Chapter 2 Measurement of Temperature

Chapter 2 Measurement of Temperature

CONTENTS

2.1 Definition and units .............................................................................................................. 1 2.1.1 Definition of atmospheric temperature......................................................................... 1 2.1.2 The international temperature scale ............................................................................... 2

2.2 Principles of Instruments ...................................................................................................... 3 2.2.1 Electrical Thermometers................................................................................................. 3 2.2.2 Liquid-in-glass Thermometers....................................................................................... 5 2.2.3 Bimetallic Thermographs ............................................................................................... 7 2.2.4 Clock-driven Drums......................................................................................................... 8 2.2.5 Louvered Screens............................................................................................................ 9 2.2.6 Ventilated Shields .......................................................................................................... 10

2.3 Exposure and Siting............................................................................................................. 11 2.3.1 Observation Fields ........................................................................................................ 11 2.3.2 Louvered Screens and Ventilated Shields ................................................................. 11 2.3.3 Thermometers .............................................................................................................. 12

2.4 Maintenance .......................................................................................................................... 12 2.4.1 Routine Maintenance .................................................................................................... 12 2.4.2 Periodic Maintenance.................................................................................................... 13

2.5 Calibration ............................................................................................................................. 14 2.5.1 Freezing-point Calibration ............................................................................................ 14 2.5.2 Calibration other than Freezing Point ......................................................................... 17

2.6 Adjustment and Repair ........................................................................................................ 18 2.6.1 Electrical Thermometers............................................................................................... 18 2.6.2 Liquid-in-glass Thermometers..................................................................................... 18 2.6.3 Bimetallic Thermographs ............................................................................................. 20 2.6.4 Clock-driven Drums....................................................................................................... 21

2.7 Transport ............................................................................................................................. 21

Chapter 2 Measurement of Temperature

2.1 Definition and units Heat balance difference of atmosphere between regions creates temperature distribution. This

temperature distribution generates wind current along with cloud and rainfall phenomena. Thus, atmospheric temperature is one of the most important meteorological elements as well as wind and precipitation.

WMO recommends to measure atmospheric temperature at the height from 1.25 to 2m above ground at a representative location of region, as standard.

2.1.1 Definition of atmospheric temperature The thermodynamic temperature is defined as one of the seven quantities (length, mass, time, electric

current, thermodynamic temperature, amount of substance and luminous intensity) in the International System of Unit (SI). The definition of unit is as described below:

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Temperature and temperature difference can be expressed in both Kelvin and Celsius. Relation of temperature in degree Celsius (t, unit : ?C) and thermodynamic temperature in Kelvin (T, unit : K) is shown as follows. Degree Celsius is commonly used in meteorological observation.

t/?C = T/K - 273.15

Phase diagram of water and outline of water triple point cell is shown in Figure 2.1 and Figure 2.2.

Pressure

about 610

Water (liquid) Ice (solid)

Triple point

273.16 K (0.01 ?C)

Temperature

Figure 2.1 Phase diagram of water

Thermometer well Water vapor (gas) Water (liquid) Ice (solid)

Figure 2.2 Water triple point

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2.1.2 The international temperature scale The international temperature scale is a temperature scale to accord with the results of thermodynamic

temperature measurements along with the definition of thermodynamic temperature in 2.1.1 and it is defined based on several fixed points of temperature (defining fixed points) such as phase transition temperature of substance and using several types of stable thermometers. First international temperature scale was established in 1927. Since then it was revised several times to expand temperature range and improve accuracy. Currently the international temperature scale of 1990 (ITS-90) is in effect.

The ITS-90 defines the fixed points of temperature, the type of instrument along with method to interpolate in between fixed points of temperatureTable 2.1, Table 2.2.

For the temperature range used in meteorological observation, a platinum resistance thermometer is specified as the interpolate instrument. To be a standard, an acceptable platinum resistance thermometer must be made from pure, strain-free platinum, and it must satisfy at least one of the following two relations:

R (29.7646 ?C) / R (0.01 ?C) >= 1.11807 R (-38.8344 ?C) / R (0.01 ?C) ................
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