Measurement of air temperature

a) Mercury or alcohol in glass thermometer:

• This thermometer is invariably used for routine observation of air temperature.
• Alcohol in glass thermometer is used in very cold region. However, mercury in glass thermometer is most common in normal and high temperature measurement.
• There is expansion or contraction of mercury or alcohol due to change in temperature which is measured by observing the graduated capillary tube of the thermometer.
• The accuracy of the measurement depends upon the types of thermometer’s constriction and calibration and method of handling.

b) Maximum minimum thermometer:

• It consists of two thermometer-one to measure maximum temperature called maximum thermometer and next to measure minimum temperature of a time period called minimum thermometer.
• Maximum thermometer is similar to mercury in glass thermometer except that it is provided with a small constriction in capillary tube. There is rise of mercury column up to the maximum temperature it attains. However, if the temperature falls, the mercury in the constriction contracts and the threads breaks but the level of mercury remains in the capillary tube which indicates the maximum temperature of the measuring period.
• Similarly, minimum thermometer is generally alcohol in thermometer which can measured temperature below freezing point also. In its capillary tube, a metal index is fitted which is made as such that alcohol can pass from inner side of the index at rising function. However, with decrease in the temperature, the alcohol cannot pass throughout the index and it pulls the index down.

c) Bimetallic Thermograph:

• It makes the use of a curved metallic strip. Bimetallic strip is formed by welding together two strips of different metals. Due to temperature increase, both expand, but two metals expand at different rates.
• So, it leads the strips to bends. And this variation in strip curvature transduces to change in temperature and calibrated. There is a rotating drum, rotating in a fixed speed (clock driven rotating drum).
• And variation in curvature due to temperature change is presented on the graph by pen arm showing the temperature record of the measure period.