Q&A on vacuum gauge technical knowledge

- May 31, 2019-

Q&A on vacuum gauge technical knowledge


1. What are the characteristics of maclehose vacuum gauge?

A: macleod vacuum gauge is an absolute vacuum gauge, which can give the absolute value of the measured pressure. But the McGuire can only measure the partial pressure of the permanent gas in the measured gas, not the pressure of the condensable gas.


2. How does a heat conduction vacuum gauge work? By what measure is the change in heat conduction reflected?

A: the heat conduction vacuum gauge is based on the phenomenon that the heat conduction of a low-pressure gas is related to pressure. It can reflect the change of temperature through the change of the resistance of the hot wire, so as to reflect the change of the heat conduction. The thermocouple vacuum gauge can also be used to measure the temperature change of the hot wire directly to reflect the change of heat conduction.


3. What is the reason for affecting the lower limit of heat conduction vacuum gauge?

A: in addition to the heat transfer of gas, the heat transfer of the hot wire can also be along the hot wire itself to both ends, that is, the solid heat transfer; It is also possible to conduct heat in the form of radiation, which have nothing to do with pressure. When the pressure is low and the heat transfer of gas is obviously lower than that of solid and radiation, the change of pressure cannot be reflected, that is, the lower limit of measurement is reached.


4. Why do heat conduction vacuum gauges age easily?

Answer: because the hot wire works in high temperature for a long time, it is easy to be oxidized and become fine under the action of gas, or lead to the change of the crystal structure of the hot wire itself, so the resistance of the hot wire will be changed and the zero point will drift, which belongs to the hot wire itself. Due to the adsorption of gas on the hot wire surface, the surface adaptation coefficient, i.e. the mechanism of heat exchange, is changed, resulting in the change of sensitivity. All of these are signs of aging.


5. How does an ionization vacuum gauge work?

A: the impact of electrons on a gas produces ionization, which is related to the pressure of the gas. When the electron current is constant, the ion stream formed by ionization is proportional to the pressure, so the pressure of the gas is measured indirectly through the ion stream.


6. What is the external control connection of the ionization vacuum gauge? What are the benefits?

A: if the regulated gate serves as the electron accelerator and the plate as the ion collector, the connection is external control. The advantage is that the electrons are not easily intercepted by the gate wire in the acceleration, and are repulsion by the potential of the filament and the plate, and can repeatedly oscillate back and forth through the grid in the space between the filament and the plate, so as to significantly improve the sensitivity of the measurement.


7. What are the reasons affecting the lower limit of the hot cathode ionization vacuum gauge? How to improve?

Answer: electronic hit the soft X-ray radiation collector, the ion collector absorbs the soft X-ray produce photoelectrons emission, the resulting electronic current is just the same as that of ion current, so the stack together, but the current has nothing to do with pressure, when pressure is reduced to the formation of the ion current are much smaller than the photocurrent, collect ion flow of pole changing with pressure, namely to measure the lower limit. The lower limit of measurement can be extended by increasing ion current and decreasing photocurrent.


8. What is the meaning of vacuum leak detection?

Answer: the ultimate vacuum of the vacuum system depends on the pumping speed and air leakage rate of the system. Therefore, when the pumping speed is fixed, the air leakage rate determines the vacuum degree. In order to make the system reach the predetermined ultimate vacuum, the air leakage rate of the system must be controlled within the allowable leakage rate through leak detection and plugging.


9. In principle, what are the two major categories of vacuum leak detection?

Answer: can be divided into: 1) pressure leak detection method, including (1) gas leak detection method, (2) halogen leak detection method, (3) ammonia leak detection method. 2) vacuum leak detection method, including (1) high frequency spark leak detection method, (2) vacuum meter leak detection method, (3) mass spectrometer leak detection method.


10. Describe the pumping principle of vacuum pump.

A: 1) use compression to remove gas from the pump. A. mechanical periodic expansion compression, b. viscous traction compression of steam flow, c. traction compression of rigid surfaces. 2) use adsorption to bind the gas in the pump. [a] ionization aspiration and [b] physical and chemical adsorption.