurement

1.INTRODUCTION




You should consider where to probe, with what, how, and for how long.

Type of product Sausages, patties, cured meats

Shape of pieces, casings, molds

Application Endpoint check (CCP)

Process validation (F-value)

Oven validation

Parameters affecting temperature measurement

1.INTRODUCTION





3/11

A BIT OF HISTORY

Temperature measurements and thermometers 1600 - thermometers (water expansion, mercury) 1650 - first attempts at temperature scales (Boyle) 1700 - standard temperature scales (Magelotti, Renaldini, Newton) -

did not catch

1708 - Farenheit scale (180 div.) 1742 - Celsius scale 1848 - Kelvin scale (based on Carnots thermodynamic work) 1927 - IPTS - International Practical Temperature Scale

1.INTRODUCTION




Temperature sensors are the oldest sensors 1821 - Seebeck effect (Thomas Johann Seebeck)

1826 - first sensor - a thermocouple - based on the Seebeckeffect (Antoine Cesar Becquerel)

1834 - Peltier effect (Charles Athanase Peltier).First peltier cell built in 1960sUsed for cooling and heating

1821 - discovery of temperature dependence of conductivity (SirHumphrey Davey)1871 - William Siemens builds the first resistive sensor made

of platinum

A BIT OF HISTORY

1.INTRODUCTION




A SIMPLE THERMAL SYSTEM

Heat Source

Temperature

Controlling

Device

Sensor

Work Load

Sensor Input

Output

1.INTRODUCTION




2. DEFINITION OF TEMPERATURE




4/11


5/11

3. TEMPERATURE SCALES

The International Temperature Scale of 1990 (ITS-90) is thecurrent standard for temperature measurement, defining theKelvin temperature scale. The standard is based on phase

transition points of various pure substances, with the Kelvindegree defined as 1/273.16 the absolute temperature of the triplepoint of water. Examples of a few other key points defined in thisscale are listed in Table 1.

The reason for defining the temperature scale on the basis offreezing and triple points is that these events can be readilyreproduced to a high degree of repeatability.

This means that there need not be a standard kilogram oftemperature locked in a vault somewhere.

To measure temperatures between reference points, you have toresort to less fundamental devices, calibrating them to the knownpoints and interpolating between those calibration points.





Two temperature scales which are the Fahrenheit and Celsius scales

These scales are based on a specification of the number increment between

the freezing point and a boiling point of water at standard atmospheric

pressure.

The Celsius scale has 100 units between these points, while Fahrenheit

scale has 180 units. The absolute Celsius scale is called the Kelvin scale, while the absolute

Fahrenheit scale is termed the Rankine scale.

Both absolute scales are defined that they will correspond as closely as

possible with the absolute thermodynamic temperature scale.

The zero point of both absolute scales represent the same physical state, and

the ratio of two values is the same, regardless of the absolute scale used; i.e.:

(T2/T1)Rankine= (T2/T1)Kelvin The boiling point of water at atm is arbitrarily taken as 100o on Celsius

scale and 212o on the Fahrenheit scale. The relationship between the scales is as follows:

oF = 32 + 9/5oCoF = 9/5oK





20

The reason for defining the temperature scale on thebasis of freezing and triple points is that these eventscan be readily reproduced to a high degree ofrepeatability.

This means that there need not be a standardkilogram of temperature locked in a vaultsomewhere.

To measure temperatures between reference points,you have to resort to less fundamental devices,calibrating them to the known points andinterpolating between those calibration points.






6/11





4. CLASSIFICATION TEMPERATURE MEASUREMENTS




1. Temperature measurements include:

change in volume of liquid

change in length of a metal bar

change in electrical resistance (of a wire)

change in the pressure of a gas at constant volume

change in the colour of a flame or heated object

2.Temperature measurement by mechanical effects

Devices operating on the basis of a change in mechanical dimension with a change in temperature.

Several temperature-measurement devices may be classified as mechanically operative including:

a) The liquid-in-glass thermometer

b) The bimetallic thermometer

c) Fluid-expansion thermometers

3.Temperature measurement by Electrical Effects

Very convenient method because they furnish a signal that is easily detected, amplified, or used for

control purposes.Usually quite accurate when properly calibrated and compensated. Three commonly

used

a) Electrical-Resistance Thermometer or Resistance Temperature Detector (RTD).

b) Thermistorc) Thermocouple

4.Temperature measurement by changes in Emitted thermal radiation

4. CLASSIFICATION TEMPERATURE MEASUREMENTS




5.TEMPERATURE MEASUREMENT

BY MECHANICAL EFFECTS





7/11

One of the most common types of temperature-measurement devices.

The construction of the device is shown in figure.

- A relatively large bulb at the lower portion of the thermometer holds

the major portion of the liquid, which expands when heated and rises

in capillary tube upon which are etched appropriate scale marking.

- At the top of the capillary tube another bulb is placed to provide a

safety feature in case the temperature range of the thermometer is

inadvertently exceeded.

- Alcohol and mercury are the most commonly used liquids. Alcohol

has advantage that it has a higher coefficient of expansion than

mercury, but it is limited a low-temperature measurement because it

tends boil away at high temperature. Mercury cannot be used below its

freezing point of 38.78oF(-37.8oC).- The size of the capillary depends on the size of the sensing bulb,

the liquid, and the desired temperature range for the thermometer.

FLUID-EXPANSION THERMOMETER

5.TEMPERATURE MEASUREMENT BY MECHANICAL EFFECTS




Method is based on the expansion of a liquid with temperature

The liquid in the bulb is forced up the capillary stem

Thermal expansion:

)1(0 TVV

During calibration this type of

thermometer is subjected to three

types of measuring environments.






The traditional thermometres

Measurement scale from -190 C to +600 C

Used mainly in calibrationMercury: -39 C +357 C

Spirit: -14 C +78 C






CAUSES OF INACCURATIES

Temperature differences in the liquid

Glass temperature also affects The amount of mmersion (vs. calibration)

For accurate measurements, thethermometer should be immersed in the

same manner as during calibration.

Otherwise stem correction needs to be

applied.







8/11

DLl 5,0






Method used forbimetallic sensors: two strips of metals

with different thermal expansion coefficient,

A and

B,bonded together.

BIMETALLIC THERMOMETERS





All metals expand withtemperature

Volume stays constant -length changes

Each metal has acoefficient of linearexpansion a.

a is usually given at T1,temperatures inC.

l2=l1 1+ T2T1 m






Tr16

rr

1

r)rr1()

2

22

))((

hr

h

EhEEhh

,h

hh

1

2r

,E

EE

2

2r

Thickness ratio

Elastic modulus ratio







9/11


10/11






Dial-type bimetallic thermometer with an adjustable stem and dial

to allow flexibility in installation and viewing. Photograph courtesy of WIKA

Alexander Wiegand GmbH & Co.






Taylor

(+2.2oF)

DeltaTrak(+1.8oF)

DeltaTrak

(+1.8oF)






Continuous measurement of temperature/deflection of cantilever

beam having bimetallic strip using reflection of Laser.

Laser beam

detected by PSD or

CCD detector

ol/l,)hh(

l,deflection21

2o

21

2o

hhIT







11/11

Represent the most economical, versatile and widely used devices for industrial

temperature-measurement applications.

Also called pressure thermometer.

The principle of operation is indicated in figure 4.23.

Consist of sensitive bulb containing a liquid, gas, or vapor is

immersed in the environment.

The bulb is connected to some type of pressure-measuring device, such as bourdon

tube, bellows or diaphragm via capillary tube.

An increase in temperature causes the liquid or gas to expand, thereby increasing the

pressure on the gauge.

The pressure is taken as an indication of the temperature.

The entire system consisting of the bulb, capillary and gauge may be calibrated

directly.

Capillary tube may be used for remote measurement.

Temperature variation along the capillary and at the pressure-sensing device generallyrequire compensation, except in the vapor-pressure type (shown in figure 4.24) where

pressure depend only the temperature liquid liquids free surface located at the bulb.






Vapor-pressure ThermometerFluid-expansion Thermometer






chapter 7 temperature measurement

Documents