Section 7. Temperature Measurement

Transcription

1 Section 7 Temperature Measurement 7/25/2017 Engineering Measurements 7 1 Working Definition Temperature is a measure of the average kinetic energy of the molecules that make of a substance. After time, temperature is the second most measured physical unit. 7/25/2017 Engineering Measurements 7 2 1

2 International Temperature Scale Reference Points triple point of hydrogen C boiling point of oxygen* C boiling point of water* C freezing point of zinc* C freezing point of silver* C freezing point of gold* +1, C (*at atmospheric pressure) T [ K ] T ( C ) T [ C ] T ( F ) 32 T [ F ] T ( C ) 9 5 T [ R ] T ( F ) /25/2017 Engineering Measurements 7 3 Common Temperature Measurement Techniques Changes in Physical Dimensions Bimetallic Thermometers Filled-Bulb and Glass-Stem Thermometers Changes in Electrical Properties Resistance Temperature Detectors (RTDS) Thermistors Thermocouples Changes in Chemical Phase Quartz Crystal Thermometry Changes in Emitted thermal radiation Radiation and Infrared Pyrometers 7/25/2017 Engineering Measurements 7 4 2

3 Bimetal Thermometer Two metals with different coefficients of thermal expansion are bonded together. As temperature changes the unequal expansion of the two metals will cause the bimetal strip to curl or bend. - Bimetal strips can be fabricated into coils, spirals, and disks. - Frequently used in on-off temperature control (thermostats) Range of use: - 65 to 430 C Accuracy varies with range; ± 0.5 to 12 C 7/25/2017 Engineering Measurements 7 5 Liquid Expansion Thermometer Volumetric expansion of liquids & solids is used for temperature measurement. - Expansion registered is actually the difference between the liquid & the glass. Mercury filled thermometers Range= -37 to 320 C, Accuracy ± 0.3 C Alcohol filled thermometers. Range =-75 to 120 C, Accuracy ± 0.6 C Upper range determined by boiling point. 7/25/2017 Engineering Measurements 7 6 3

4 Thermal Expansion Methods liquid-in-glass thermometer bimetallic thermometer capillary tube needle scale scale bimetallic strip bulb containing fluid motion of free end 7/25/2017 Engineering Measurements 7 7 Thermocouples 7/25/2017 Engineering Measurements 7 8 4

5 Thermocouples: Principle of Operation When two wires composed of dissimilar metals are joined at both ends & one of the ends is heated, an emf is generated. This is referred to as the Seebeck effect. Combined result of two phenomenon, Thomson & Peltier effects: - Thomson observed the existence of an emf due to the contact of two dissimilar metals. - Peltier discovered that an emf is generated when a temperature gradient along a conductor exists. 7/25/2017 Engineering Measurements 7 9 Seebeck Voltage If this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals. Metal A T abs V AB Metal B V AB = T abs For small T 7/25/2017 Engineering Measurements

6 Temperature Characteristics Thermoelectric Voltage [mv] Chromel-Constantan Chromel-Alumel VT ( ) at at 2 at 3... Platinum/13%Rhodium- Platinum 10 0 Platinum/10%Rhodium- Platinum Temperature [ C] 7/25/2017 Engineering Measurements 7 11 Thermocouple System Junctions V 3 J 3 Cu A A V 1 V Cu B B J 1 V 2 J 2 7/25/2017 Engineering Measurements

7 Law of Intermediate Metals A B C T 1 T 2 T 1 = T 2 =T A C T 7/25/2017 Engineering Measurements 7 13 Combined Junctions V 3 J 3 Cu A A V 1 V Cu B B J 1 V 2 J 2 V V V 2 V 1 V 3 T CuB J2 AB T J1 CuA T J3 7/25/2017 Engineering Measurements

8 Probe Configurations Ungrounded Junction is detached from the probe. Electrical isolation is obtained at the cost of response time. Grounded Junction is physically attached to probe wall. Good heat transfer to junction. Exposed Junction is exposed to the surrounding environment. Best response time. 7/25/2017 Engineering Measurements 7 15 Law of Intermediate Temperatures T2 T3 T1 T2 If a thermocouple circuit develops: i) a net emf 1-2 when the junctions are placed between temperatures T 1 and T 2, and, ii) a net emf 2-3 when junctions are between temperatures T 2 and T 3. Then the same thermocouple circuit would develop an emf 1-3 when the junctions are at temperatures T 1 and T 3. T3 T1 emf 1-3 = emf 1-2 +emf 2-3 7/25/2017 Engineering Measurements

9 Isothermal Block Thermistor or IC Temperature Reference V R V Cu Cu T R A B Isothermal Block (same temperature across) A B V1 T1 T R V1 T1 T 1 AB f (V R AB ) T R f (V R ) 7/25/2017 Engineering Measurements 7 17 RTD Sensors 7/25/2017 Engineering Measurements

10 RTD Principle of Operation An RTD is a precision temperature sensing device that utilizes metal conductors (typically a fine platinum wire or thin metallic layer applied to a substrate). Increases in temperature produce positive increases in the resistance, i.e., the sensing element is referred to as having a positive temperature coefficient (PTC). The RTD system error is minimized when the nominal value of the RTD resistance is large. This implies a metallic wire with a high resistivity. 7/25/2017 Engineering Measurements 7 19 Metal Resistivity Each metal has a specific resistivity, (ohm*m) which varies with temperature. R T o 1 ( T To ) Where: T L A L= metal wire length A=cross-sectional area Temperature Coefficient of resistivity ( o C o C Gold (Au) Aluminum(Al) Copper(C) Platinum (Pt) Tungsten (W) Nickel (Ni) Nichrome Lead (Pb) Iron (Fe) /25/2017 Engineering Measurements

11 RTD Resistance Temperature Curves R R Nickel Copper Platinum Tungsten C Temperature General temperature-dependence RT ( ) R 2 3 0(1 at 1 at 2 at 3...) Linearized temperaturedependence RT ( ) R0(1 at 1 ) 7/25/2017 Engineering Measurements 7 21 RTD Platinum Sensor Example The temperature coefficient of resistivity for pure platinum 20ºC is /ºC. A typical platinum RTD has resistance of 100. What is the sensitivity of a typical platinum RTD temperature 20ºC. 7/25/2017 Engineering Measurements

12 RTD Sensor Circuit A V exc + _ calibrated variable resistance D R 4 _ V m R 1 + RTD B R 3 R 2 C 7/25/2017 Engineering Measurements 7 23 RTD s Characteristics Low resistance: 100 (most common) to 1000 Wide operating range: -200 C to 850 C High sensitivity: compared to thermocouples High accuracy: ± C to 0.1 C High Repeatability & Stability: Low drift ( C/year) Industrial models drift < 0.1 C/year Slow response time Sensitive to shock and vibration Internal/self-heating: measure resistance by passing current through sensor. Joule Heating = I 2 R 7/25/2017 Engineering Measurements

13 Thermistors 7/25/2017 Engineering Measurements 7 25 Thermistors Thermistors are made from semiconductor materials and in general have a negative temperature coefficient (NTC). So as the temperature increases the resistance of a thermistor decreases (NTC). However, thermistors with a PTC are also available. The resistance-to-temperature relationships for these devices are highly non-linear. 7/25/2017 Engineering Measurements

14 Thermistors Thermistors have resistances from 1 k to 100 k - This high resistance eliminates most issues due to high lead resistance. Highly non-linear resistance-to-temperature relationships. - Thermistors are mostly made from metal oxides and have NTC, but barium and strontium titanate mixtures are models available and have a PTC Small physical size - Fast response time Very high sensitivity and resolution - Up to 1000 times more sensitive than RTD s Thermistors are not sensitive to shock and vibration 7/25/2017 Engineering Measurements 7 27 Thermistor Sensor Circuits Vexc (+) R 1 I t R t V Power Dissipation P d =I t V Thermistor 7/25/2017 Engineering Measurements

15 Infrared Temperature Sensing F F 7/25/2017 Engineering Measurements 7 29 Infrared Thermometers What is infrared (IR)? - Thermal energy which spans the wavelength range between 0.7 to 1000 m of the Electromagnetic (E&M) spectrum. - All substances above absolute zero emit thermal energy in the IR range. Infrared thermometers are non-contact devices that are able to estimate an object s temperature (T) by measuring the emitted IR energy. Modern IR instruments can readily measure between 0.7 & 50m due to their low sensitivity outside this wavelength range. 7/25/2017 Engineering Measurements

16 Electromagnetic Spectrum 7/25/2017 Engineering Measurements 7 31 Measurement Principles Energy traveling at the speed of light is transmitted as electro-magnetic (E&M) waves or photons. Radiation striking a surface is partially reflected, absorbed, and transmitted. P radiated Pincident Preflected Ptransmitted Pabsorbed P incident Pradiated Pabsorbed P absorbed emissivity absorption P reflected P transmitted Radiative Heat Transfer (Q): q=q/a q=f BA (T A4 -T B4 ) P radiated where the Stefan-Boltzmann constant, = ( x 10-8 W/m 2 K 4 ) 7/25/2017 Engineering Measurements

17 Thermal Radiation Spectral Radiance [W/m 3 srad] visible K K K K Wavelength [µm] Planck s law: I b 5 2hc e hc kt I b - black body spectral intensity (W/m 3 srad) h = x10-34 (Js); Planck constant c = x 10 8 (m/s); speed of light ~ 0.1 to 1000 m; wavelength k = x (J/K); Boltzmann constant T - absolute temperature [K] 2 1 7/25/2017 Engineering Measurements 7 33 Thermal Radiation Heat Source Testpiece Infrared Film or Camera Stefan-Boltzmann law E b 0 I b ( ) d T 4 where E b is the blackbody emissive power and 5.67x10-8 [W/(m 2 K 4 )] 0 E ( ) I b ( ) d T 4 7/25/2017 Engineering Measurements

18 Thermal Radiation Example The energy emitted from a piece of metal is measured, and assuming an emissivity of 0.82 the temperature is estimated to be 1050 C. It is later found that the true emissivity is only Determine the corrected temperature? 7/25/2017 Engineering Measurements 7 35 Overview of IR Thermometers IR thermometers are remote sensing and can measure objects that translate, rotate, or vibrate. They do not damage or contaminate the surface of interest (i.e., food, painted surfaces, or other delicate surfaces). A typical response time is in the millisecond range. However, material emissivity is subject to a great amount of uncertainty because it depends on surface finish, color, oxidation, aging, etc. IR thermometers generally have a low dynamic range and low sensitivity. 7/25/2017 Engineering Measurements

19 Non-Electronic Indicators 7/25/2017 Engineering Measurements 7 37 Liquid Crystals Certain cholesteric liquid crystals produce changes in reflectance and hence color as a function of temperature. The range and resolution of liquid crystal thermometers is varied by adjustment of the formulation. Temperature ranges from 0 C to several hundred C are available. Typical temperature resolution is on the order of ~ 0.1 C. 7/25/2017 Engineering Measurements

20 Liquid Crystals Liquid crystals are long chain molecules which exist in a helical type configuration. Due to their thermal sensitivity, the pitch of these helical structures change with temperature, producing a corresponding change in its optical property. 7/25/2017 Engineering Measurements 7 39 Temperature Sensitive Indicators The maximum temperature of an object can also be detected using various crayons and paints. These devices are produced with pigments dispersed throughout the material, with an approximate melting temperature. As the melting temperature is approached, the color of the pigment deepens due to changes in the light scattering properties of the pigment powder. Temperature sensitive glass tiles work in a similar fashion. 7/25/2017 Engineering Measurements

21 Temperature Measurement Errors Conduction Convection Radiation Response Time Electrical Noise or Interference Sensor Self-Heating Thermal Gradients in the material being measured Grounding issues and shorts, especially on metal surfaces Calibration stability (static or dynamic) 7/25/2017 Engineering Measurements