Vacuum and Vacuum Degree Concepts

In vacuum technology, the state of a gas below one atmospheric pressure is called vacuum. This is a rarer gas state than normal atmosphere.
The degree of rarity of residual gases in vacuum is the degree of vacuum, that is, the degree of vacuum.
According to the definition of vacuum degree, the most direct physical quantity to measure the vacuum degree is the number of molecules per unit volume. However, due to historical and technical reasons, the vacuum degree is expressed by the absolute pressure of the residual gas in the evacuated container. “Pressure” refers to the pressure on the wall of a unit area container. It is a universal physical quantity to express the vacuum degree in the world at present. In 1958, the first International Technical Conference recommended that Torr be used as the unit of vacuum measurement. It was defined as “1 Torr is 1/760 of 1 atmosphere”. Because 1 atmosphere is 760 mmHg, Torr can be used in common with mmHg. The international unit system (SI) stipulates that the unit of pressure is Pa, which is the special name of (N/m2). At present, Pa has gradually replaced Torr as the standard basic unit of vacuum pressure in many countries, including China.
The higher the vacuum degree is, the lower the gas pressure is, and the less the number of gas molecules in the furnace. On the contrary, the higher the gas pressure is, the lower the vacuum degree is. It can be seen that the pressure is inversely proportional to the vacuum.
In various literatures, besides Pa and Torr, there are standard atmospheric pressure, bar, kgf/cm2. Conversion relationships among several common units are as follows.
1 Torr = 133.3 Pa
1 Pa = 7.5 x 10-3 Torr
1 Torr = 1 mmHg
1 Torr = 1/760 atmospheric pressure
Division of Vacuum Region
There are many methods to divide the vacuum area. At present, China divides the vacuum area into low vacuum, medium vacuum, high vacuum and ultra-high vacuum. The corresponding vacuum values for each vacuum region are:
Low vacuum: 105-102 Pa (760-1 Torr)
Medium Vacuum: 102-10-1 Pa (1-10-3 Torr)
High vacuum: 10-1-10-5 Pa (10-3-10-7 Torr)
Ultra-high vacuum: <10-5 Pa(<10-7 Torr)
The physical phenomena of molecular motion are different in different vacuum regions. Ion nitriding is carried out in medium and low vacuum, and its pressure range is usually 7-1000 Pa.
Brief Introduction of Common Vacuum Meters
A tool for measuring the pressure of a gas below atmospheric pressure is called a vacuum gauge. Vacuum gauge can directly measure the pressure of gas, or indirectly measure the pressure by physical quantities related to pressure. The former is called absolute vacuum gauge, and the latter is called relative vacuum gauge.
There are many kinds of vacuum gauges. The following is a brief introduction of several vacuum gauges commonly used in vacuum furnace in China.

Compressed vacuum gauge
Compression vacuum gauge is also called McLaugh vacuum gauge, which is called McLaugh vacuum gauge for short. The wheat vacuum gauge belongs to the category of absolute vacuum gauge. Its advantage is that it has high accuracy and can be used as a standard instrument for vacuum measurement. Its disadvantage is that it is not convenient to use, slow reaction and can not be continuously measured.

Resistance Vacuum Meter
Resistance vacuum gauge is made by using the principle that the number of molecules in the vacuum system is related to the heat conduction. Resistance vacuum gauge belongs to the category of relative vacuum gauge.
Resistance vacuum gauge has the advantages of simple structure, continuous measurement, total pressure measurement and easy use (long-distance measurement can be carried out by wire). The disadvantage is that the measurement value of this type of vacuum gauge is related to the type of gas measured and the composition of the gas. This is because most of the vacuum gauges are calibrated by air when they leave the factory. Because of the existence of hydrogen in the ion nitriding furnace, the thermal conductivity of hydrogen differs greatly from that of nitrogen and air, so the measurement error of vacuum value is large. However, due to its convenient use, intuitive reading and relatively low price, this kind of vacuum gauge is still widely used in many enterprises in China because it is accurate and reliable in measuring the ultimate vacuum and pressure rise rate of furnace body.

Absolute Pressure Transmitter-Absolute Vacuum Meter
This is an absolute vacuum gauge, which measures vacuum by means of deformation caused by direct action of gas pressure. According to the specific structure, it can be divided into many forms. Among them, capacitive film absolute pressure transmitter is one of the most widely used products. It works on the principle of capacitance variation. Its most remarkable advantage is that the detection is not affected by the type and composition of the medium under test. It can be used continuously for a long time and the reading is real and reliable. But its price is relatively high.