Measurement of temperatures of soil profile

A. Contact type: It includes Contact type includes thermometers in which the thermal expansion of solid, liquid or gas is measured

• Mercury or alcohol in thermometer
• Bimetallic strip thermometer
• Constant pressure or constant volume thermometer
• Electric resistance thermometer
• Thermocouple

B. Non-contact type: In non-contact type, the temperature measurement is based on the measurement of radiation from an emitter. This includes;

• Optical pyrometer
• Total intensity radiometer

Bimetallic thermometer

• These are made by welding together two bars of the different metals like invar-bronze and rolling the resulting compound bar into a strip.
• Because of difference in linear expansion of two metals, strips bend in response to temperature changes.
• A pointer to indicate the amount of angular deformation is attached to strip. The angular deflection is calibrated in terms of temperature.

Mercury or alcohol in glass thermometer

• In these thermometers, expansion of liquid (mercury or alcohol) due to change in temperature is measured on a temperature calibrated scale.
• These have been modified by meteorologists for measuring soil temperature at different depths under field conditions.
• These are generally of two types: those with straight stem having a bend of about 1200 just above the bulb and those in which stem is bent above the soil surface and capillary tube lies parallel to the ground.

Expansion thermometers (Bourdon Gauge) using liquid and gases

• Thermometers that use a liquid are of dial type in which the change in volume of liquid in a metallic bulb due to changes in temperature is indicated by a gauge to which the bulb is connected.
• In a constant volume gas thermometer, the change in pressure of a constant volume of an inert gas is measured by Bourdon type pressure gauge on temperature scale. In vapour pressure thermometers, a volatile liquid partially fills a metal bulb, which is connected by a Bourdon type pressure gauge by means of capillary tubing. Variation in saturation vapor pressure of liquid due to changes in temperature is measured by Bourdon pressure gauge on temperature scale.

Electrical resistance thermometer

• There are two types of resistance thermometers in a general use. One type depends upon the increase in resistance with an increase in temperature of a wire such as nichrome, copper, silver or platinum.
• Resistance changes are measured with bridge or in some cases a potentiometer and are related to temperature changes. Second type of the resistance thermometer is semiconductor called thermistor. The thermistors have a high negative temperature coefficient of resistance.

Thermocouples

• The thermoelectric junctions called thermocouples are made by joining two dissimilar metals at two different places to form two junctions.
• If the entire circuit is composed of only two metals, the total electromotive force in the circuit is proportional to the difference in the temperature of two junctions. One junction is called measuring (hot) junction and another junction is called reference (cold) junction.
• For measurement of temperature, reference junction is kept at constant temperature. The electric potential produced is usually measured with a potentiometer
• Thermistors and thermocouples give a continuous record of temperature and are smaller in size and almost instantaneous in response to temperature changes.