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ATMS 320 Meteorological Instrumentation

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ATMS 320 Meteorological Instrumentation

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    1. Thermometry objectives: Understand the various methods used to measure air temperature Appreciate the similarities and differences of these methods Learn the issues related to the proper exposure of instrumentation for sensing air temperature ATMS 320 – Meteorological Instrumentation

    2. Review – what is temperature of an ideal gas? proportional to the mean kinetic energy of random motion of the molecules of the gas ATMS 320 – Thermometry

    3. Why worry about temperature? Errors of just 1oC in a mesoscale model have been shown to be the deciding factor between storms/no storms. Small errors can change the prediction of a global climate model ATMS 320 – Thermometry

    4. Temperature sensor categories: Thermal expansion (X) Thermoelectric (V) Electrical resistance (R) Electrical capacitance (C) ATMS 320 – Thermometry

    5. Thermal expansion – bimetallic strip A pair of metals with different thermal expansion coefficients that have been bonded together The strip maintains its original shape at the reference temperature (when bonding took place) The strip bends in a circular arc when the temperature changes ATMS 320 – Thermometry

    6. Bimetallic strip Common in thermostats Static sensitivity ATMS 320 – Thermometry

    7. Thermal expansion – liquid-in-glass thermometer Glass tube with a bulb at one end filled with the liquid and a scale fastened or etched on the glass tube Classified according to the immersion required Special types; minimum and maximum thermometers ATMS 320 – Thermometry

    8. Liquid-in-glass thermometer - minimum Liquid flows around the dumbbell as the temperature increases and leaves the dumbbell in a fixed position When the temperature decreases, the meniscus of the alcohol does not let the dumbbell pass but drags it down to indicate the minimum temperature ATMS 320 – Thermometry

    9. Liquid-in-glass thermometer - maximum As the temperature increases the volume of liquid in the bulb increases and the liquid is forced through the constriction When the temperature decreases, the column of liquid breaks at the constriction The remaining column above the constriction indicates the maximum temperature ATMS 320 – Thermometry

    10. Liquid-in-glass thermometer – immersion types Partial* Total* Complete – used for air temperature measurement ATMS 320 – Thermometry

    11. Liquid-in-glass thermometer – change of volume and static sensitivity ATMS 320 – Thermometry

    12. Thermoelectric sensors – the junction of two dissimilar metals forms a thermocouple. When the two junctions are at different temperatures, a voltage is developed across the junction ATMS 320 – Thermometry

    13. Thermoelectric sensors – thermocouple advantages Provide a useful temperature range Rugged Reliable Inexpensive Fast response ATMS 320 – Thermometry

    14. Thermoelectric sensors – thermocouple disadvantages Low output (requires an amplifier) Slight nonlinearity Need for calibration ATMS 320 – Thermometry

    15. Thermocouple Measures differential temperature Absolute temperature measurements can be made only if one of the junctions is held at a known temperature or if an electronic reference junction is used ATMS 320 – Thermometry

    16. Thermocouple – example Desire temperature range of -50 to 50oC Desire corresponding sensor output of 0 and 5 V (volts) at 0 and 50oC, respectively. “Seebeck effect” at 50oC is 2.036 millivolts Hence, we need a gain* of 5/(2036 x 10-6) = 2456 ATMS 320 – Thermometry

    17. Electrical resistance sensor – a sensor whose resistance varies as a function of temperature ATMS 320 – Thermometry

    18. Electrical resistance sensors – Resistance Temperature Detectors (RTDs) made of platinum Advantages Stable Resists corrosion Easily workable Has a high melting point Capable of high-level purity Disadvantage Changing shape changes the resistance ATMS 320 – Thermometry

    19. Electrical resistance sensors – RTDs Because the RTD resistance is fairly low and the change with temperature is small, a bridge circuit is often used. Converts resistance to voltage and can be amplified to a reasonable level ATMS 320 – Thermometry

    20. Electrical resistance sensors – RTDs bridge circuit ATMS 320 – Thermometry

    21. Electrical resistance sensors – RTDs Need to control sensor self-heating due to current flow through the RTD. Self-heating spec: PD divided by the tolerable temperature error ATMS 320 – Thermometry

    22. RTD - example Given self-heating spec of 5.9 mW/oC Willing to tolerate temperature error of 0.1oC At temperature of 0oC; PD(0) = VR2/(4R0) and must be less than 5.9 x 10-4 W If R0 is 500 Ohms, then VR must be less than 1.09 V ATMS 320 – Thermometry

    23. RTD – example (cont.) Desire a temperature range of -50 to 50oC Desire a voltage range of -5.00 to 5.00 V over temperature range V3 (at -50oC) = -5.00V so G = 93[Eq (4.7)] ATMS 320 – Thermometry

    24. RTDs Non-linearity is due almost entirely to the bridge circuit Copper wires supplying power to the sensor have non-negligible resistance and have some impact on the accuracy of the circuit ATMS 320 – Thermometry

    25. RTD calibration Function of resistors, reference voltage, amplifier gain Impacted by drift, temperature sensitivity, supply voltage sensitivity Microprocessor can be used to control these effects ATMS 320 – Thermometry

    26. Electrical resistance sensors – thermistors Temperature-sensitive semiconductors Large and nonlinear temperature sensitivity Less sensitive to the resistance of lead (pronounced l-EE-d) wires ATMS 320 – Thermometry

    27. Thermistors – circuit to linearize (example) ATMS 320 – Thermometry

    28. Thermistors – example (cont.) V2 = 0.65107 VR Set VR = 1.00 V Want V3 = 5.00 V at T = 50oC, so G = 14.7 ATMS 320 – Thermometry

    29. Comparison of temperature sensors Ratings made on basis of cost, reliability, size, and ease of use Ratings change as manufacturing technology improves (e.g. RTDs once were too large and expensive) Amplifier gain required to bring the sensor output into conformance with the input requirements of an ADC ATMS 320 – Thermometry

    30. Exposure of temperature sensors To accurately measure the air temperature the sensor must be in good thermal contact with the air Air circulation is required to promote heat transfer by convection Sensor must be protected from conductive heat flow along the mechanical support, and from radiative heat transfer ATMS 320 – Thermometry

    31. Exposure of temperature sensors Heat transfer by conduction, radiation, and convection ATMS 320 – Thermometry

    32. Exposure of temperature sensors Radiosondes Agricultural applications Small, highly reflective sensors ATMS 320 – Thermometry

    33. ATMS 320 – Thermometry Exposure of temperature sensors Trade-off between shielding from radiative effects and allowing natural air-flow Smallest possible diameter and lowest possible absorptivity

    34. Exposure of temperature sensors Example for conditions of maximum solar radiation, light winds, and snow (highly reflective ground surface) Uses multi-plate shield as shown in Fig. 4-14a ATMS 320 – Thermometry

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