Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Thermal conductivity wikipedia , lookup
Dynamic insulation wikipedia , lookup
Hypothermia wikipedia , lookup
Radiator (engine cooling) wikipedia , lookup
Thermal comfort wikipedia , lookup
Intercooler wikipedia , lookup
R-value (insulation) wikipedia , lookup
Underfloor heating wikipedia , lookup
Solar air conditioning wikipedia , lookup
Thermal conduction wikipedia , lookup
Temperature Measurement In microscopic world, temperature measures the average kinetic energy of its atoms as they move. In macroscopic world, it a physical property of a system that underlies the common notions of hot and cold. Temperature scales mixture of water and ice boiling water Celsius (Centigrade) 100o C 0o C Fahrenheit 212 o F 32o F To F aTo C b T o F 95 T o C 32o F Absolute temperature (Kelvin) Tk T oC 273.15 Thermometer Calibrations Three reference points T ax b Ice Kelvin 273.15 Celsius 0.00 Triple Point Steam 273.16 0.01 373.15 100.00 Types of Thermometers Primary thermometers: temperature can be calculated without any unknown quantities. Secondary thermometers: knowledge of the measured property is not sufficient to allow direct calculation of temperature. Need to be calibrated against a primary thermometer. 1. Gas thermometer Vconst T P mR m: air mass; R: the gas constant. Environment to be measured 2. Liquid in glass thermometer V V Volume expansion of glass:1.2-2.7x10-5 per 1.00oC, Volume expansion of Hg:18x10-5 per 1.00oC 3. Maximum thermometer 4. Minimum thermometer 5. Six's thermometer 6. Bi-metallic strip 7. Electrical thermometers Thermocouples Seebeck effect: any conductor that is subjected to a thermal gradient will generate a voltage. Emf) output ~ temperature difference Absolute temperature Resistance Electrical resistance of some materials changes with changing temperature. Resistance thermal sensor can be made with small wires to give fast response film Wire-wound Thermistors A thermistor is a type of resistor whose resistance varies with temperature. Thermistors differ from resistance temperature sensors in that the material used in a thermistor is generally semi-conductor, such as ceramic or polymer, while the latter uses pure metals. The temperature response is also different; the resistance temperature sensors are used for larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range Radiation thermometers Based on the principle that the radiation emitted from an object depends on its temperature. By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can be determined. They are, sometimes, called laser thermometers if a laser is used to help aim the thermometer to enhance the device’s ability to measure temperature from a distance. Thermometer Response Time The time a system or sensor takes to react to a given input Factors that control the time constant Cs cs V AH conv inversely proportional to heat transfer rate. proportional to the heat capacity csV cs d AH conv H conv aU b , b 0.5 Making temperature measurements in the air 1. Air is a poor conductor, thus, a good flow over the sensor should be maintained. 2. Sensor to be thermally insulated from the mounting. 3. To prevent radiation, sensors can be polished or coated to reflect solar radiation and to reduce the absorption of infrared radiation. A shield can also be used to shelter the sensor, but it needs to be aspirated to ensure proper ventilation. 4. Heating by adiabatic compression may occur when a sensor is exposed to air moving at very high rates, e.g., aircraft measurements. Adiabatic heating needs to be corrected. 5. Wetting of a temperature sensor will lower the measured temperature due to evaporative cooling. Upper air measurements can be affected as a sensor goes though a cloud. A special device is needed to prevent sensor wetting. For surface measurements, the radiation shield should keep the sensor dry.