Why Is Mercury Used In Thermometers?

(digital thermometer, HS-21---click here https://www.cnhealthshining.com/digital-thermometer/flexible-thermometer/flexible-thermomete.html)

Today, electronic thermometers are the temperature of choice for most people. They are cheaper and safer than mercury thermometers. But why does expensive mercury have to be in the thermometer? 

Similarly, when the temperature rises or falls, the water in a small glass tube rises or falls. This is how every sealed thermometer with liquid works. However, if something as rich and cheap as water can be so effective, why do we insist on using something as rare and expensive as mercury?

Reason 1：History 

Centuries after the first thermometer was invented, humans learned about thermodynamic temperatures. Therefore, initially the thermometer defined the temperature. However, the first thermometer is not a thermometer or a calorimeter, but a thermometer ranger, indicating only whether the temperature is high or low. These devices are not calibrated to a standard range, they can only make rough or vague assessments.

The first thermometer was invented by Hero in Alexandria. He was an intellectual engineer and considered to be the greatest experimenter in ancient times. The device he designed consisted of an air-filled tube whose end was submerged in a small bowl of water. When the thermometer touches the hot or cold surface, the air will expand or shrink, resulting in fluctuation of air-water interface.

Even Galileo's invention followed the same principle. However, these inventions are not only scale-free, but also sensitive to air pressure. To design a device that responds only to heat, Galileo's student Joseph Delmedigo invented the first sealed liquid built-in glass thermometer. This is the first thermometer because it has been scaled. However, the liquid he sealed was not water, but alcohol.

Reason 2：Coefficient of expansion

At constant pressure, matter expands under heating, because heat increases the kinetic energy of atoms, causing them to move violently, thus separating them from each other. The volume increase is evident in metal rails, rubber tires and other solids and liquids such as water, alcohol, mercury and halogen. However, the degree of expansion of each substance varies with each degree of temperature rise. This constant of matter is called the coefficient of expansion.

Alcohol is preferred over water for the simple reason that it has a higher coefficient of expansion. Even a small change in temperature will result in a drastic change in the volume of the tube. However, alcohol is so susceptible that these changes can lead to almost turbulent alcohol performance in the tube. The liquid interface will change with the slight change of temperature. This unpredictability is worrying, because when the thermometer is removed from a pot of boiling water, the readings immediately change to the temperature of the new environment, unable to measure the desired boiling water temperature.

To avoid this unreliability, Dutch inventor Daniel Warrenheit replaced alcohol with mercury. The expansion coefficient of mercury is higher than that of water, which means that the volume of mercury varies more obviously with temperature. However, it changes almost six times less than alcohol. That is to say, every time the temperature rises, the volume of alcohol rises six times larger than that of mercury.

This means that mercury rises much slower than alcohol in sealed glass tubes, but it also means that Mercury falls as slowly as it does when the thermometer is removed from a pot filled with boiling water. Its readings will be effectively undisturbed, making the thermometer very reliable.

Before the invention, thermometers were unique and their readings did not conform to any standardized scale. However, Warrenheit provided a range of readings that were applied to all mercury thermometers manufactured and eventually to all thermometers. This transformation is not very cumbersome, because almost all mercury thermometers are made by Warren Height himself. Later, the scale changed slightly, and now the thermometer is called a Fahrenheit thermometer.

Reason 3：Examples of better use of alcohol

Alcohol hypersensitivity can be compensated for by its advantages. Compared with mercury, alcohol is much cheaper, less rare and non-toxic. If the mercury thermometer is damaged, the laboratory may need to be sealed up for several hours because inhaling mercury can cause serious health problems. Alcohol has no such threat.

More importantly, the freezing point of mercury is - 40 C, and the freezing point of alcohol is - 115 C. This means that mercury thermometer can not measure the temperature of - 40 C, so it can not be often used in scientific laboratories or superconducting technology manufacturing industry.

But alcohol is colorless, which forces manufacturers to add artificial dyes to alcohol to make it visible. In addition, although alcohol can measure very low temperatures, it cannot measure temperatures above 78.37 C, because this is the boiling point of alcohol. Compared with the boiling point of mercury at 356.7 degrees Celsius, it is pitifully low.

Although mercury's rare, expensive and toxic properties cannot be changed, we can still break through its temperature limit. To further enhance its boiling point, mercury is usually sealed with inert gases, such as nitrogen. Inert gases increase the pressure on liquid mercury, which further increases its boiling point.

We can also reduce its freezing point by combining it with thallium. These new mercury-thallium thermometers are now able to measure temperatures of - 62 degrees Celsius. Mercury thermometer is considered one of the most accurate available thermometers, despite its small flaws.

Nowadays

Instead of mercury thermometers, which give accurate but more troublesome readings, people now use electronic thermometers. Electronic thermometers are more convenient, fast and safe. Electronic thermometers come in a variety of styles, including hard-tip, soft-tip and waterproof versions, which can cater to the needs of different groups of people.

Richard

Sales Manager

Hangzhou Health Shining Electronic Technology Co., Ltd.

tao@hzhealthshining.com

wechat/whatsapp: 008618658896852

www.cnhealthshining.com/www.hzhealthshining.com