1. Introduction
The instruments commonly used for temperature measurement are listed below:
- Liquid-in-glass thermometers (Mercury-in-glass, Organic fluid-in-glass)
- Resistance thermometers
- Thermocouples
- Thermistors
- Bimetallic thermometers
- Pressure-bulb thermometers
- Optical pyrometers
- IR radiometers
- IR thermography
- Seger cones (fusion pyrometers)
In this article we will learn why mercury is preferred over other fluids in Liquid-in-glass thermometers.
In the temperature measurement techniques by using liquid-in-glass thermometer, mercury thermometers have held a position of prominence for centuries.
Despite the advent of digital and alcohol-based alternatives, the specificity and reliability of mercury thermometers have kept them in use in various applications. This article delves into the reasons why mercury has been the fluid of choice in thermometers, examining its unique properties and the science behind its continued preference.
2. Components of Mercury-in-glass thermometers
- Glass tube
- Bulb
- Scale
- Expansion chamber (optional), for high temperature applications
3. Working Principle
A mercury-in-glass thermometer operates based on the “thermal expansion property” of the mercury.
When the temperature changes, mercury expands or contracts, and its level rises or falls in a calibrated glass tube to indicate the corresponding temperature.
3.1 Temperature Exposure
The bulb comes into contact with the medium whose temperature is to be measured (air, liquid, or body surface).
3.2 Thermal Expansion
- When the temperature increases, the mercury in the bulb absorbs heat and expands.
- Due to the narrow capillary tube, this expansion pushes mercury up the tube.
3.3 Reading the Temperature
The level of mercury in the tube aligns with the calibrated scale, giving the temperature reading.
3.4 Contraction on Cooling:
When the temperature decreases, the mercury contracts and descends in the tube, indicating a lower temperature.
4. Advantages of Mercury
Mercury is used in Liquid-in-glass thermometers because of its unique physical and chemical properties, which make it ideal for precise temperature measurement.
4.1 Wide Temperature Range
Mercury (Hg) remains in a liquid state over a broad temperature range, from −39°C to 356°C. This makes it suitable for measuring extreme temperatures in both negative and positive directions.
Other liquids, like alcohol, freeze at lower temperatures or evaporate at higher temperatures.
4.2 High Density and Thermal Expansion
Mercury has a relatively low coefficient of thermal expansion (181 x 10⁻⁶ per °C, volumetric expansion), meaning it expands uniformly and predictably with temperature changes. This makes readings accurate and consistent.
4.3 High Thermal Diffusivity (α)
Thermal diffusivity of liquid mercury is high compared to other liquids, refer below (reference temperature is 25 OC):
| Sl. No | Liquid | Thermal Conductivity (k) W/m.K | Density (ρ) Kg/m3 | Specific Heat Capacity (Cp) J/kg.K | Thermal Diffusivity (α = k / (ρ x Cp)) mm2x10-7 |
| 1 | Mercury (Hg) | 8.3 | 13600 | 138 | 44.224 |
| 2 | Water (H2O) | 0.606 | 1000 | 4187 | 1.447 |
| 3 | Ethanol (C₂H₅OH) | 0.171 | 789 | 2440 | 0.888 |
| 4 | Methanol (CH₃OH) | 0.201 | 791 | 2510 | 1.012 |
| 5 | Isopropanol (C₃H₇OH) | 0.135 | 786 | 2530 | 0.679 |
| 6 | Toluene (C₆H₅CH₃) | 0.135 | 867 | 1720 | 0.905 |
| 7 | Isoamyl Alcohol (C₅H₁₂O) | 0.135 | 810 | 2480 | 0.672 |
Thermal diffusivity (α) measures how quickly heat propagates through a material during temperature changes over time. A higher thermal diffusivity indicates that heat can penetrate the material faster, allowing it to reach thermal equilibrium in a shorter period.
Among liquids, mercury exhibits much higher thermal diffusivity compared to substances like water and alcohol. This property, combined with its excellent thermal conductivity, allows mercury to efficiently transfer heat and respond rapidly to temperature fluctuations. As a result, mercury-based thermometers provide quicker and more reliable reading.
4.4 Non-Adhesiveness
Mercury does not adhere or wet glass. It forms a distinct meniscus and moves smoothly inside the thermometer. This non-adhesive property ensures that mercury remains in a consistent column within the thermometer tube, allowing for clear and unobstructed readings.
In contrast, other fluids like alcohol require dyes for visibility and can stick to the glass, compromising accuracy.
4.5 Visibility
Mercury is shiny, metallic appearance makes the mercury column easily visible against the thermometer’s background.
4.6 Ease of Calibration
Mercury thermometers are relatively easy to calibrate, a practical advantage in maintaining accurate measurements. The predictable and uniform expansion of mercury allows for straightforward calibration processes, ensuring the reliability of the thermometer. This ease of calibration has made mercury thermometers a preferred choice in laboratories and industrial settings where precise temperature measurements are critical.
5. Why Not Other Fluids?
- Alcohol-based Thermometers: Alcohol is used in some thermometers, especially for low-temperature applications, but it evaporates easily, requires dyes for visibility, and adheres to the glass, making it less accurate.
- Water: Water has a limited temperature range (freezing at 0°C and boiling at 100°C) and is unsuitable for most practical thermometer applications.
- Other Metals: Other metals do not remain in a liquid state at normal temperatures.
6. Limitations of Mercury Thermometers
Despite its advantages, mercury thermometers are being phased out due to:
6.1 Toxicity
Despite its advantages, mercury is a toxic substance, posing significant health and environmental risks.
Exposure to mercury can lead to severe health issues, including neurological damage and respiratory problems. The toxicity of mercury has led to increased regulations and restrictions on its use, prompting the development of safer alternatives.
6.3 Breakability
Mercury thermometers are made of glass, making them susceptible to breakage. A broken mercury thermometer can result in mercury spills, posing contamination risks and requiring careful cleanup procedures. The fragility of glass thermometers has been a driving factor in the shift towards more durable and safer digital and alcohol-based thermometers.
Modern thermometers, such as digital or alcohol-based ones, are now widely used as safer and more environmentally friendly alternatives.
7. Conclusion
Mercury thermometers have long been a gold standard in temperature measurement due to their unique combination of properties, including high thermal diffusivity, wide temperature range, excellent thermal conductivity, and non-adhesiveness to glass. These characteristics ensure fast, accurate, and reliable readings, which have made mercury the fluid of choice for liquid-in-glass thermometers.
However, the toxicity of mercury and the fragility of glass thermometers have prompted a global shift toward safer and more environmentally friendly alternatives. Despite this, understanding the science and historical significance behind mercury’s use remains essential for appreciating the evolution of temperature measurement technologies.
In laboratory and industrial applications that demand high precision, advancements in thermometer technology continue to provide new solutions that balance accuracy, safety, and environmental sustainability.
While the era of mercury thermometers may be coming to an end, their contribution to the field of thermometry remains invaluable.
8. References
ASHRAE Handbook—Fundamentals, Chapter “Measurement and Instruments”.
9. Abbreviations
| ASHRAE | American Society of Heating, Refrigerating, and Air-Conditioning Engineers |
| IR | Infra Red |