Introduction To Bimetallic Thermometer

 

A liquid-in-glass thermometer is widely used due to its accuracy for the temperature range -200 to 600°C. Compared to other thermometers, it is simple and no other equipment beyond the human eye is required. The LIG thermometer is one of the earliest thermometers. It has been used in medicine, metrology and industry. The first thermometer appeared around 1650 and was a development from the thermoscope. The liquid used was spirit from wine. By 1714, thermometers with mercury were found to give a more linear scale than spirits. By 1742, a centigrade scale using 100 steps from the point of boiling water to the melting point of water was suggested by Anders Celsius.

In the LIG thermometer the thermally sensitive element is a liquid contained in a graduated glass envelope. The principle used to measure temperature is that of the apparent thermal expansion of the liquid. It is the difference between the volumetric reversible thermal expansion of the liquid and its glass container that makes it possible to measure temperature.

 

The liquid-in-glass thermometer comprises:
 

1. A bulb, a reservoir in which the working liquid can expand or contract in volume
2. A stem, a glass tube containing a tiny capillary connected to the bulb and enlarged at the bottom into a bulb that is partially filled with a working liquid. The tube's bore is extremely small - less than 0.02 inch (0.5 millimetre) in diameter
3. A temperature scale is fixed or engraved on the stem supporting the capillary tube to indicate the range and the value of the temperature. It is the case for the precision thermometers whereas for the low accurate thermometers such as industrial thermometer, the scale is printed on a separate card and then protected from the environment. The liquid-in-glass thermometers is usually calibrated against a standard thermometer and at the melting point of water
4. A reference point, a calibration point, the most common being the ice point
5. A working liquid, usually mercury or alcohol
6. An inert gas is used for mercury intended to high temperature. The thermometer is filled with an inert gas such as argon or nitrogen above the mercury to reduce its volatilization.

 

The accuracy of measurement depends mainly on the extent of immersion of the thermometer into the medium - not just the bulb but also the stem. There are three types of immersion, as shown in the following figure: total, partial and complete immersion, depending on the level of contact between the medium and the sensor. The response of the thermometer depends on the bulb volume, bulb thickness, total weight and type of thermometer. To reduce the response time, the bulb should be small and the bulb wall thin. The sensitivity depends on the reversible thermal expansion of the liquid compared to the glass. The greater the fluid expansion, the more sensitive the thermometer.

Mercury was the liquid the most often used because of its good reaction time, repeatability, linear coefficient of expansion and large temperature range. But it is poisonous and so other working liquids are used. Common organic liquids are toluene, ethyl alcohol, pentane; their expansion is high but not linear and they are limited at high temperature. They need to be dyed, the most common colours being red, blue and green. During calibration the thermometer must be correctly immersed in the calibration bath to ensure accurate measurements and to avoid a systematic error linked to the height of the emergent column above the surface of the bath. For precise work, the measurement can be performed with a microscope attached to the thermometer. For very high or very low temperatures, as well as for very accurate measurements, liquid-in-glass thermometers are not suitable. Various types of electrical thermometers are used instead because they are more robust and can be digitised and automated.