Molecular-kinetic theory considers the pressure as a result of blows of the gas molecules that are in continuous random motion, the wall of the vessel. In other words, its value is associated with the average kinetic energy of translational motion of molecules mv2 (where m is the mass of the molecule, and v2 is the average square speed of the molecules) and their number N in the volume V:P = Nmv2/3V.
The concept of "absolute pressure" is used in the calculation of partial pressure of gases (that is, the calculation pressure of each gas separately), and is measured from absolute zero pressure (absolute vacuum). Regardless of less or more pressure vessel of atmospheric pressure, absolute pressure is calculated by the formula:P = Ro + Ri.
Pressure (Ri) is called the difference between the pressure of the gas (fluid) and the ambient pressure. In order to determine the value of the excess pressure, use a manometer. If you are going to measure the oxygen pressure, select a pressure gauge whose body is blue, hydrogen is a dark green, etc. in addition, usually the manufacturers of these devices point to the inner surface of the manometer for measuring the overpressure of a gas it is intended. Pressure gauges made in the USSR until 1982, specify the pressure in kgf/cm2. 1 kgf/cm2 equals 98, kPa (kPa), the units of measurement adopted in the USSR since 1982.
For the measurement of barometric pressure use a mercury barometer. One millimeter of mercury is equal to 133,32 PA. However, for convenience of calculations the constant atmospheric pressure is considered to be normal. Normal atmospheric pressure - 760 mm of mercury (i.e., 101, 32 kPa).
Use the formula:P = Ro + Ri.Determine the absolute value of the pressure.
Advice 2: What is the absolute pressure
Pressure is an important physical quantity that characterizes the behavior of liquid and gaseous substances. Is called the absolute pressure, measured relative to the temperature equal to absolute zero. Such pressure creates the ideal gas on the vessel wall.
From the point of view of science, the absolute pressure is the pressure in the system to pressure in a vacuum. The most common expression for the absolute pressure is the sum of the sensor system and atmospheric pressure. The expression takes the form:
Absolute pressure = Gauge pressure + atmospheric pressure.
Atmospheric pressure is defined as the ambient air pressure on the Earth's surface. This value is not fixed or constant and may vary depending on temperature, altitude and humidity.
Gauge pressure represents the pressure in the system, which was measured using a measuring device. These devices or sensors can be classified according to structural features. The most common types are sensors based on elastic elements, sensors with the fluid column and electrical appliances. If the sensor does not take into account the pressure of the atmosphere in his testimony, the calculation of absolute pressure made by hand.
Units of measurement and practical application
In practice, absolute and gauge pressure are not the same characteristics of the system. Therefore, each of them uses its own designation. The most common technique is to add indexes. After the letter denoting the absolute pressure, put the index "a", and after gauge – "m".
Such designations are most often used in engineering calculations. In their implementation it is necessary to use a proper marking pressure to avoid mistakes. The difference between absolute and gauge pressure is much more pronounced in the case when the atmospheric pressure has the same order of magnitude as the gauge.
Neglect atmospheric component of the absolute pressure in the calculations also leads to serious errors in the design. This can be demonstrated by examining a closed cylinder with an ideal gas at a temperature of 25oC and a volume of 1 cubic meter. If the pressure gauge on the cylinder shows a pressure of 100 KPa, and the pressure of the atmosphere is not taken into account, the estimated number of gas moles in the cylinder is approximately 40,34.
When the atmospheric pressure is 100 KPa, the absolute pressure is actually equal to 200 KPa and the correct number of moles of gas will 80,68. The actual number of moles of gas will be two times more compared to the original calculation. This example shows the importance of using the correct algorithm for calculating the pressure.