First, the concept of vacuum and vacuum degree In vacuum technology, the state of the gas below one atmosphere is collectively referred to as vacuum. Compared to the normal atmosphere, this is a leaner gas state.
The thinness of the residual gas in the vacuum is the level of vacuum, that is, the degree of vacuum.
Second, the unit of vacuum degree According to the definition of vacuum degree, the most direct physical quantity to measure the degree of vacuum should be the number of molecules per unit volume. However, due to historical and technical reasons, the degree of vacuum is left in the evacuated container. The absolute pressure of the gas. "Pressure" refers to the pressure on the wall of the container per unit area. It is a physical quantity that is commonly used internationally to indicate the degree of vacuum. The first International Technical Conference in 1958 proposed to use "Torr" as a unit for measuring the degree of vacuum, defined as "1Torr is 1/760 of 1 atmosphere", because 1 atmosphere is 760mmHg, so Torr can be used with mmHg Universal. The unit of pressure specified in the International System of Units (SI) is Pa (Pa), which is a special name for Newton / meter 2 (N / m2). At present, many countries including China have gradually replaced Torr with Pa as the standard basic unit of vacuum pressure.
The higher the degree of vacuum, the lower the pressure of the gas and the smaller the number of gas molecules in the furnace; on the contrary, the higher the pressure of the gas, the lower the degree of vacuum. It can be seen that the magnitude of the pressure is inversely proportional to the level of vacuum.
In various literatures, in addition to Pa and Torr, there are standard atmospheric pressure, bar, kgf / cm2. The conversion relationship between several common units is as follows.
1 Torr = 133.3 Pa
1 Pa = 7.5 × 10-3 Torr
1 Torr = 1 mmHg column
1 Torr = 1/760 atmosphere
Third, the division of the vacuum area There are many ways to divide the vacuum area. At present, China divides the vacuum area into: low vacuum, medium vacuum, high vacuum and ultra high vacuum. The vacuum values corresponding to each vacuum area 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 phenomenon of molecular motion is different in different vacuum regions. Ion nitriding is carried out in a medium-low vacuum, and its pressure range is usually 7 to 1000 Pa.
Fourth, the introduction of commonly used vacuum gauge The tool for measuring gas pressure below atmospheric pressure is called a vacuum gauge. The vacuum gauge can directly measure the pressure of the gas, and it can also measure the pressure indirectly through the physical quantity related to the pressure. The former is called an absolute vacuum gauge, and the latter is called a relative vacuum gauge.
There are many types of vacuum gauges. The following briefly introduces several vacuum gauges commonly used in domestic ion nitriding equipment.
1. Compression type vacuum gauge Compression type vacuum gauge is also called McLaugh vacuum gauge, referred to as Mc type vacuum gauge for short. Micro vacuum gauge belongs to the category of absolute vacuum gauge. The advantage is that the measurement accuracy is high, and it can be used as a standard for vacuum measurement. The disadvantages are that it is not convenient to use, the response is slow, and continuous measurement cannot be performed.
2, resistance vacuum gauge resistance vacuum gauge is made by the principle of the number of molecules in the vacuum system and the conduction of heat. Resistance vacuum gauge belongs to the category of relative vacuum gauge.
The advantages of the resistance type vacuum gauge are simple structure, continuous measurement, total pressure measurement, and easy use (long-distance measurement can be performed with a wire). The disadvantage is that the measurement value of this type of vacuum gauge is related to the type and composition of the gas being measured. This is because such vacuum gauges are mostly calibrated with air when leaving the factory. Due to the presence of hydrogen in the ion nitriding furnace, the heat of hydrogen is The conductivity is too different from the thermal conductivity of nitrogen, air, etc., so the error of the measured vacuum value is large. However, this type of vacuum gauge is easy to use, intuitive to read, and relatively inexpensive. It is used to measure the ultimate vacuum and pressure rise rate of the furnace body accurately and reliably, so it is still widely used by many enterprises in China.
3. Absolute pressure transmitter-Absolute vacuum gauge This is an absolute vacuum gauge that measures the vacuum produced by the direct effect of gas pressure. Depending on the specific structure, it can be divided into various forms. Among them, the capacitive film absolute pressure transmitter is one of the most widely used products. It works based on the principle of capacitance change. Its most significant advantage is that the test is not affected by the type and composition of the measured medium, can be used continuously for a long time, and the reading is true and reliable. But its price is relatively high.