Excessive pressure - what and how ...

It happens that remembering the school years, things like chemistry, physics come to mind. They expanded the boundaries of the world outlook for me through acquaintance with these or other physical concepts. Some of them did not want to "take root". Probably, because most often voiced by the teacher definitions did not have a specific physical image, but were, at first, abstract concepts.

Here's a meter - everything is clear. Time, too, like as there are no questions, the clock is a favorite toy of childhood, tick-by, without tire. Meter, hour, second - these physical entities "grow" into consciousness. But how to imagine 10 km of air, which "presses" on you day and night without tire, and called lyrically and mysteriously - atmospheric pressure. And it turns out that this is not all, there is excessive pressure, and osmotic, and blood, and ... do not have a number, all these pressures. Then, however, everything falls into place, but judging by Internet forums, questions on various aspects of the concept of "pressure" were, are and will be. So what is this - pressure?

The practical meaning of the concept of "pressure" is that it is the specific force characteristic - the load per unit area. Any object that surrounds us, necessarily "leans", presses on any surface - a table, a window sill, a stand, a road. And how to determine who is harder pressed on the road - a tank or a man walking along it? Here the notion 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. We divide the weight by the area and get the pressure value - the weight fraction per unit area, say, the square centimeter. Now we can say confidently who is more "crushing".

And do not say now who first suggested that the "column" of air with an area of one square centimeter and a height of 100 km, it is such a conventionally consider the height of the earth's atmosphere, called atmospheric pressure. If the level of the world ocean is taken as the zero value of the altitude, then the numerical value of atmospheric pressure Ratm, also called barometric pressure, will be approximately 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 is widely used in millimeters of mercury - 1 atmosphere is equal to 760 mm Hg. At different heights from the Earth, the "pressing" 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 variants of the pressure name are classified according to their relationship to the barometric pressure. Thus, for example, a pressure gauge widely used in the technique of a gauge measures excess pressure. The definition is that the excess pressure is considered to be above the barometric pressure and denote it by Rati. In technical applications, it is often the excess pressure that is measured.

Or maybe less than atmospheric pressure? Of course, in that case it is also referred to as a vacuum gauge. Absolute vacuum - a pressure equal to absolute zero - less does not happen. Therefore, if we take it as a reference point, then any pressure greater than zero is called absolute, changes from zero to any value, and is denoted by 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 a difference:

Rati = Rabs - Ratm

Pressure, which has a simple analytical expression, unfortunately, has such an enormous number of units of measurement that only specialists are able to understand them. The most applicable of them is bar, pascal, kg / cm square. And their derivatives with prefixes mega, hecto, giga.

In the C system, a unit of pressure - pascal - is a force of one newton per square meter square. By itself, n / m square. Such a small amount that in practice its derivative bar is used, which is approximately 100 kilopascals or 750 mm Hg, i.e. Almost 1 kg / cm square. Here it is, the circle, closed - we again came to a beautiful and poetic unit of pressure - the atmosphere. True, in this case, it is called a "technical atmosphere," and it is this unit that, often from old memory, excess pressure is measured. A complete list of compliance used in various areas of pressure units takes a page of concise text.

So, the simple and understandable pressure has developed into an intricate labyrinth of its own units of measurement. But there is no reason to worry - there is always a directory or ... on the Internet.