1. Met 163: Lecture 4 Chapter 4 Thermometry

2. Air Temperature The measurement of air temperature dates back to the time of Galileo(1564-1642). It is perhaps the premier measurement of meteorological parameters. Measurements of temperature are not free of errors or problems due to response time, accuracy of instrument, and operational need. Errors in the measurement of air temperature in excess of 2-3 °C are not uncommon.

3. Air Temperature Errors in the measurement of air temperature in excess of 2-3 °C are not uncommon. These errors are acceptable for the general public. However, numerical models of all scales of motion are greatly affected by errors as large as 1 °C. Errors of just 1 °C in mesoscale models have been shown to be the deciding factor between no storms initiated and intense storms. Additionally, errors as small as 0.2 °C can change the prediction of a global climate model.

4. Air Temperature Temperature sensors can be categorized according to the physical principle that they use: Thermal expansion Thermoelectric (thermocouples) Electrical resistance Electrical capacitance and some other effects… Direct indicating instruments can use displacement directly: (liquid in glass thermometers, bimetallic strips) Sensors designed to work with data loggers usually convert the output to a voltage signal.

5. Air Temperature Raw output of a temperature sensor may be displacement, voltage, resistance, capacitance, etc. Temperature Sensor T Y 1 = X, V, R, C

6. Air Temperature: Thermal Expansion One way thermal expansion is exploited to sense temperature is through the use of bimetallic strips and liquid-in-glass thermometers. In both cases the difference between expansion coefficients of two materials is used. Linear expansion is given by: ΔL=αL 0 ΔT Where α is the coefficient of linear expansion, L 0 is the length of the material when ΔT = 0, ΔL is the change in length and ΔT is the temperature change from some arbitrary temperature where L 0 was measured.

7. Air Temperature: Thermal Expansion

8. Bimetallic Strip: A bimetallic strip is a pair of metals with different thermal expansion coefficients that have been bonded together. At the reference temperature, the temperature at which bonding took place, the strip maintains its original shape. When the temperature changes, the strip bends in a circular arc, for small deflections, due to the differential expansions of the two components of the strip, where one end is held in a fixed position.

9. Air Temperature: Thermal Expansion

10. Air Temperature: Liquid-in-glass Thermometer A Liquid-in-glass thermometer is a glass tube with a bulb at one end filled with the liquid and a scale fastened to or etched on the glass tube. The liquid is usually mercury or alcohol. Mercury can be used on above -39°C, its freezing point, alcohol can be used down to -62°C.

11. Air Temperature: Liquid-in-glass Thermometer

12. One way to classifying these thermometers is by the immersion required. Immersion types are partial, total and complete. A partial immersion thermometer should be placed in the bath liquid until the bulb and a small portion of stem, indicated by an immersion line, are immersed in the liquid to be measured. For a total immersion thermometer, the bulb and the portion of the stem containing the thermometric fluid are immersed. Partial and total types are used for calibrating other sensors.

13. Air Temperature: Liquid-in-glass Thermometer A complete immersion thermometer, the bulb and the entire stem are immersed. This type is used for air temperature measurement. Two special liquid-in-glass thermometers are used to measure the minimum and maximum temperature. The minimum thermometer uses alcohol with a dumbbell in the stem. The thermometer is mounted horizontally, the alcohol 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.

14. Air Temperature: Liquid-in-glass Thermometer The maximum thermometer uses mercury and has a constriction in the stem. The bulb is mounted slightly higher than the rest of the column, and as the temperature increases the volume of mercury in the bulb increases and mercury is forced through the constriction. When the temperature decreases, the column breaks at the constriction. The remaining column above the constriction indicates the maximum temperature.

15. Air Temperature: Liquid-in-glass Thermometer