What do relative pressure and absolute pressure mean?

The pressure can be measured in an absolute or relative manner, depending on whether the atmospheric pressure of 1 bar at sea level is taken into account. We are all subjected to a pressure of 1 bar given by the air column of the atmosphere above and around us. Since this is the normal condition in which we all find ourselves, point 0 can be considered from which to measure the pressure and in this case we speak of relative pressure since it does not take into account the atmospheric pressure. The absolute pressure, on the other hand, takes it into account and therefore the starting condition is no longer 0 but 1 bar.

If we closed an airtight container at sea level in it we would have 0 bar relative pressure and 1 bar absolute pressure. If we could subtract air from inside the container we would have negative relative pressure and absolute pressure lower than 1. It is not possible to reach a perfect relative pressure or absolute pressure of 0 because it is impossible to obtain the perfect vacuum. If we reopen and close the same container and then return to the starting conditions (0 bar relative, 1 bar absolute), we could increase the internal pressure by 1 bar, we would have 1 bar of relative pressure (0 + 1) and 2 bar of absolute pressure (1 + 1) and so on.

In order for a sensor to measure the relative pressure it must be able to measure the pressure in a given environment and compare it with the atmospheric pressure which cannot always be considered at 0 bar for the relative and at 1 bar for the absolute as it varies according to weather conditions. altitude.