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

2 Lecture 21. Temperature measurement

3 Temperature measurement Temperature scales - three temperature scales in use today, Fahrenheit (  F), Celsius (  C) and Kelvin (K) Fahrenheit temperature scale for the boiling point of water - 32 for the freezing point of water - interval divided into 180 parts Celsius, or centigrade, scale 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 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 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 Temperature measurement Liquid-in-glass thermometers - Typical resolution: 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 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 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 Temperature measurement Thermocouples Sensor & measuring circuit

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 K - non-linear response fitted with low-order polynomials e.g. resistance R pt of platinum RTD in the range 0f  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 Discussion on homework assignment - Questions and Problems: 8 on page 286

12 Homework - Questions and Problems: 6 on page Read textbook on page Due on 10/14