Invented in 1724 by Daniel Gabriel Fahrenheit, the mercury thermometer has largely been replaced by safer, more precise alternatives. Today, the five most common types—liquid expansion, bimetallic, resistance temperature detectors (RTDs) or thermistors, thermocouples, and infrared devices—offer reliable ways to measure temperature across diverse applications.
These thermometers operate on a straightforward principle: a reservoir connected to a narrow capillary tube. As heat expands the liquid in the reservoir, it rises through the calibrated capillary. The tube's fineness and the liquid's thermal expansion coefficient amplify small volume changes into visible movement, providing accurate readings.
Using liquid over gas minimizes atmospheric pressure effects. For safety and cost, colored alcohol has replaced mercury, though silver-colored liquids may be galinstan—an alloy of gallium, indium, and tin with mercury-like properties.
Featuring a needle on a dial, these use a coiled strip of two metals with differing expansion rates. Heat causes the coil to unwind predictably, moving the needle across the graduated scale.
Affordable and durable, they require full immersion of the coil—often several centimeters long—for accuracy and can take minutes to stabilize.
These rely on heat's effect on electrical resistance. RTDs (e.g., platinum-based) increase resistance linearly with temperature, enabling precise measurements up to 800°C, though at higher cost.
Thermistors decrease resistance with heat, offering faster response and lower cost but limited to under 300°C—ideal for many practical uses.
Based on the Seebeck effect, these join dissimilar metals (e.g., Type K: nickel-chromium; Type T: copper-constantan; Type J: iron-constantan), generating voltage proportional to temperature differences at the junction.
Balancing cost, speed, and reasonable accuracy, thermocouples suit industrial and specialized applications effectively.
These non-contact devices detect bodies' natural infrared emissions, calibrated to temperature. They provide instant readings via digital interfaces, perfect for hygiene-sensitive health settings or hazardous industrial environments with extreme heat or radiation.
