Absolute pressure - formula and calculation examples
Any substance can be described by its physicochemical parameters. Unlike liquid and solid substances, whose state can be characterized by temperature and density, gases have another indicator, which is called “pressure”. This physical quantity for a gaseous substance can be represented by the total value of the forces of the impacts of molecules against the walls of the vessel containing the gas. The more molecules hit the walls, the greater their mass, speed and force of impact on the walls of the vessel - the higher the pressure.
Physicists distinguish atmospheric, absolute and excessive pressure. These kinds of quantities are interconnected by means of physical formulas.
There are many traditional units of pressure that have developed as a result of the development of physical disciplines. The most common of them are “bar”, “atmosphere”, “mm Hg” and other values derived from them.In physical processes, this parameter is denoted by the letter P, measured in pascals and units derived from it. In writing, the pascal is displayed like this: [Pa].
The concept of atmospheric pressure
The air surrounding us consists of constantly moving molecules that collide with the earth's surface, objects on it and between themselves. From the blows of tiny particles is the total pressure. This parameter is called atmospheric, or barometric pressure.
But, as shown by the measurements, Patmlargely depends on the ambient temperature and the altitude above sea level. Therefore, to explain the physical processes and solve problems, the current parameters of atmospheric pressure are reduced to normal conditions. Initial parameters Patmdetermined at a temperature of 0 ° C above zero sea level.
What is absolute pressure
Standard pressure measurement methods typically use atmospheric pressure as a reference point. Usually this parameter is measured by various instruments. The most popular of which are barometers.
In other cases, the ratio of observed pressure to vacuum or to another selected mark is applied. To designate selected categories, the following definitions apply:
- Absolute gas pressure: is a parameter of the transition point between vacuum and observed pressure.
- Overpressure: for it, the atmospheric pressure becomes the reference point. This indicator is calculated as the difference between absolute and atmospheric pressure.
- Differential pressure - is the difference in performance between two arbitrary points of measurement.
Differential, absolute and excessive pressure can be visualized as follows:
Excess and absolute pressure are logically related. The value of absolute pressure can be obtained by measuring the observed pressure and adding to it the value of atmospheric R.
In the case of overpressure, the reference point is the value of atmospheric P. Thus, this value can be represented as the difference between absolute pressure and atmospheric. Absolute and excessive pressure can not be negative. When Pabs= 0 pressure becomes equal to the atmospheric indicator of this value.To be precise, Pabscannot be equal to vacuum - there always remains some value formed, for example, by the pressure of saturated vapors in a liquid. But in the case of heavy liquids, this parameter is very insignificant, therefore, in the initial calculations that do not require precise calculations, it is quite acceptable.
What is absolute air pressure
The absolute air pressure can be measured only in vessels with other substances - with liquids or gases. So, this parameter is quite often measured in closed vessels with liquids. As in the first case, the absolute air pressure in a closed vessel can be measured as the difference between the observed P and atmospheric.
As is often the case, along with the generally accepted units of measurement of physical quantities, historical ones are also used. Piezometric height is one of these quantities. It can be measured by a special device, which is a glass tube, the upper part of which is unsealed and openly communicates with the atmosphere, and the lower one is attached to the vessel in which pressure is measured.