Pressure is a quantity from physics that shows the numerical value of the ratio of the force acting on an accepted unit of a certain surface. It is believed that the direction of this force is strictly perpendicular to the surface area. If in the SI system, which is adopted in the vastness of the CIS countries, the force is Newton or kilogram-force, and the area is square centimeters (millimeters, meters), then we get:
P = F / S = N / cm2 = Pa
Pascal is one of the varieties of quantities (used in engineering and physics). As a rule, the values are indicated in MPa - MegaPascals. This is useful for reducing the number of zeros. In technology and everyday life, the term "atmosphere" is often used. Absolute pressure, as well as relative, can be measured in "atmospheres", Pa and other units.
In practice, the term "atmosphere" also has several concepts: standard and technical. The standard, physical or normal atmosphere has the designation "atm" (ata) and the standard value is 101325 Pa, which means the pressure of the mercury column at sea level and at a temperature of 0 degrees Celsius. You can also find the expression 760 mm Hg. (mmHg). The technical atmosphere has nothing to do with the term absolute pressure, since it has a different definition and units of measure "at" (ati). The definition reads: "a force perpendicular to the mass (1 kg) with gravity acceleration (1g) uniformly per unit surface area (1 cm2)."
There are also definitions:
Absolute pressure - the measured pressure relative to absolute zero - vacuum (ata).
Relative pressure is the measured pressure relative to the standard atmosphere (ati).
Excessive pressure - can both be equal to relative, and be its difference (ati).
Very often, the concepts of absolute and excess pressure are distinguished as two single types. In fact, there is no significant difference in excess and relative pressure. This is more verbal speed, which is more common for specialists in a particular industry to use.
Back to the concept of absolute pressure. As already mentioned, it has an absolute indicator, i.e. measured from physical zero. It's like a scale on a ruler - you measure from 0 and how much you need. In nature, this concept is rarely resorted to in calculations, since there is always some kind of pressure "overboard". Therefore, it is more convenient to measure from it. For example, the tire pressure of your car the master at the service station tells you in atmospheres (for cars about 2.2 atm). Cylinders also have a unit of measure - the atmosphere, but already technical. This is because the difference is important when using.
It should be noted separately that air pressure naturally changes with height. Absolute and relative pressure changes conditionally with each kilometer. To determine it, all external factors should be taken into account: temperature, density and gravitational acceleration . This data is standard for each height. They are summarized in a table of pressure distribution by height or can be presented in the form of graphs. This information is mainly useful to aircraft designers. Since the aircraft has a range of working heights from 0 to 12 km (passenger). And, therefore, all systems, assemblies and operation of aircraft engines must be designed taking into account different external conditions.
Various quantities are used in the world, which measure both excess and absolute pressure. It all depends on the geographic region and area of application. The following is a chain of basic pressure units:
1 Pa = 9.87 * 10-6 atm. = 1.02 at. = 1.02 kg / cm2 = 10-5 Bar = 1.45 * 10-4 PSI =
= 7.5 * 10-3 mmHg = 0.120 mm water
As you can see, the range of pressure units is quite large. Of this list, in the CIS countries the most common are Pa, atm., At., Kg / cm2, mm Hg
Among other things, in nature and processes there is static and total pressure. Static pressure is the pressure that is measured from the flow of the working fluid (i.e. its natural pressure). The total pressure (pressure according to the braked parameters) is the pressure measured directly in the flow (i.e., when the flow decelerates in front of the sensor).
Pressure affects a person to a greater extent than temperature, because the temperature of the human body has a certain margin, and the pressure is very small. That is why older and sick people are very sensitive to pressure changes. For technical needs, pressure measuring instruments - manometers are used, for hydrometeorological services - barometers.