THERMOMETER

There are four main types available:

1. Those which rely on the expansion of a liquid or solid as the temperature rises. The best known of these are the mercury-in-glass type, of which there are many variations, notably the maximum reading clinical thermometer.

2. Chemical thermometers which indicate temperature by change of state or colour. Liquid crystal material can be fabricated to change state (and optical properties) at a particular temperature. If combined with a dye the colour of the material will indicate whether temperature is above or below the transition temperature. An array of spots of such material may be arranged to form a graduated thermometer. Such devices are available for clinical use, as are special paints for the skin which produce a colour temperature map of parts of the body.

3. Electrical thermometers. These have the advantage that the temperature sensing element can be very small (e.g. 0.5 mm) and remote from the indicating part of the instrument, or even placed inside a hypodermic needle. Electrical thermometers may use thermocouples which produce a voltage proportional to the temperature difference between two bi- metal junctions by the Seebeck effect, or they may use thermistor types which sense the change in resistance due to temperature rise which occurs in some materials.

4. Radiation thermometers. There is a known relationship between the surface temperature of an object and its radiant power. Thus temperature can be sensed remotely by detecting the infrared radiation, provided the surface in question approximates to a black body radiator. Human skin meets this requirement. Radiation thermometry is employed in thermography to produce maps of the surface temperature of parts of the human body.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details