There are two major types of Liquid-in-glass thermometers which are mentioned below:
- Mercury Thermometers
- Alcohol Thermometers
This type of thermometer was developed by a German based physicist named Daniel Gabriel Fahrenheit. It consists of mercury as a liquid filled in a glass tube. On the body of the glass tube, calibrated marks are provided which facilitates the reading of temperature. A bulb is formed at one end of the thermometer which contains the largest part of mercury. The expansion and contraction of this mercury size is then further increased in the extremely thin bore of the glass tube. It aids in increasing the sensitivity of the thermometer. In general, the area over the mercury is filled with inert gases like nitrogen. However, this area can be left evacuated too.
Various types of mercury-in-glass thermometers are available. “A maximum thermometer is a unique kind of mercury thermometer which functions by having a constriction in the neck close to the bulb. The mercury is forced up through the constriction by the force of expansion as the temperature increases. When there is a decrease in the temperature, the column of mercury breaks at the constriction and cannot return to the bulb and will remain stationary in the tube.”1
By means of a maximum thermometer, one can measure the maximum temperature over a predetermined time span. Resetting of the maximum thermometer is a very simple process which just requires the sharp swinging of the thermometer.
The freezing point of mercury is – 38.83°C at which it gets solidified. However, it doesn’t result in expansion upon solidification and hence there will be no danger of glass breakage of the thermometer tube. Whenever there is rise in temperature, the nitrogen gas filled above the mercury in glass tube usually comes down the column and gets stuck there. This whole action may affect the functioning of thermometer. To prevent this difficulty, one must manages to get mercury thermometers inside when the temperature drops to – 37°C. In areas, where the upper temperature limit is – 38.83°C, one may employ a mercury-thallium alloy thermometer having a freezing i.e. solidification point of – 61.1°C.
Advantages of Mercury Thermometers
Key benefits offered by mercury thermometers include:
- Fast response time
- Good repeatability
- Linear thermal coefficient of expansion
- Wide range of temperature
Following are the major applications of mercury-in-glass thermometers:
- They are largely employed in meteorological applications.
- One of the popular mercury thermometers called ASTM-type is commonly used in industrial, scientific and commercial purposes.
- They are also applied in “industries related to the aviation fuel freezing point, petrolatum melting point, butadiene boiling point range, coolant freezing point, stormer viscosity, oxidation stability and weathering test”.2
- They can also be employed in cooking and food processing applications for temperature measurement.
However, their use has been banned by many countries for medical applications owing to their hazardous and toxic effects. Besides, “the use of mercury thermometers in sling psychrometers is strongly discouraged, since users of sling psychrometers sometimes strike objects during the spinning operation, breaking the glass and releasing mercury.”3
In case of thermometer breakage, the discharge of even very little quantity of mercury called “mercury spill” can prove very dangerous to human health. In addition, it can be noxious to marine life forms and extremely corrosive to electronic parts, if used in naval and marine applications. Hence, necessary precautions must be taken while handling mercury thermometers.
Few alternatives to mercury have now been developed. One of which includes galinstan that is a liquid alloy of gallium, indium and tin.
Alcohol thermometers are replacing mercury thermometers in many applications. They employ alcohol as the filled in liquid in glass tubes. The temperature measurement range of alcohol thermometers varies from 115°C to 785°C, where former is the freezing temperature point of alcohol and latter is boiling temperature point of alcohol. A typical thermometer for getting environmental temperature readings is required to have a temperature range of -20°F to 120°F i.e. -30°C to 50°C.
Alcohol, being a volatile substance may result in parting of the column in the thermometer due to mechanical shock. The column can then be joined again by making the thermometer fall over a wooden surface enveloped with lots of paper sheets. “Hold the thermometer in a vertical position, bulb end down, 4 to 6 inches over the wood surface covered with paper, and then drop. After the thermometer bulb strikes the paper, catch the thermometer with your other hand to prevent breakage. Small amounts of alcohol may cling to the portion of the capillary tube above the alcohol column.”4
High temperature portion of the thermometer can be warmed up beneath an incandescent lamp which will bring the alcohol down to the column. One can also mount the thermometer vertically for overnight which will let the drawing back of alcohol to the column. This thermometer should never be employed with a parted fluid column since it generally leads to incorrect temperature measurements.