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) Fahrenheit Absolute temperature (Kelvin) Thermometer Calibrations Ice Triple Point Steam Kelvin 273.15 273.16 373.15 Celsius 0.00 0.01 100.00 Three reference points
Primary thermometers: temperature can be calculated without any 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 m: air mass; R: the gas constant. Environment to be measured
2. Liquid in glass thermometer 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. 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 , sometimes it is also called sensor’s time constant Factors that control the time constant proportional to the heat capacity inversely proportional to heat transfer rate. U: ventilation rate
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.