Mechanics

# Manometer and Barometer

Manometer used to measure the oil pressure of a machine or vehicle.

The concept of pressure is directly linked to force and area. Let us consider a surface of area A, on which a system of forces whose resultant is F is distributed perpendicularly. We can define pressure on the surface as the relation between the intensity of the acting force F and the area A of the surface.

Some industries, or even automobiles, make use of instruments to measure pressure, be it air pressure in car tires, oil pressure in a truck’s engine, etc. The devices we use to measure pressure are basically of two types: the Manometer and the Barometer.

pressure gauge

The manometer is most commonly used for measuring the pressure of a fluid, liquid, or gas. We can see in the figure below a basic type of manometer. This type can measure the pressure of a given gas.

Type of manometer used to measure the pressure of a fluid.

Starting from the principle of Stevin’s theorem, we can deduce that the pressure at point 1 is equal to the pressure at point 2. Thus, we have:

1 = P 2

According to the figure above, we have:

1 = P atm + µ. gh

So:      gas = P atm + P hid

Hydraulic engineers make use of this type of device. They use the manometer in order to measure the pressure difference between points of a liquid contained in a pipe.

Barometer

The barometer is a device invented by the physicist Torricelli. The barometer is most commonly used for measuring atmospheric pressure. We know that these measurements are very important, as they are directly related to obtaining weather forecasts. Using this instrument, the physicist Torricelli managed to obtain, through experiments, the value of 76 cmHg for the atmospheric pressure at sea level.

The measure of atmospheric pressure measured by Torricelli

He used a one-meter-long glass tube. This tube was closed at one end, so Torricelli put mercury inside. Capping the open end, he dipped the tube into a bowl of mercury. Upon uncapping the open end, he verified that the liquid column had descended. Measuring the length of the liquid column, he verified that the height was 76 cm above the level of mercury contained in the vessel.

After performing the experiment, he was then able to conclude that the atmospheric pressure that was acting on the surface of the liquid contained in the vessel was balancing the column of mercury. Thus, it was possible to state that the value of atmospheric pressure is equivalent to the pressure exerted by a column of mercury 76 cm high.

The same experiment was also carried out by Pascal, but he noticed that the height of the column of mercury balanced by atmospheric pressure was a little smaller than that obtained by Torricelli.