Đo nhiệt độ: Bài giảng Sensors và analytical devices

Nguyễn Công Phương

Sensors and Analytical Devices

Some Basic Measurement Methods, Temperature Measurement

A. Introduction B. Sensors Characteristics C. Some Basic Measurement Methods D. Measurement Systems

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Some Basic Measurement Methods

I. Sensor Technologies II. Temperature Measurement III. Pressure Measurement IV.Flow Measurement V. Level Measurement VI.Mass, Force, and Torque Measurement VII.Translational Motion, Vibration, and Shock

Measurement

VIII.Rotational Motion Transducers

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Introduction

• Temperature measurement is very important in all aspects of life. In engineering applications, it is the most commonly measured • process variable.

• Difficulty: any given temperature cannot be related to a fundamental

standard of temperature.

• 10 classes of instrument based on 10 principles:

– Thermoelectric effect – Resistance change – Sensitivity of semiconductor device – Radiative heat emission – Thermography – Thermal expansion – Resonant frequency change – Sensitivity of fiber-optic devices – Color change – Change of state of materials

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Thermoelectric Effect Sensors (1)





Metal 1

Metal 2





...  

E a T a T 1

a T 3

a T n

1a T

Thermocouples

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Thermoelectric Effect Sensors (2)

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Thermoelectric Effect Sensors (3)





E 1

E 5



E   1

E 5

http://www.pcbheaven.com/wikipages /How_Thermocouples_Work/?p=1

E   1

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Thermoelectric Effect Sensors (4)

• Chromel – constantan (type E)

– Highest measurement sensitivity: 68 μV/oC – Inaccuracy: ±0.5% – Range: –200oC up to 900oC • Iron – constantan (type J) – Sensitivity: 55 μV/oC – Inaccuracy: ±0.75% – Range: –40oC up to 750oC • Copper – constantan (type T)

– Sensitivity: 43 μV/oC – Inaccuracy: ±0.75% – Range: –200oC up to 350oC

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Thermoelectric Effect Sensors (5)

• Chromel – alumel (type K)

– Highest measurement sensitivity: 41 μV/oC – Inaccuracy: ±0.75% – Range: –200oC up to 1300oC – Applications: 700oC up to 1200oC – Widely used, general – purpose

• Nicrosil – nisil (type N) – Sensitivity: 39 μV/oC – Inaccuracy: ±0.75% – Range: up to 1300oC – Long – term stability & life

• Nickel/molybdenum – nickel – cobalt

– One wire made from a nickel – molybdenum alloy with 18% molybdenum &

the other wire made from a nickel – cobalt alloy with 0.8% cobalt

– Range: up to 1400oC – Rarely used except for special applications such as temperatuer measurement in

vacuucm furnaces

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Thermoelectric Effect Sensors (6)

• Chromel – alumel (type K) – Sensitivity: 41 μV/oC – Inaccuracy: ±0.75% – Range: –200oC up to 1300oC – Applications: 700oC up to 1200oC – Widely used, general – purpose

• Nicrosil – nisil (type N) – Sensitivity: 39 μV/oC – Inaccuracy: ±0.75% – Range: up to 1300oC – Long – term stability & life

• Nickel/molybdenum – nickel – cobalt (type M)

– One wire made from a nickel – molybdenum alloy with 18% molybdenum

& the other wire made from a nickel – cobalt alloy with 0.8% cobalt

– Range: up to 1400oC – Rarely used except for special applications such as temperatuer

measurement in vacuucm furnaces

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Thermoelectric Effect Sensors (7)

Platinum (type B)

•

– One wire made from a platinum – rhodium alloy with 30% rhodium & the other wire made from a platinum –

rhodium alloy with 6% rhodium Sensitivity: 10 μV/oC – – Range: 50oC up to 1800oC

Platinum (type R)

•

– One wire made from pure platinum & the other wire made from a platinum – rhodium alloy with 13%

rhodium Sensitivity: 10 μV/oC – Inaccuracy: ±0.5% – – Range: 0 up to 1700oC

Platinum (type S)

•

– One wire made from pure platinum & the other wire made from a platinum – rhodium alloy with 10%

rhodium Sensitivity: 10 μV/oC – Inaccuracy: ±0.5% – – Range: 0 up to 1750oC

Tungsten (type C)

•

– One wire made from pure tungsten & the other wire made from a tungsten/rhenium alloy Sensitivity: 20 μV/oC – – Range: 0 up to 2300oC

Chromel – gold/iron

•

Sensitivity: 15 μV/oK

For very low temperature applications

– One wire made from chromel & the other wire made from a gold/iron alloy – – Range: from 1.2oK –

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Thermoelectric Effect Sensors (8)

• Manufactured by connecting together two

wires of different materials – Welding (the most common technique), or – Soldering, or – Twisting the wire ends together

• Diameter

– Between 0.4 & 2 mm (usually) – Some special case: 0.1 μm (for fast response time)

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Varying Resistance Devices

• Rely on the physical principle of the variation

of resistance with temperature. • 2 types: resistance thermometers &

thermistors.

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Varying Resistance Devices, Resistance Thermometers (1)

• A.k.a. resistance temperature

devices.

• R = R0(1 + a1T + a2T2 + ... + anTn) • R ≈ R0(1 + a1T) • Platinum:

– The most linear characteristic & the

most commonly used.

– Inaccuracy: ±1.2%. – Very expensive

• Platinum thermometers are made in

3 forms: – A film deposited on a ceramic

substrate.

– A coil mounted inside a glass or

ceramic probe.

– A coil wound on a mandrel

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Varying Resistance Devices, Resistance Thermometers (2) • The nominal resistance (platinum) at 0oC is

typically 100 or 1000Ω. • Sensitivity (platinum): – 0.385Ω/oC (100Ω type) – 3.85Ω/oC (1000Ω type)

• The working range:

–270 to 1000oC –200 to 260oC –200 to 430oC –270 to 1100oC

– Platinum: – Copper: – Nickel: – tungsten:

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Varying Resistance Devices, Thermistors

• Made from beads of semiconductor material prepared from oxides of the iron group of metals (chromium, cobalt, iron, manganese, & nickel).

• Have a negative temperature coeficient, that is, resistance decreases as temperature increases:

(1/

)

1/ 

T 0

R R e 

0 • Disadvantages: – Nonlinear – Low sensitivity

• Advantages: – Low cost – Small size

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Semiconductor Devices

• Consist of either diodes or integrated circuit

transistors.

• Advantage: inexpensive. • Disadvantage: require and external power

supply to the sensor.

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Semiconductor Devices, Diodes • The forward voltage across the device varies

with temperature. • Output is in μA range. • Small size. • Good output linearity. • Inaccuracy: ±0.5%. • Silicon diodes: from –50 to 200oC. • Germanium diondes: from –270 to 40oC.

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Semiconductor Devices, Integrated Circuit Transistors • Produce an output proportional to the absolute

1μA/oK 10mV/oK

temperature: – Current: – Voltage: • Very low cost. • Good output linearity. • Inaccuracy: ±3%. • Range: from –50 to 150oC. • Widely used in monitoring pipes & cables.

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Radiation Thermometers (1)

• A.k.a. radiation pyrometers. • All objects emit electromagnetic radiation as a function of their temperature above absolute zero.

• Radiation thermometers

measure this radiation in order to calculate the temperature of the object.

• The total rate of radiation

emission per second is: E = KT4

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Radiation Thermometers (2)

• Range: –100 to 10,000oC. Inaccuracy: ±0.05%. • • Versions: portable, battery – powered, hand-held. • Easy to use. • The important advantage: there is no contact between the hot body

& the meter, hence: – The measured system is not disturbed. – No possibility of contamination (important in food, drug, etc). – Suitable for measuring high temperatures. – Capable of measuring moving bodies.

• The use of radiation thermometers is complicated due to the

absorption & scattering of the energy between the emitting body & the radiation detector – Absorption: by carbon dioxide, ozone, water vapor molecules. – Scattering: by atmospheric dust & water droplets.

• Types: optical & radiation pyrometers.

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Radiation Thermometers, Optical Pyrometer

• To measure temperatures above 600oC (up to 10,000oC). • Contains a heated tungsten filement within its optical system. • The current in the filament is increased until its color is the same as

the hot body (the filament disappears).

• Temperature measurement is obtained in terms of the current

flowing in the filament.

• Can not be used in automatic systems because of the human eye.

http://www.machineryspaces.com /temperature-measurement.html

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Radiation Thermometers, Radiation Pyrometers

Detector

Light from hot body

• A detector, not eye. • Range: from –100 to 3600oC. • Detector: a thermal detector, or a photon detector. • Thermal detectors:

– Respond equally to all wavelengths in the frequency spectrum – Consist of resistaces thermometers & thermistors – Time constant: several miliseconds

• Photon detectors:

– Respond selectively to a particular band within the full spectrum – Consist of photoconductive or photovoltaic type – Time constant: a few microseconds

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Thermography

Control Control unit unit

Processor Processor

Display Display unit unit

Scanning Scanning radiation radiation detector detector

• A.k.a. thermal imaging. • Scan an infrared radiation detector

across an object.

• The output is in the form of the

temperature distribution. • Range: –20oC up to 1500oC. • Radiation detector: the same principles of operation as a radiation pyrometer.

http://www.thermalvisionresearch.co.uk/thermography-for-the-detection-of-raynauds-disease/cst-bilat-hyperaemia-example/ sites.google.com/site/ncpdhbkhn 30

Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods

a) Liquid-in-Glass Thermometers b) Bimetallic Thermometer c) Pressure Thermometers

8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Liquid-in-Glass Thermometers

• A well – known temperature – measuring

instrument used in a wide range of applications.

• The fluid is normally either mercury or

colored alcohol, contained within a bulb & capillary tube.

• As the temperature rises, the fluid expands

along the capillary tube & the level is read.

• Range: –200 to 1000oC. • Inaccuracy: ±0.15%.

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Bimetallic Thermometer

• Based on the fact that if two strips of different metals are bonded together, any temperature change will cause the strip to bend. • In the bimetallic thermostat,

it is used as a switch.

• If the magnitude of bending

is measured, it is a thermometer.

• Range: –75 to 1500oC. • Inaccuracy: ±0.5%.

http://www.daviddarling.info/encyclopedia/B/ bimetallic_strip.html

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Pressure Thermometers

• Consist of a stainless-steel bulb

containing a liquid or gas.

• Temperature rises will cause the fluid’s pressure increases (not its volume, because the fluid is constrained). • The change in pressure of the fluid is

measured by a suitable pressure transducer (such as the Bourdon tube).

http://image.china- ogpe.com/newsimages/ Pressure_type_thermo meter_2_glossary-1.gif

• Range: –250 up to 2000oC. • Inaccuracy: ±0.5%. • A particulary long time constant.

Bourdon tube

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Quartz Thermometers

• Make use of the principle that the resonant frequency of a material such as quartz is a function of temperature.

• Temperature changes are translated into frequency

change.

• Measurement of the oscillator frequency allows the

measured temperature to be calculated. • Have a very linear output characteristic. • Range: –40 up to 230oC. • Inaccuracy: ±0.1%. • Resolution: 0.0003oC!!! • Very expensive.

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Fiber – Optic Temperature Sensors • Fiber – optic can be used as either intrinsic or

extrinsic temperature sensors.

• Range: 250 up to 3000oC. • Inaccuracy: ±1.0%. • Applications:

– Measuring temperatures in hard-to-reach locations. – Very high measurement accuracy is required.

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Color Indicators

• The color of various substances & objects changes as a

function of temperature.

• One use of this is in the optical pyrometer (discussed). • The other main use is in special color indicators, widely used in industry to determine whether objects placed in furnaces have reached the required temperature. • Such color indicators consist of special paints or

crayons that are applied to an object before it is placed in a furnace.

• At a certain temperature, a chemical reaction takes

place & a permanent color change occurs in the paint or crayon.

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Change of State of Materials

• Seger cones or pyrometric

cones.

• Used commonly in the ceramics industry.

• Consist of a fused oxide &

glass material that is formed into a cone shape. • The tip of the cone softens

& bends over when a particular temperature is reached.

• Range: 600 up to 2000oC.

http://en.wikipedia.org/wiki/ File:Segerkegel.jpg

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Temperature Measurement

Introduction

1. 2. Thermoelectric Effect Sensors (Thermocouples) 3. Varying Resistance Devices 4. Semiconductor Devices 5. Radiation Thermometers 6. Thermography (Thermal Imaging) 7. Thermal Expansion Methods 8. Quartz Thermometers 9. Fiber – Optic Temperature Sensors 10. Color Indicators 11. Change of State of Materials 12. Choice between Temperature Transducers

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Choice between Temperature Transducers (1)

• Depends substantially on whether the medium to be

measured is a solid or a fluid. – Solid:

• It is essential that good contact is made between the body & the

transducers (unless a radiation thermometer)

•  thermocouples, resistance thermometers, thermistors,

semiconductor devices, & color indicators

– Fluid: any

• The most commonly used device in industry for

temperature measurement is the base metal thermocoupe. – Relatively inexpensive – Inaccuracy: ±0.5% – Range: –250 to 1200oC – Low-level output voltage  prone to noise  unsuitable for

measuring small temperature differences.

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Choice between Temperature Transducers (2)

• Resistance thermometers – Also commonly used – Range: –270 to 650oC (smaller than thermocouples) – Inaccuracy: ±0.5% – More stable (compared to thermocouples) & can

measure small temperature differences

• Thermistors

– Also commonly used – Small & inexpensive – Fast output response to temperature changes – Good measurement sensitivity – Range is quite limited

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