MEASUREMENT OF TEMPERATURE. Temperature is the condition which determines the direction of the net flow of heat between two bodies. Meteorological requirements for temperature measurements primarily related to : The air near the Earth’s surface, The surface of the ground,
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Meteorological requirements for temperaturemeasurements primarily related to :
The liquid used depends on the required temperature range; mecury is generally used the temperatures above its freezing point (-38.3°C), While ethyl alcohol or other pure organic liquids are used for lower temperatures.
A. Bimetallic Thermograph
The general arrangement is similar to that of the bimetallic type but its temperature-sensitive element is in the form of a curved metal tube of flat, elliptical section, filled with alcohol.
In the thermograph mechanism itself, friction is the main source of error.
A. Electrical resistance thermometers
B. Semiconductor thermometers
The most frequently used sensors are electrical resistance, semiconductor thermometers (thermistors) and thermocouples.
A measurement of the electrical resistance of a materialwhose resistance varies in a known manner with the temperature of the material can be used to represent the temperature.
Pure platinum best satisfies the foregoing requirements. Copper is a suitable material for use in secondary standards. Practical thermometers are artificially aged before use and are commonly made from platinum alloys, nickel, or copper (and occasionally tungsten) for meteorological purposes. Usually they are hermetically sealed with either glass or ceramic. Nevertheless, their time constant is smaller than that of the liquid-in-glass thermometers.
This is a semiconductor with a relatively large temperature coefficient of resistance, which may be either positive or negative depending upon the actual material. Mixture of sintered metallic oxides are suitable for making practical thermistors, which usually take the form of small disc, rods, or spheres and are often glass-coated.
If a simple circuit is made with two metals and with the conjuction at the same temperature there will be no resultant electromotive forces, one at each junction, will exactly oppose and cancel one another. If the temperature of one junction is altered, the two electromotive forces no longer balance and there is a net electromotive force set up in the circuit; a current then flow. For meteorology, thermocouples are mostly used when a temperature of very small time constant, of the order of one or two seconds, and capable of remote reading and recording. Copper-constantan or iron-constantan combinations are suitable for meteorological work.
The most common types of thermometers used in an AWOS are pure metal resistance thermometers or thermistors. The platinum resistance thermometer (PT100, 100Ω at 0°C) shows a very good long time stability and can be considered as a first candidate sensor.