The Liquid in glass thermometer is widely used for measuring temperature. Temperature is defined as the degree or intensity of heat that is present within a substance or system. Temperature is expressed based on a comparative scale.
To measure temperature, different temperature measuring methods are used like expansion thermometers, filled-system thermometers, electrical thermometers, and pyrometers.
The SI unit of temperature is Kelvin (K). In general, the Celsius(C) and Fahrenheit (F) units are used in temperature measurement.
Classification of Expansion Thermometers
Expansion thermometers are classified as;
- Expansion of solids – Bimetallic Thermometer
- Expansion of Liquids- Liquid in glass thermometer
- Expansion of Gases – Gas Thermometer
In this article, we will discuss Liquid in Gas Thermometers.
Working Principle of Liquid in Glass Thermometer
The liquid in glass thermometers is a type of expansion thermometer. The liquid in a glass thermometer work on the principle of apparent thermal expansion of liquids. Some liquids have the tendency to expand when the temperature rises.
Thus in the liquid in glass thermometers, the liquid inside the thermometer’s bore capillary will rise when the temperature rises and will decrease when the temperature decreases.
Construction of Liquid in Glass Thermometer
The liquid in glass thermometer is the simplest thermometer. The liquid in glass thermometers has a small bore glass tube with a thin-wall glass bulb at the lower end. The thermometric liquid which is contained in the bulb is generally mercury; however, some other liquids like blue or red-colored ethanol (alcohol) can also be used. A temperature scale is indicated on the capillary.
The heat is transferred from the well to the glass stem and then into the mercury (or any other thermometric liquid used in the thermometer). Thus, the mercury will expand and thus push the column of mercury higher in the capillary above which the temperature scale is indicated.
The liquid in glass thermometer type of thermometer is used for temperatures ranging from -18.4 to 608° F (-120 to 320°C). When mercury is used as the thermometric liquid, it will freeze at -38.2°F (-39°C). In applications involving the measurement of very low temperatures, alcohol is used as a thermometric liquid.
In applications involving the measurement of high temperatures, the glass stems above the mercury can be charged with nitrogen at a pressure of 30 to 300 psi. This would help in preventing mercury from evaporating or boiling.
Selection of Thermometric Liquids
The selection of the thermometric liquids depends on certain factors such as:
- They should have particular properties that are suitable for use for example they should not freeze at lower temperatures.
- The thermometric liquid should have a sufficiently high boiling point so as to avoid vaporization at high temperatures.
- They must expand uniformly with respect to temperature in the measurement range.
- Earlier the mercury which was used was very toxic and poisonous. It has a solidification temperature of -39°C and a boiling temperature of 357°C.
- Conventional thermometers used mercury as the thermometric liquid. Mercury is highly toxic. It has a solidification temperature of -39 °C and a boiling temperature of 357 °C.
- Hence, blue or red-colored ethanol (alcohol) is now used instead of mercury as the thermometric liquid in the liquid in glass thermometers.
- This ethanol has a melting point of -115°C, & a boiling point of 78°C.
- For everyday use, the temperature generally ranges between -20°C and +50°C.
- Some other liquids can also be used as thermometric liquids. These are mentioned below along with their temperature range.
| Liquid in thermometer | Temperature range |
| Mercury | -90 to 100-degree Centigrade |
| Toulene | -110 to 100-degree Centigrade |
| Ethyl Alcohol | -200 to 20 degrees Centigrade |
| Pentane | -200 to 20 degree Centigrade |
Types of Liquid in Glass Thermometers
There are two types of liquid in glass thermometers that are available these days for everyday use. They are:
- Mercury thermometers
- Alcohol thermometers
Mercury Thermometers
The mercury thermometers were developed by a German-based physicist named Daniel Gabriel Fahrenheit. In mercury thermometers, mercury is used as the liquid filled in a glass tube. The body of the glass tube has calibrated markings on it to facilitate the reading of temperature.
The bulb present at one end of the thermometer contains the largest part of mercury. When there is a change in temperature, the expansion or contraction of mercury will take place in an extremely thin bore of the glass tube. This will help in increasing the sensitivity of the thermometer.
Generally, the area that is above the mercury is filled with inert gases like nitrogen, however, the space above the mercury can be kept evacuated also.
Alcohol Thermometers
Alcohol thermometers are used instead of mercury thermometers in many applications. The alcohol thermometers have alcohol filled inside the bulb of the thermometer. The alcohol thermometers operate in a temperature range from 115°C to 785°C.
Hence, the freezing point is 115°C and the boiling temperature point of alcohol is 785°C. Since alcohol is a volatile substance, there are chances of a parting of the column in the thermometer due to mechanical shock. The alcohol thermometers possess a disadvantage in that they cannot be employed with a parted fluid column because they generally lead to incorrect temperature measurement.
Application of Liquid in Glass Thermometers
- Liquid in glass thermometers can be used to measure the temperature of the human body.
- Liquid in glass thermometers can be used to measure the temperature of liquids in open tanks or containers.
- Liquid in glass thermometers can be used to measure the temperature of air ducts.
- Liquid in glass thermometers can be used to measure temperature in various fluid flow pipes and other industrial applications.
Advantages of Liquid in Glass Thermometers
- Liquid in glass thermometers is less expensive than other temperature-measuring equipment.
- Liquid in glass thermometer is convenient and efficient to use.
- The liquid in glass thermometers does not need a power supply or battery to charge, unlike electrical thermometers.
- The liquid in glass thermometers has good repeatability.
- The liquid in the glass thermometer’s calibration stays unaffected.
Disadvantages of Liquid in Glass Thermometers
- Liquid in glass thermometers cannot be used in areas where very accurate readings are required.
- Liquid in glass thermometers cannot give digital results.
- The liquid in glass thermometers is very weak and delicate as compared to other temperature sensors.
- Liquid-in-glass thermometers can be used only where manual reading is sufficient.
