1. 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

2. 2 Lecture 21. Temperature measurement

3. 3 Temperature measurement Temperature scales - three temperature scales in use today, Fahrenheit (  F), Celsius (  C) and Kelvin (K) Fahrenheit temperature scale - 212 for the boiling point of water - 32 for the freezing point of water - interval divided into 180 parts Celsius, or centigrade, scale - 100 for the boiling point of water - 0 for the freezing 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

4. 4 Temperature measurement Thermometers - liquid-in-glass thermometers Thermal expansion thermometers - based on the thermoelectric effect Thermocouples - based on the relationship between temperature and electric resistance Resistance thermometers - include metallic resistance sensors (RTDs), and semiconductor resistance sensors - bimetallic thermometers Coil elements

5. 5 Temperature measurement Liquid-in-glass thermometers - constriction may be used to measure maximal or minimal temperature - 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.

6. 6 Temperature measurement Liquid-in-glass thermometers - Typical resolution: 0.05-1 K - 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) - 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: - disadvantage: poor spatial and temporal resolutions - advantage: excellent laboratory standards for calibration of other instruments

7. 7 Temperature measurement Bimetallic thermometers - two thin plates of different materials - bonded together tightly with one end fixed & another free - vastly different thermal expansion coefficient - 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

8. 8 Temperature measurement Thermocouples Thermocouple configuration Seebeck effect - Any electrical conductor will develop a potential difference (thermoelectric voltage) between two of its points that have a temperature difference. - two dissimilar metallic wires (e.g. A and B) joined firmly at two junctions - one junction exposed to the temperature of interest (e.g. T 1 ) - the other one (reference junction) kept at known constant temperature (e.g. T 2 ) - 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

9. 9 Temperature measurement Thermocouples Sensor & measuring circuit

10. 10 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 R pt of platinum RTD in the range 0f 100-700  C described by Callendar-Van Dusen equation R pt0 – resistance at 0  C Cold-wires - similar construction to HW Thermistors - semiconductor elements whose resistance is a very strong function of temperature. - high frequency response in temperature measurement - extremely high sensitivity to temperature - non-linear response

11. 11 Discussion on homework assignment - Questions and Problems: 8 on page 286

12. 12 Homework - Questions and Problems: 6 on page 305 - Read textbook 12.1-12.2 on page 290 - 296 - Due on 10/14