Definition
Atmosphere is a shell of air that surrounds the earth. Its thickness is several thousand kilometers.
As a result of the action of gravity, the upper layers of air compress the lower ones. The layer of air near the Earth experiences the greatest compression. According to Pascal's law, this layer of the atmosphere transmits the pressure that is exerted on it in all directions. As a result, the surface of the Earth and all objects located on it experience the pressure of the entire thickness of the air. The pressure exerted by the atmosphere on all bodies is called atmospheric pressure. A person does not notice the pressure of the atmosphere, since the pressure inside is equal to the pressure outside.
Pascals (Pa) - units of atmospheric pressure.
As with any other type of pressure, pascals (Pa) are units of atmospheric pressure.
It is not possible to calculate the value of atmospheric pressure using the formula for finding the pressure of a liquid column. For such a calculation, you need to know the height of the atmosphere and the density of the air. But the atmosphere does not have a clearly defined boundary, and the air density changes with height. Atmospheric pressure is found experimentally. Torricelli's experience in measuring atmospheric pressure is well known. The scientist took a glass tube 1 meter long, sealed at one end. Filled it with mercury. He tightly closed the open end of the tube, turned it over, lowered the open end into a vessel with mercury, and opened it. Part of the mouth spilled out, but part remained in the tube. The height of the remaining mercury column was measured. It turned out that it is approximately 760 mm. Torricelli suggested that the atmosphere exerts pressure on the surface of the mercury in the cup. The mercury in the cup and tube is in equilibrium, which means that the pressure of the mercury column is equal to the pressure of the atmosphere. With an increase in atmospheric pressure, the height of the vertical column of mercury increased. In the case under consideration, it is logical to take one millimeter of mercury (1 mm Hg) as a unit of pressure.
And so, pascal and millimeter of mercury are units of atmospheric pressure. Using the formula to calculate the pressure ($p$) of a liquid column:
where $\rho $ is the density of the liquid (we have mercury $\rho =13600\ \frac(kg)(m^3)$), $g$ is the free fall acceleration; $h$ - the height of the liquid column (we have mercury). We get that the pressure exerted by a mercury column of 1 mm is:
Consequently:
Normal atmospheric pressure is considered equal to 760 mm Hg. Art. or 1013 hPa (hPa - hectopascal).
If the Torricelli tube is provided with a vertical scale, then the simplest mercury barometer will be obtained, which can be used to measure atmospheric pressure.
There is an off-system unit of pressure, which is called the atmosphere, this is the pressure on the Earth's surface at the level of the World Ocean. A distinction is made between the technical atmosphere ($p=98066.5\ Pa$) and the physical atmosphere ($p=101325\ (\rm Pa).$).
Sometimes a non-systemic unit of pressure is used bar. Normal atmospheric pressure is:
Water column meters (m.w.c.) are also used to measure pressure, including atmospheric pressure, while:
We got: pascals, millimeters of mercury, meters of water, bars - units of atmospheric pressure.
Examples of problems with a solution
Example 1
Exercise. At the temperature $t_1=$300С the barometer showed the atmospheric pressure $p_1=$730 mmHg. At the temperature $t_2=$-300С the barometer readings were: $p_2=$780 mm Hg. Art. Find the ratio of air density at given temperatures ($\frac((\rho )_2)((\rho )_1)$). Consider air as an ideal gas under given conditions.
Solution. Let us take the Mendeleev-Clapeyron equation as the basis for solving the problem:
Let us express the air density from (1.1) for the first and second states:
\[(\rho )_1=\frac(p_1\mu )(RT_1);;(\rho )_2=\frac(p_2\mu )(RT_2)\ \ \left(1.2\right).\]
Find the ratio of densities:
\[\frac((\rho )_2)((\rho )_1)=\frac(p_2\mu RT_1)(p_1\mu RT_2)=\frac(p_2T_1)(p_1T_2).\]
To calculate the ratio of densities, the given temperatures should be converted using the ratio:
then $T_1=303\ K;;\ T_2=243\ K.$ It is not necessary to convert the pressure to SI units, since the same factor will be in the numerator and denominator. Let's do the calculations:
\[\frac((\rho )_2)((\rho )_1)=\frac(780\cdot 303)(730\cdot 243)\approx 1.33.\]
Answer.$\frac((\rho )_2)((\rho )_1)\approx 1.33$
Example 2
Exercise. The aneroid barometer shows that the atmospheric pressure is 101300 Pa. What is the height of the mercury column installed vertically (Fig. 1)?
Solution. Aneroid barometer shows normal atmospheric pressure $p=$101300 Pa. Since the liquid in the tube and the cup is in equilibrium, therefore, the pressure of the mercury column in the tube is equal to the pressure that the atmosphere exerts on the surface of the mercury in the cup, which means that the pressure of the mercury column in the tube is $p=$101300 Pa, based on the formula :
Express the height of the mercury table in the tube:
The density of mercury is equal to $\rho =13600\ \frac(kg)(m^3)$, $g=9.8\ \frac(m)(s^2)$, calculate the height of the mercury column:
Answer.$h=760$ mm
Different manufacturers use different designations and standards to indicate the water resistance of watches. Some waterproof watch manufacturers use bars (bar), others in meters, and still others in atmospheres. There are also many ISO standards that determine the water resistance and water resistance of not only watches, but also other devices. This article will help you deal with all these subtleties.
First, let's look at the units of measure for water resistance.
Bar
Bar - international designation: bar. The term comes from the Greek word βάρος, which means heaviness. The bar is a non-systemic unit of pressure, that is, it is not included in any measurement system. The value of a bar is approximately equal to one atmosphere. That is, the pressure of "one bar" is the same as the pressure of one atmosphere.
Atmosphere
Well, everything is clear from the name, and, perhaps, from school course physics. This pressure is equal to the force with which the layer of air above the earth presses on the earth itself. In nature, pressure is of course constantly changing, but in physics it is generally accepted that the pressure of one atmosphere is equal to the pressure of 760 millimeters of mercury (mmHg). Pressure in atmospheres is abbreviated as "atm" or "atm".
m or meters
Most often, the water resistance of watches is indicated in meters, but these are not the meters that you can dive under water. This is the equivalent of the pressure measured by the water column. For example, at a depth of 10 meters, water will press with a force of one atmosphere. That is, a pressure value of 10 m is equal to a pressure of one atmosphere.
So, there are different systems for indicating the water resistance of watches - in meters, bars and atmospheres. But they all mean about the same thing: 1 bar is equal to 1 atmosphere and is approximately equal to immersion by 10 meters.
1 bar = 1 atm = 10 m
Watch water resistance standards
There are many different standards by which the water resistance of watches and other electronic devices (such as phones) is determined. Waterproof watches are very popular among hikers, climbers and extreme sports enthusiasts.
Watch water resistance standard ISO 2281 (GOST 29330)
This standard was adopted in 1990 to standardize the water resistance of watches. It describes the procedure for checking the water-resistance of a watch during a test run. The standard specifies the requirements for water pressure, or air, at which the watch must maintain its tightness and performance. However, the standard states that it can be carried out selectively. This means that not all watches produced according to this standard undergo mandatory water resistance testing - the manufacturer can selectively check individual items. This standard is used for watches not specifically designed for diving or swimming, but only for watches for daily use with possible short-term immersion in water.
Testing a watch against this water resistance standard consists of the following steps:
- Immerse the watch in water to a depth of 10 cm for one hour.
- Immersion of the watch in water to a depth of 10 cm with a water pressure of 5 N (Newtons) perpendicular to the buttons or to the crown for 10 minutes.
- Immersion of the watch in water to a depth of 10 cm with temperature changes between 40°C, 20°C and again 40°C. At each temperature, the clock is within five minutes, the transition between temperatures is no more than five minutes.
- Immersion of watches in water in a pressure chamber and exposure to their nominal pressure for which they are designed for 1 hour. Do not allow condensation inside the watch and water penetration into the case.
- Checking watches with an excess of nominal pressure by 2 atm.
Well, additional checks that are not directly related to the water resistance of the watch:
- The watch must not exhibit a flow rate exceeding 50 µg/min.
- No strap test required
- No corrosion test required
- No negative pressure test required
- Resistance test magnetic fields and no punches required
ISO 6425 standard - diving and diving watches
This standard was developed and adopted in 1996 and is designed specifically for watches that require increased water resistance, such as watches for diving, spearfishing and other types of underwater work.
All watches produced under the ISO 6425 standard are subject to a mandatory water resistance test. That is, unlike the ISO 2281 standard, where only individual watches are tested for water resistance, in the ISO 6425 standard, absolutely all watches are tested at the factory before they are sold.
Moreover, the check is also performed with an excess of the calculated indicators by 25%. That is, watches designed for diving up to 100 meters will be tested at a pressure as at a depth of 125 meters.
According to the ISO 6425 standard, all watches must pass the following water resistance tests:
Prolonged stay under water. The watch is immersed in water to a depth of 30 cm for 50 hours. The water temperature can vary from 18°C to 25°C. All mechanisms must continue to function, no condensation should appear inside the watch.
Check for condensation in the watch. The watch heats up to 40°C - 45°C. After that, cold water is poured onto the watch glass for 1 minute. Watches that have condensation on the glass on the inside of the glass must be destroyed.
Resistance of crowns and buttons to increased water pressure. The watch is placed in water and pressurized in water 25% above its rated water resistance. Within 10 minutes in such conditions, the watch should maintain its tightness.
Prolonged exposure to water under pressure exceeding the calculated pressure by 25%, for two hours. The clock must continue to work, maintain tightness. There must be no condensation on the glass.
Immersion in water to a depth of 30 cm with a change in water temperature from 40°C to 5°C and again 40°C. The transition time from one dive to another should not exceed 1 minute.
25% over design pressure provides a safety margin to prevent wetting during dynamic pressure increases or changes in water density, e.g. sea water 2 - 5% denser than fresh water.
Watches that have passed ISO 6425 testing are marked with the inscription DIVER "S WATCH L M. The letter L indicates the diving depth in meters guaranteed by the manufacturer.
Water Resistant watch table
| Watch water resistance (Water Resistant) | Purpose | Restrictions |
| Water Resistant 3ATM or 30m | for everyday use. Withstands light rain and splashes | not suitable for showering, swimming, diving. |
| Water Resistant 5ATM or 50m | Withstand short-term immersion in water. | swimming is not recommended. |
| Water Resistant 10ATM or 100m | Water sports | do not use for diving and snorkeling |
| Water Resistant 20ATM or 200m | Professional water sports. Scuba diving. | duration of stay under water no more than 2 hours |
| Diver's 100m | ISO 6425 minimum requirement for scuba diving | This marking is worn by obsolete watches. Not suitable for long dives. |
| Diver's 200m or 300m | Suitable for scuba diving | Typical markings for modern diving watches. |
| Diver's 300+m for mixed gas diving. | Suitable for long-term scuba diving with mixed gas in scuba gear. | They are additionally marked DIVER'S WATCH L M or DIVER'S L M |
IP water resistance standard
The IP standard adopted for various electronic devices, including smart smart watches, regulates two indicators: protection against dust ingress and protection against liquid ingress. The marking according to this standard is IPXX, where instead of "X" there are numbers indicating the degree of protection against dust and water ingress into the case. The numbers may be followed by one or two characters that carry auxiliary information. For example, a sports watch with an IP68 rating is a dust-proof device that can withstand long-term immersion in pressurized water.
First digit in the code IPXX indicates the level of protection against ingress of dust. Sports GPS trackers and smartwatches tend to use the most high levels dust protection:
- 5 Dust-proof, some dust may enter the case, but this does not interfere with the operation of the device.
- 6 Dust-proof, dust does not get inside the device.
The second digit in the IPXX code indicates the level of water protection. Changes from 0 to 9 - the higher the number, the better the water resistance:
- 0 No protection
- 1 Vertically dripping water must not interfere with the operation of the device.
- 2 Vertically dripping water must not interfere with the operation of the device if it is tilted up to 15° from the working position.
- 3 Rain protection. Water flows vertically or at an angle up to 60°.
- 4 Protected against splashes falling in any direction.
- 5 Protected against water jets from any direction.
- 6 Protection against sea waves or strong water currents. Water entering the housing must not impair the operation of the device.
- 7 Short-term immersion to a depth of 1 m During short-term immersion, water does not enter in quantities that disrupt the operation of the device. Permanent job in immersed mode is not expected.
- 8 Long-term immersion to a depth of more than 1 m Completely waterproof. The device can work in immersed mode.
- 9 Long-term pressure immersion. Completely waterproof under pressure. The device can operate in immersed mode at high water pressure.
Common watch water resistance designations
Watches not waterproof
This is a watch that is not designed to be used in water. Try not to keep them in damp places and keep them away from accidental water or splashes, steam, etc.
Please note that non-water resistant watches usually do not have any special markings on the dial or case back.
Normal water resistance - up to 30 m -3 ATM - 3 bar - 3 bar
On such hours there is an inscription "WATER RESISTANT" ("water-resistant"). This means that the watch is able to withstand the static pressure of a 30-meter water column (3 atmospheres), but does not mean that they can dive to a depth of 30 m. The meaning of this inscription is that the watch will not be damaged by drops when washing, rainy season etc. . The design of these watches allows them to be used in Everyday life- for example, when washing your face or in the rain, but you should not swim, take a bath or wash your car in such a watch.
Normal water resistance - up to 50 m- 5 ATM - 5 bar - 5 bar
On such watches there is an inscription "WATER RESISTANT 50M" or "50M" (or "5 bar"). This means that the watch can withstand the static pressure of a 50-meter water column (5 atmospheres), but does not mean that it can dive to a depth of 50 m. Such water resistance allows you to work with water in the watch. This watch cannot be used for diving, diving, windsurfing, etc.
Water resistant up to 100 m- 10 ATM - 10 bar - 10 bar
The watch is labeled "WATER RESISTANT 100M" or "100M" (or 10 bar). This also means that the watch can withstand the static pressure of a 100-meter water column, but note that you cannot dive to a depth of 100 m in it. In practice, this water resistance allows the watch to be exposed to water or even submerged in water, but does not allow the watch to withstand the pressure of water when swimming in a pool or sea, where waves can hit the watch.
Water resistant up to 200 m- 20 ATM - 20 bar - 20 bar
Watches with such water resistance are called "diver" ("diver's watches"). You can safely swim in the sea or in the pool while wearing this watch, but you need to be careful when taking a pressure shower or diving into the water. In addition, it is best to avoid bathing in hot water, as hot water can damage the lubricating oil inside the watch.
It is determined by the weight of the air. 1 m³ of air weighs 1.033 kg. For every meter of the earth's surface, there is an air pressure of 10033 kg. By this is meant a column of air from sea level to upper layers atmosphere. If we compare it with a column of water, then the diameter of the latter would have a height of only 10 meters. That is, atmospheric pressure is created by its own mass of air. The value of atmospheric pressure per unit area corresponds to the mass of the air column above it. As a result of an increase in air in this column, an increase in pressure occurs, and with a decrease in air, a drop occurs. Normal atmospheric pressure is the air pressure at t 0 ° C at sea level at a latitude of 45 °. In this case, the atmosphere presses with a force of 1.033 kg for every 1 cm2 of the earth's area. The mass of this air is balanced by a mercury column 760 mm high. This relationship is used to measure atmospheric pressure. It is measured in millimeters of mercury or millibars (mb), as well as in hectopascals. 1mb = 0.75 mm Hg, 1 hPa = 1 mm.
Measurement of atmospheric pressure.
measured with barometers. They are of two types.
1. A mercury barometer is a glass tube that is sealed at the top and immersed with an open end in a metal bowl with mercury. A scale is attached next to the tube, showing the change in pressure. Mercury is affected by air pressure, which balances the column of mercury in a glass tube with its weight. The height of the mercury column changes with pressure.
2. A metal barometer or aneroid is a corrugated metal box that is hermetically sealed. Inside this box is rarefied air. The change in pressure causes the walls of the box to oscillate, pushing in or out. These vibrations by a system of levers cause the arrow to move along a scale with divisions.
Recording barometers or barographs are designed to record changes atmospheric pressure. The pen detects the vibration of the walls of the aneroid box and draws a line on the tape of the drum, which rotates around its axis.
What is atmospheric pressure.
Atmospheric pressure on the globe varies over a wide range. Its minimum value - 641.3 mm Hg or 854 mb was registered over Pacific Ocean in Hurricane Nancy, and the maximum is 815.85 mm Hg. or 1087 mb in Turukhansk in winter.
Air pressure on the earth's surface changes with height. Average atmospheric pressure value above sea level - 1013 mb or 760 mm Hg. The higher the altitude, the lower the atmospheric pressure, as the air becomes more and more rarefied. In the lower layer of the troposphere, up to a height of 10 m, it decreases by 1 mm Hg. for every 10 m or 1 mb for every 8 meters. At an altitude of 5 km, it is 2 times less, 15 km - 8 times, 20 km - 18 times.
Due to air movement, temperature change, season change Atmosphere pressure constantly changing. Twice a day, morning and evening, it rises and falls the same number of times, after midnight and in the afternoon. During the year, due to cold and compacted air, atmospheric pressure has a maximum value in winter, and a minimum in summer.
Constantly changing and distributed over the surface of the earth zonally. This is due to uneven heating by the sun. earth's surface. The change in pressure is affected by the movement of air. Where there is more air, the pressure is high, and where the air leaves, the pressure is low. The air, warmed up from the surface, rises and the pressure on the surface decreases. At altitude, the air begins to cool, condenses and sinks to nearby cold areas. There, the pressure rises. Therefore, the change in pressure is caused by the movement of air as a result of its heating and cooling from the earth's surface.
Atmospheric pressure in the equatorial zone constantly lowered, and in tropical latitudes - increased. This is due to the constantly high air temperatures at the equator. The heated air rises and goes towards the tropics. In the Arctic and Antarctic, the surface of the earth is always cold and the atmospheric pressure is high. It is caused by air that comes from temperate latitudes. In turn, in temperate latitudes, due to the outflow of air, a zone of low pressure is formed. Thus, there are two belts on Earth atmospheric pressure- low and high. Decreased at the equator and at two temperate latitudes. Upgraded to two tropical and two polar. They can shift slightly depending on the time of year following the Sun towards the summer hemisphere.
High-pressure polar belts exist throughout the year, however, in summer they are reduced, and in winter, on the contrary, they expand. All year round areas of low pressure persist near the Equator and in southern hemisphere in temperate latitudes. Things are different in the northern hemisphere. In temperate latitudes northern hemisphere the pressure over the continents increases greatly and the low pressure field seems to "break": it is preserved only over the oceans in the form of closed areas low atmospheric pressure- Icelandic and Aleutian lows. Over the continents, where the pressure has noticeably increased, winter maxima are formed: Asian (Siberian) and North American (Canadian). In summer, the low pressure field in the temperate latitudes of the northern hemisphere is restored. At the same time, a vast area of low pressure is formed over Asia. This is the Asian low.
In the belt elevated atmospheric pressure- tropics - the continents heat up more than the oceans and the pressure over them is lower. Because of this, subtropical highs are distinguished over the oceans:
- North Atlantic (Azores);
- South Atlantic;
- South Pacific;
- Indian.
Despite large-scale seasonal changes in their performance, belts of low and high atmospheric pressure of the Earth- formations are quite stable.
If you have thought about new system heating, or water supply, then you willy-nilly will meet with such a concept as "BAR". Personally, I encountered when I installed a heating boiler. For experienced physicists, or for those who studied well at school, this abbreviation is nothing complicated, and even more so they can easily translate it into atmospheres, but if you believe the Internet, then others who do not quite remember everything from school curriculum also a lot! Therefore, today a useful and informative article on the translation of this meaning ...
I'll start with a definition
BAR - (from the Greek "baros" is translated as gravity) is a non-systemic unit of pressure. I would also like to emphasize that not only liquid is measured, but also other quantities, for example, atmospheric pressure, although there it is in “millibars” mBAR.
In simple terms, this is just another abbreviation that characterizes pressure, and for some reason many manufacturers have adopted it in their systems, it seems to me, to distinguish it from other devices.
So different inside
Do you know what - now in Russia they use two categories of units, which are meant by "BAR".
- Used in the physical system of units - centimeter, gram, second, abbreviated CGS. The definition is 1 DIN/cm2, where DIN is the measurement of force (as applied to physics).
- A more common unit, often referred to as the "meteorological" unit, is roughly equal to one standard atmosphere, or 106 DYN/cm2.
If we dig deeper, we get even more atmospheres, for example, there are technical and physical ones.
Technical, or "measuring", also known as "metric" - used mainly in technical systems, equal to the produced force of 1 kgf directed perpendicularly and evenly, to a surface equal to 1 cm2.
Physical (normal) is a unit of pressure on the surface of the earth. Measured with mercury at 0 degrees Celsius. If we connect it with a bar, then we get a ratio of 0.9869 atm.
Applied in practice
A little confusing, but it was necessary to display all the pressure indicators. Now let's go down "from heaven to earth" and decide already on the "BAR" which is used in our boilers, water supply systems, etc.
To exaggerate, all manufacturers use the technical BAR - and it is equal to 1.0197 kgf / cm2, or approximately 1 atmosphere.
Now in many double-circuit boilers, the measurement of pressure is precisely in “BARS”, the recommended operating range is from 1 to 2. That is, in fact, if you translate this, it turns out from one to two atmospheres, the pressure is about the same as in a car wheel, only this pressure water (or antifreeze) not air.
Transfer toPSI
There is also such a bourgeois concept as PSI (the ratio of gas pressure, which is measured in pounds per square inch), in fact, these are the same atmospheres, only they are not measured according to our accepted units of measurement. Why are so many interested in these units? Again, it’s simple - many boilers, especially Asian ones, have an indicator in PSI. Therefore, below is a small translation.
1 BAR ≈ 1 ATM (technical) ≈ 14.5 PSI
Why is it approximately equal, but because there is a small error, it is no more than 1 - 2%.
About heating boilers
To be honest, I started all this reasoning for the sake of a heating boiler, it is in modern models that need pressure in their system, they have indicators on the side or on a digital display.
"Why is it needed?" - you ask. YES, everything is just guys, there is a pump that drives water through the system, and the more pressure the easier it is for him to do it! That is why if it drops to a minimum level (usually below 0.9 BAR), the boiler will automatically turn off - it will not work.
That is, in order for it to function normally, you need to monitor the “bars”. However, you shouldn’t “borscht” either - if you bring the pressure to more than 2.7 BAR, then the boiler will also turn off (protection will work), because the heat exchangers are made of copper or brass - and this is a soft material, it can simply break! Therefore, pressure relief systems have been installed.
That is why it is mandatory to take out a sensor with an indicator.
Wow, a great article turned out, I tried to reveal the topic to the maximum. I think it worked.
Imagine an air-filled sealed cylinder with a piston mounted on top. If you start to put pressure on the piston, then the volume of air in the cylinder will begin to decrease, the air molecules will collide with each other and with the piston more and more intensively, and the pressure of compressed air on the piston will increase.
If the piston is now abruptly released, then the compressed air will abruptly push it up. This will happen because with a constant piston area, the force acting on the piston from the compressed air will increase. The area of the piston remained unchanged, and the force from the side of the gas molecules increased, and the pressure increased accordingly.
Or another example. A man stands on the ground, stands with both feet. In this position, a person is comfortable, he does not experience inconvenience. But what happens if this person decides to stand on one leg? He will bend one of his legs at the knee, and now he will lean on the ground with only one foot. In this position, a person will feel some discomfort, because the pressure on the foot has increased, and about 2 times. Why? Because the area through which gravity now presses a person to the ground has decreased by 2 times. Here is an example of what pressure is and how easy it is to detect in everyday life.
From the point of view of physics, pressure is called physical quantity, numerically equal to strength acting perpendicular to the surface per unit area of this surface. Therefore, to determine the pressure at a certain point on the surface, the normal component of the force applied to the surface is divided by the area of the small surface element on which this force acts. And in order to determine the average pressure over the entire area, the normal component of the force acting on the surface must be divided by the total area of this surface.
Pressure is measured in pascals (Pa). This pressure unit got its name in honor of the French mathematician, physicist and writer Blaise Pascal, the author of the basic law of hydrostatics - Pascal's Law, which states that the pressure exerted on a liquid or gas is transmitted to any point unchanged in all directions. For the first time, the unit of pressure "pascal" was put into circulation in France in 1961, according to the decree on units, three centuries after the death of the scientist.
One pascal is equal to the pressure exerted by a force of one newton, evenly distributed, and directed perpendicular to a surface of one square meter.
In pascals, not only mechanical pressure (mechanical stress) is measured, but also the modulus of elasticity, Young's modulus, bulk modulus of elasticity, yield strength, limit of proportionality, tear resistance, shear strength, sound pressure and osmotic pressure. Traditionally, it is in pascals that the most important mechanical characteristics of materials in the strength of materials are expressed.
Atmosphere technical (at), physical (atm), kilogram-force per square centimeter (kgf / cm2)
In addition to the pascal, other (off-system) units are also used to measure pressure. One such unit is the “atmosphere” (at). A pressure of one atmosphere is approximately equal to atmospheric pressure on the Earth's surface at sea level. Today, “atmosphere” is understood as the technical atmosphere (at).
The technical atmosphere (at) is the pressure produced by one kilogram-force (kgf) distributed evenly over an area of one square centimeter. And one kilogram-force, in turn, is equal to the force of gravity acting on a body with a mass of one kilogram under conditions of free fall acceleration equal to 9.80665 m/s2. One kilogram-force is thus equal to 9.80665 Newton, and 1 atmosphere turns out to be equal to exactly 98066.5 Pa. 1 at = 98066.5 Pa.
In atmospheres, for example, the pressure in automobile tires is measured, for example, the recommended pressure in the tires of a GAZ-2217 passenger bus is 3 atmospheres.
There is also the "physical atmosphere" (atm), defined as the pressure of a column of mercury, 760 mm high at its base, given that the density of mercury is 13595.04 kg / m3, at a temperature of 0 ° C and under conditions of a gravitational acceleration of 9, 80665 m/s2. So it turns out that 1 atm \u003d 1.033233 atm \u003d 101 325 Pa.
As for the kilogram-force per square centimeter (kgf/cm2), this non-systemic unit of pressure is equal to normal atmospheric pressure with good accuracy, which is sometimes convenient for assessing various effects.
The non-systemic unit "bar" is approximately equal to one atmosphere, but is more accurate - exactly 100,000 Pa. In the CGS system, 1 bar is equal to 1,000,000 dynes/cm2. Previously, the name "bar" was carried by the unit, now called "barium", and equal to 0.1 Pa or in the CGS system 1 barium \u003d 1 dyn / cm2. The word "bar", "barium" and "barometer" come from the same Greek word for "gravity".
Often, to measure atmospheric pressure in meteorology, the unit mbar (millibar), equal to 0.001 bar, is used. And to measure pressure on planets where the atmosphere is very rarefied - microbar (microbar), equal to 0.000001 bar. On technical pressure gauges, most often the scale has a graduation in bars.
Millimeter of mercury column (mm Hg), millimeter of water column (mm of water column)
The non-systemic unit of measure "millimeter of mercury" is 101325/760 = 133.3223684 Pa. It is designated "mm Hg", but sometimes it is designated "torr" - in honor of the Italian physicist, a student of Galileo, Evangelista Torricelli, the author of the concept of atmospheric pressure.
The unit was formed in connection with a convenient way to measure atmospheric pressure with a barometer, in which the mercury column is in equilibrium under the action of atmospheric pressure. Mercury has a high density of about 13,600 kg/m3 and is characterized by low saturated vapor pressure at room temperature, which is why mercury was chosen for barometers at one time.
At sea level, atmospheric pressure is approximately 760 mm Hg, it is this value that is now considered to be normal atmospheric pressure, equal to 101325 Pa or one physical atmosphere, 1 atm. That is, 1 millimeter of mercury is equal to 101325/760 pascals.
In millimeters of mercury, pressure is measured in medicine, meteorology, and aviation navigation. In medicine, blood pressure is measured in mmHg; in vacuum technology, it is graduated in mmHg, along with bars. Sometimes they even simply write 25 microns, meaning microns of mercury, when it comes to evacuation, and pressure measurements are carried out with vacuum gauges.
In some cases, millimeters of water column are used, and then 13.59 mm of water column \u003d 1 mm Hg. Sometimes it is more expedient and convenient. A millimeter of a water column, like a millimeter of a mercury column, is an off-system unit, equal in turn to the hydrostatic pressure of 1 mm of a water column, which this column exerts on a flat base at a column water temperature of 4 ° C.