When employed as a measuring liquid in a barometer tube, mercury offers several advantages. The density of mercury is very high. Mercury is also an excellent heat conductor. As a result, if the system's temperature changes, mercury can reach that temperature faster. Finally, its specific gravity decreases as it approaches room temperature, so any pressure change caused by rising or falling air temperatures will cause a corresponding change in the height of the mercury column.
These properties allow mercury to act as a reliable and accurate gauge of atmospheric pressure. A depression in the weather above the point where the barometer tube is located would cause a decrease in the surface area of contact between the mercury and the surrounding atmosphere. This reduction in interference would show up as a drop in the mercury's level in the tube.
The depth of this drop would be proportional to the severity of the storm. Thus, mercury barometers are useful in tracking storms' movements and determining their intensity.
Mercury also has some disadvantages as a barometric fluid. It is toxic if inhaled or ingested. Also, because of its high density, it occupies a large volume of space when stored or transported. Finally, its melting point is -38 degrees Celsius, which makes it difficult to use without additional cooling equipment.
Despite these limitations, mercury barometers remain important tools for monitoring severe weather.
Mercury is the finest choice for liquid-filled thermometers because to its strong thermal conductivity, great sensitivity to temperature, and superb visibility. Alcohol thermometers may readily leak and get wet on the wall, resulting in less accurate and slower readings. Thermometers that use antifreeze or other chemicals as their heat-transfer fluid can be dangerous if they break down under pressure.
The advantages of mercury thermometers are their great accuracy and wide measuring range, from 0°C to +50°C (32°F to 122°F). They need to be replaced only once every few years, unlike alcohol thermometers which must be changed annually or more often depending on usage. Mercury thermometers are also very durable; most modern models will stay accurate after being exposed to temperatures as high as +500°C (932°F). Alcohol thermometers tend to malfunction at much lower temperatures.
The main disadvantage of mercury thermometers is their cost. However not only do they offer the best quality measurement, but they also last for many years too! If you live in an area with no pollution controls on your furnace or air conditioner, then using a mercury thermometer is recommended. These devices remove pollutants by filtration, while alcohol thermometers cannot. Without proper treatment, gases such as hydrogen cyanide can be released into the atmosphere when breaking down mercury thermometers.
Mercury is especially suitable for use in manometers (and barometers) since it has a low vapor pressure at ambient temperature, does not wet glass, and has a high density. Manometers have also employed other liquids such as linseed oil or water. The choice of liquid affects the reading on the meter.
Boyle's law states that the ratio of the partial pressures of two gases varies in proportion to their absolute temperatures. At constant pressure, the volume of one gas relative to another changes if the temperature changes. For example, if the volume of an air bubble in water increases, this shows that the air is becoming less dense and thus its pressure is dropping. On the other hand, if the volume of the air bubble decreases, this shows that the air is becoming more dense and thus its pressure is rising.
In order to measure the absolute pressure of a single gas, you need to know its partial pressure. This can be done using an instrument called a pneumatic gauge. These are very accurate devices that employ an amount of mercury in a sealed container. As the pressure of the gas applied to one side of the container increases, the mercury rises to compensate. When the pressure reaches 100 kPa (10 mmHg), no more mercury will rise. The pressure is recorded by some form of pointer attached to a scale.
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