What is the barometer for?

What is the barometer for?

The barometer is used to measure atmospheric pressure . It was invented in the 17th century by Evangelista Torricelli, who built the first model from a bucket with mercury and a graduated tube , open at one end. The first step in its operation is simple: the open end of the graduated tube is inserted into the mercury-filled cuvette.

The pressure exerted by the atmosphere on the mercury in the bucket makes this liquid metal rise through the tube, until it reaches a certain level . This level will indicate that the two forces acting are balanced: the weight of mercury in the tube, downward, and that of the atmosphere, which ends up pushing it upward in the tube.

The graduation of the tube will show us the millimeters that the mercury has risen in the tube, to the level where the forces were balanced. These millimeters of mercury are the atmospheric pressure of the place where the experiment is being carried out. Torricelli thus determined that the value equivalent to 1 atmosphere is 760mmHg (millimeters of mercury).

The standard atmospheric pressure (1 atm) is equal to the pressure supported by a column of mercury of exactly 760 millimeters.

Units to measure with the barometer

In the barometer, the glass tube is graduated in millimeters, so atmospheric pressure can be measured in millimeters of mercury (mmHg) . The measured value can be translated into millimeters of any other liquid, such as ethyl alcohol, using its density as calculation data.

For instance:

Pressure in millimeters of mercury: 760 mmHg

Mercury density: 13.6 g / mL

Pressure in millimeters of ethyl alcohol 😕 mmC ₂ H ₅ OH

Ethyl alcohol density: 0.789 g / mL

The calculation is carried out with a relation of densities:

This is the ratio by which the standard value of 760mmHg will be multiplied.

Then:

This result is interpreted like this:

Alcohol is very thin compared to mercury, so it will receive much more thrust and its column would rise to 13100.12 mm. This is of little use for practical purposes, so it is limited to experimental aspects.

Other units of standard atmospheric pressure

Thanks to the barometer, it has been possible to start from a base to experimentally determine other units of standard atmospheric pressure, among which are:

• 1 atm (1 atmosphere)
• 760 mmHg
• 101325 Pascals (or Newton / square meter)
• 013 bar
• 033 Kgf / cm ² (kilogram-force each square centimeter)
• 7 lb / in ² or psi (pounds per square inch or pounds per square inch)

All of these units are equivalent, and are used on both analog pressure gauge faces and digital gauge displays. When the gauges indicate a value of the pressure that is inside a container, they are called pressure gauges . The value of this pressure is called gauge pressure .