# How can a manometer measure absolute pressure

### The manometer

A manometer can be used to measure the pressure of a medium such as a liquid or a gas. There are two main types of manometers, those that use the ambient air pressure as a reference pressure and those that use a vacuum as a reference to determine the absolute pressure. Such a manometer is also known as a barometer. Pressure gauges, which use the ambient pressure as a reference pressure, are mostly open pressure gauges in the form of a U-tube.
Furthermore, manometers are divided into two classes:
On the one hand there are manometers which measure the pressure directly by determining the force per area. And manometers with which the pressure is measured indirectly via heat conduction or particle number density.
In the internship, U-tube manometers filled with a liquid are mostly used, which allow a direct pressure measurement. The liquid is usually water, oil or mercury, which is the same level in both legs (the liquid levels are at the same level). If there is a pressure difference between the two legs of the U-tube, the liquid in the tube shifts to one side. The liquid shifts to the side with the lower pressure in the thigh. By reading the new height of the liquid, the pressure can be determined from the height difference.
The relationship between height difference and pressure difference is the following:

with the density the liquid and the gravitational acceleration .
It should be noted that the legs do not have to have the same radii to determine the pressure. If the radii and thus the cross-sectional areas of the legs are of different sizes, but both are constant, the following relationship applies:

which results from the continuity theorem for a constant volume.

In addition to the U-tube manometer, steam pressure manometers are also used in the internship, which show the pressure changes via a digital display or mechanical device. Always pay attention to the scaling of the display. In the experiment "Vapor pressure of water", for example, a vapor pressure manometer is used, which shows the change in pressure as a function of temperature.