Measurements in Fluid Mechanics
058:180:001 (ME:5180:0001)
Time & Location: 2:30P - 3:20P MWF 218 MLH
Office Hours: 4:00P – 5:00P MWF 223B-5 HL
Instructor: Lichuan Gui
lichuan-gui@uiowa.edu
http://lcgui.net
Lecture 21. Temperature measurement
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Temperature measurement
Temperature scales
- three temperature scales in use today, Fahrenheit (F), Celsius (C) and Kelvin (K)
Fahrenheit temperature scale
- 32 for the freezing point of water
- 212 for the boiling point of water
- interval divided into 180 parts
Celsius, or centigrade, scale
- 0 for the freezing point of water
- 100 for the boiling point of water
- conversion formula: F = 9/5C + 32
Kelvin temperature scale
- base unit in International System (SI) of measurement
- zero point at absolute zero
- difference between the freezing and boiling points
of water is 100 degrees
- conversion formula: K = C + 273
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Temperature measurement
Thermometers
Thermal expansion thermometers
- liquid-in-glass thermometers
- bimetallic thermometers
Thermocouples
- based on the thermoelectric effect
Resistance thermometers
- based on the relationship between temperature and electric resistance
- include metallic resistance sensors (RTDs), and semiconductor resistance sensors
Coil elements
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Temperature measurement
Liquid-in-glass thermometers
- a bulb, a reservoir in which the working liquid can expand or contract in volume
- a stem, a glass tube containing a tiny capillary connected to the bulb and
enlarged at the bottom into a bulb that is partially filled with a working
liquid. The tube's bore is extremely small - less than 0.5 mm in diameter
- a temperature scale is fixed or engraved on the stem supporting the
capillary tube to indicate the range and the value of the temperature.
The liquid-in-glass thermometers is usually calibrated against a standard
thermometer and at the melting point of water
- a reference point, a calibration point, the most common being the ice point
- a working liquid, usually mercury or alcohol
- an inert gas is used for mercury intended to high temperature. The
thermometer is filled with an inert gas such as argon or nitrogen above the
mercury to reduce its volatilization.
- constriction may be used to measure maximal or minimal temperature
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Temperature measurement
𝑻𝒏𝒐𝒏−𝒊𝒎𝒎
Liquid-in-glass thermometers
- Typical resolution: 0.05-1 K
𝑵
- Immersion types:
partial Immersion (inserted in fluid up to marked line)
total immersion (inserted in fluid up to liquid column)
complete immersion (entirely immersed in fluid)
𝑻𝒊𝒎𝒎
- total immersion required to avoid errors due to temperature
difference between immersed and non-immersed sections
- stem correction necessary with partial immersion thermometers
e.g. for mercury-in-glass thermometers:
∆𝑇 = 0.00016𝑁 𝑇𝑖𝑚𝑚 − 𝑇𝑛𝑜𝑛−𝑖𝑚𝑚
𝑁- number of degrees (K) of the non-immersed portion
𝑇𝑖𝑚𝑚 - temperature of the immersed portion
𝑇𝑛𝑜𝑛−𝑖𝑚𝑚 - temperature of the non-immersed portion
- disadvantage: poor spatial and temporal resolutions
- advantage: excellent laboratory standards for calibration of other instruments
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Temperature measurement
Bimetallic thermometers
- two thin plates of different materials
- vastly different thermal expansion coefficient
- bonded together tightly with one end fixed & another free
- curvature of the assembly changed due to temperature variation
- helically or spiral coiled assembly used to amplify motion resulting from temperature change
Bimetallic thermometer (flat, spiral strip)
- bimetallic assemblies also used in thermostatic controls
- typical resolution about 1% of full scale, and maximal operation temperature around 500C
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Temperature measurement
Thermocouples
Seebeck effect
- Any electrical conductor will develop a potential difference
(thermoelectric voltage) between two of its points that
have a temperature difference.
Thermocouple configuration
- two dissimilar metallic wires (e.g. A and B) joined firmly at two junctions
- one junction exposed to the temperature of interest (e.g. T1)
- the other one (reference junction) kept at known constant temperature (e.g. T2)
- reference junction conventionally immersed in an ice bath for reference temperature of 0C
- constant reference temperature also provided with an electronically controlled heated block
- common types of thermocouples and their properties
- typical resolution in the order of 1C
- highest temperature of 2930C by tungsten-rhenium type
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Temperature measurement
Thermocouples
Sensor & measuring circuit
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Temperature measurement
Resistance thermometers
Resistance temperature detectors (RTDs)
- pure metal thermometers of platinum (most popular and accurate), nickel, or copper
- typical resolution of 0.1 K, possible high resolution of 0.0001 K
- non-linear response fitted with low-order polynomials
e.g. resistance Rpt of platinum RTD in the range 0f 100-700C described by Callendar-Van Dusen equation
Rpt0 – resistance at 0C
Cold-wires
- similar construction to HW
- high frequency response in temperature measurement
Thermistors
- semiconductor elements whose resistance is a very
strong function of temperature.
- extremely high sensitivity to temperature
- non-linear response
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Discussion on homework assignment
- Questions and Problems: 8 on page 286
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Homework
- Read textbook 12.1-12.2 on page 290 - 296
- Questions and Problems: 6 on page 305
- Due on 10/14
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