The liquid-in-glass thermometer measures temperature by relying on the thermal expansion of a confined fluid. As the surrounding temperature rises, the liquid expands in volume, pushing a column up a narrow glass tube. This requires a liquid that expands predictably and remains in its liquid state across the intended measurement range. Historically, one substance was the standard, but modern safety concerns have led to the adoption of several alternatives.
Mercury: Properties and Phasing Out
Mercury, a dense, silvery liquid metal, was historically the preferred fluid for liquid-in-glass thermometers due to its unique physical properties. It possesses a wide operational temperature range because of its low freezing point of approximately \(-39\) degrees Celsius and a high boiling point of about \(357\) degrees Celsius. This span made it suitable for most medical and meteorological measurements.
The metal is also highly visible and reflective, making the rising column easy to read against the engraved temperature scale. Mercury does not “wet” the glass walls of the capillary tube, which prevents it from sticking and ensures a precise reading as the column moves. This combination of traits made it the industry standard for centuries, following its invention in the early 18th century by Daniel Gabriel Fahrenheit.
Despite its functional advantages, the primary reason for discontinuing mercury thermometers is the toxicity of the liquid element. If a thermometer breaks, the spilled mercury can vaporize, posing a serious health risk if inhaled. The metal can also convert into highly toxic forms in the environment, leading many countries to ban its use in medical and household thermometers.
Alcohol and Organic Fluid Thermometers
The most common non-toxic replacement for mercury in many general-purpose thermometers is an organic fluid, often ethanol (ethyl alcohol) or isopropanol (isopropyl alcohol). These liquids are typically dyed a bright red or blue color to make the otherwise transparent column easily visible against the white background of the thermometer scale. Ethanol is the most widely used choice due to its low cost and the relatively low hazard it poses in the event of a breakage.
Alcohol thermometers are non-toxic and evaporate quickly if spilled, offering a significant safety benefit. They have a very low freezing point; pure ethanol remains liquid down to approximately \(-115\) degrees Celsius, making them excellent for measuring extremely cold temperatures. Their main limitation is a low boiling point, such as ethanol’s \(78\) degrees Celsius, which makes them unsuitable for measuring high temperatures.
Galinstan and Non-Toxic Metallic Fillings
For applications requiring the performance of a metal without the toxicity of mercury, specialized alloys have been developed. Galinstan is one such metallic fluid, a brand name for a eutectic alloy composed of gallium, indium, and tin.
Performance and Limitations
Galinstan is liquid at room temperature, with a melting point as low as approximately \(-19\) degrees Celsius and a high boiling point exceeding \(1300\) degrees Celsius. This wide range provides performance similar to mercury without the associated environmental and health risks. Galinstan is now frequently used in clinical and high-precision glass thermometers as a direct, non-toxic metallic substitute. A minor challenge is that Galinstan tends to wet or stick to the glass walls, unlike mercury. Manufacturers often coat the interior of the capillary tube to prevent this effect.