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Characterization of vacuum degree, unit and division of vacuum region

Dec 19, 2018

Characterization of vacuum degree, unit and division of vacuum region


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It is not very reasonable to use pressure to represent vacuum degree, which has been used in the history of measuring vacuum by u-type manometer.

In general vacuum system, the hydrostatic physical quantity of isotropic neutral gas pressure is usually used to express the vacuum degree. Therefore, the measurement of vacuum degree only comes down to the measurement of pressure. However, special attention should be paid to the measurement conditions, which are the stationary (random movement), steady state and isotropic single neutral gas in a finite container. In this case, maxwell velocity distribution, cosine scattering law and hydrostatic pressure concept (p = nkT, v= 1/4nc, p = ρgh) are relatively consistent with the objective reality, the vacuum degree measurement is relatively simple and easy.

 

According to the definition of vacuum degree, it is better to express the vacuum degree by molecular density n, and it is not contradictory to express the vacuum degree by pressure. When pressure is measured, the gas is generally in an equilibrium state and satisfies maxwell's law of velocity distribution, that is, p = nkT is true. The temperature of the gas is constant at the time of measurement, so the pressure of the gas p is proportional to the molecular density n. In other words, the pressure is a measure of molecular density, so the vacuum can be expressed as pressure.

 

In space research, the research object is the movement of the infinite space (1 ~ 10 KMS - 1 or higher) and under the action of complex unsteady, integrated environment atmosphere, the Max speed distribution law and cosine scattering law does not necessarily, also lost the original physical meaning, so the pressure of vacuum measurement is more complex and difficult.

In general, it is popular and used to express vacuum degree by pressure, but it is not the only one. The following parameters can also be used to express vacuum degree:

 

When the vacuum degree is very high, that is, when the molecular density is very small, the statistical fluctuation is very obvious. For example, when the pressure p = 10-12pa, the statistical fluctuation is greater than 5 times 10-2, and the pressure has lost its real significance. Thus, in some cases, pressure is only a relative indicator of other quantities.

 

1. Q&A

 

Vacuum units?

 

The pressure of a gas defined by the collision of a gas molecule on a surface is the rate of time change of the vertical component of momentum of a gas molecule colliding with a unit surface area, that is, the force received per unit area in Pascal or Pa.

1 pa = 1 Nm - 2

In engineering, sometimes the value of pa is too small, kPa and MPa are often used to express pressure. Low vacuum, sometimes expressed as "vacuum percent", such as water ring vacuum pump, reciprocating vacuum pump and straight air roots vacuum pump often expressed in this unit vacuum. When the pressure p > 102Pa, the vacuum percent is delta.

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Where: p0 -- standard atmospheric pressure, Pa

 

Vacuum zoning

 

With a unit for measuring the vacuum degree, the vacuum degree can be quantitatively expressed. However, at present, the pressure range involved in vacuum technology has reached 20 orders of magnitude, in order to facilitate the use, sometimes it is only necessary to roughly point out the general range of vacuum degree, usually qualitatively divided into several regions roughly vacuum. The physical characteristics of gas molecular motion in vacuum state and the effective working range of vacuum pump and vacuum gauge are considered in the basis of the division of regions.

As for the division of vacuum regions in China, terms of vacuum technology (gb3163-1982) stipulates as follows:

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Coarse vacuum, low vacuum and high vacuum are divided according to the average free path of gas molecules compared with the characteristic size d of the container. The main consideration is whether the collision between gas molecules or the collision between gas molecules and the wall of the device plays a decisive role in the occurrence of physical phenomena.

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The boundary between high vacuum and ultra-high vacuum is set as 10-6Pa. The main reason is to consider the vacuum physical adsorption mechanism. Only when the pressure p < 10-6Pa is obvious, can it be obtained by the diffusion pump for extraction, and the pressure is measured by the b-a meter. As for the boundary between ultra-high vacuum and ultra-high vacuum, it is because statistical fluctuation (greater than 5 * 10-2) occurs when p < 10-12 Pa.

 

The above vacuum regions are divided into regions representing a pressure range, for the reason that pressure is still expressed in degrees. However, in a low vacuum, a high vacuum, a very high vacuum and a very high vacuum, the pressure referred to is essentially different, only a relative indication of other quantities.

 

The physical characteristics of vacuum, vacuum pump and vacuum gauge used in each region are detailed in table 1.

 

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With the development of vacuum technology, the division of vacuum regions has also changed.

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