It happens that remembering school years, chemistry, physics come to mind. For me, they expanded the boundaries of understanding of the world through acquaintance with certain physical concepts. Some of them did not want to “take root” in any way. Probably because most often the definitions voiced by the teacher did not have a specific physical image, but were, at first, abstract concepts.
Here is a meter - everything is clear here. Time, too, it seems like there are no questions, a watch is a favorite childhood toy, tick-and-ticker, without getting tired. Meter, hour, second - these physical entities "grow" into consciousness. But how to imagine 10 km of air that “crush” you day and night tirelessly, and are called lyrically and mysteriously - atmospheric pressure. But it turns out that this is not all, there is excess pressure, and osmotic, and blood, and ... there is no number for them, all these pressures. Then, however, everything falls into place, but judging by the Internet forums, questions on various aspects of the concept of “pressure” were, are and will be. So what is pressure?
The practical meaning of the concept of "pressure" is that it is a specific power characteristic - the load per unit area. Any object surrounding us is sure to “lean”, presses on any surface - a table, window sill, stand, road. But how to determine who presses harder on the road - a tank or a person walking along it? This is where the concept of the physical quantity “pressure” helps, which is calculated on the basis of other physical quantities. To do this, determine the weight of the object and the area of ​​its support. Divide the weight by area and get the pressure value - the fraction of weight per unit area, say, a square centimeter. Now we can say with confidence who is more “pushing”.
And now, who was the first to propose that the “column” of air with an area of ​​one square centimeter and a height of 100 km, not so say the height of the earth’s atmosphere, is called atmospheric pressure. If we take the sea level as the zero value of the height, then the numerical value of the atmospheric pressure Ratm, it is also called barometric, will be about 1 kgf / cm2. It so happened historically that this remarkable physical quantity is one atmosphere equal to 1 kgf / cm2. - has several prototypes. In particular, the measurement option in millimeters of mercury is widely used - 1 atmosphere is 760 mm Hg. At different heights from the Earth, the “squeezing” column of air is different, and the pressure measured by the barometer changes with altitude. For example, at an altitude of 5 km, the barometric pressure is 405 mm Hg.
Other variations of the pressure name are classified by their ratio to barometric. So, for example, a pressure gauge widely used in engineering is measuring overpressure. The definition just sounds like that excess pressure is considered to be in the range above the barometric pressure and is designated Rati. In technical applications, it is precisely overpressure that is most often measured.
Or maybe the pressure is less than atmospheric? Of course, in this case it is also called vacuum gauge. Absolute vacuum - a pressure equal to absolute zero - does not happen less. Therefore, if we take it as a reference point, then any pressure greater than zero is called absolute, it changes from zero to any value, and it is labeled Rabs.
How to calculate the excess pressure? The formula for calculating the absolute pressure is:
Rabs = Ratm + Rati
Hence, it is not difficult to calculate the excess pressure as the difference:
Rati = Rabs - Ratm
Unfortunately, a pressure that has a simple analytical expression has such a huge number of units of measurement that only specialists can understand it. The most applicable of them is bar, pascal, kg / cm2. and their derivatives with the prefixes mega, hecto, gig.
In the C system, the unit of pressure, Pascal, is a force of one Newton per square meter. By itself, n / m square. such a small value that its derivative bar, which is approximately 100 kilopascals or 750 mm Hg, is used in practice, i.e. almost 1 kg / cm2 Here it is, the circle is closed - we again came to a beautiful and poetic unit of pressure - the atmosphere. True, in this case it is called the “technical atmosphere”, and this unit is often, from the old memory, measured overpressure. A full list of correspondence of pressure units used in different areas takes a page of long text.
So, a simple and understandable pressure grew into a tangled labyrinth of its own units of measurement. But there is no reason to worry - there is always a guide at hand or ... the Internet.