Vacuum leak detection

Vacuum leak detection is the process of detecting the leak location and size of the vacuum system.

Air leakage is the phenomenon in which gas flows from the high pressure side to the low pressure side through leaks or gaps in the system.

False leakage is a physical phenomenon relative to real leakage. This phenomenon is caused by the increase of gas pressure in the vacuum system due to material outgassing, desorption, re-evaporation of condensed gas, infiltration of gas through the wall of the device, and outflow of gas in the dead space in the system.

vacuum system

Air tightness is the performance of the vacuum system wall to prevent gas penetration, which includes gas leakage through leak holes (or gaps) and material leakage.

The minimum detectable leak rate refers to the minimum value of the leak rate that a certain leak detection method can detect.

Optimum sensitivity refers to the smallest leak rate that a leak detection instrument or leak detection method can detect under optimal conditions. For leak detection instruments, the optimum sensitivity is also called instrument sensitivity.

Leak detection sensitivity refers to the minimum leak rate that can be detected by a certain leak detection method under specific conditions. Leak detection sensitivity is also called effective sensitivity.

The reaction time is the time from the implementation of the leak detection method (such as starting to blow the leak gas) to the response time of the indicating method (such as the meter).

Elimination time is the time from the stop of the leak detection method (eg, stop blowing and start pumping out the leaking gas) to the disappearance of the indication of the indication method.

The leak rate is the amount of gas flowing through the leak (including the gap) per unit time.

1. Leaks, leak rates and their units

Due to the tiny size, complex shape and various forms (as shown in Figure 1) of the leak hole referred to in vacuum technology, its size cannot be expressed by geometric dimensions. Therefore, the equivalent conductance or leak rate (referred to as leak rate) is generally used to represent the size of the leak.

When the leak rate is used to indicate the size of the leak, if no special instructions are given, it means that the flow rate per unit time is under the standard conditions that the inlet pressure of the leak is 1.01×105Pa, the outlet pressure is lower than 1.33×103Pa, and the temperature is 296±3K. The amount of air passing through the leak with a dew point temperature lower than 248K.

The unit of leak rate is Pascal × cubic meters per second, recorded as Pam3/s. For convenience, sometimes Pascal × liter/second is used, and it is recorded as PaL/s.

2. Maximum allowable leak rate

Leakage of vacuum system is absolute, and no leakage is relative. The “leakage” referred to in vacuum leak detection technology is related to the concept of the maximum allowable leak rate.

For a dynamic vacuum system, as long as its equilibrium pressure can reach the required degree of vacuum, even if there is a leak, the leak rate of the system can be considered to be allowable. In this case, the leak rate of the system is called the maximum allowable leak rate. . The maximum allowable leak rate qLmax of the dynamic vacuum system should meet:

qLmax≤1/10PwS        (1)

Formula Pw—-System working pressure

S—-Effective pumping speed of the system

For a static vacuum system, it is required to maintain its pressure below the allowable pressure within a certain period of time. At this time, even if there is a leak, the leak rate of the system is considered to be allowable. In this case, the leak rate of the system is called Maximum allowable leak rate. If the pressure of the system with volume V increases from p to pt within time t, the maximum allowable leak rate qLmax should meet:

qLmax≤(pt-p)V/t        (2)

3. Airflow characteristics of the leak

The process of gas flowing through the leak is very complicated, and may include three flow states: viscous flow, transition flow and molecular flow. The dominant flow state is related to the geometry of the leak hole, the type of gas, the pressure at both ends of the leak hole and the ambient temperature. Set the ambient temperature T=296K, the inlet pressure p2=1.01×105Pa, the outlet pressure p1<p2, the length of the leak hole L, and the diameter d of the uniform circular cross-section conduit-type leak, its leakage rate to air and the visible flow state.

4. Classification of leak detection methods

There are many leak detection methods, which can be divided into pressure leak detection method, vacuum leak detection method and other leak detection methods according to the state of the tested parts.

Pressurized leak detection method: The leak-indicating substance of a certain pressure is filled inside the inspected part. If there is a leak on the inspected part, the leak-indicating substance will leak out from the leak hole, and a certain method or instrument is used to detect the outside of the inspected part. The leak-indicating substance leaking from the leak hole can be used to determine the existence, location and leak rate of the leak hole. This is the charging leak detection method.

Vacuum leak detection method: The sensitive element of the tested part and the leak detector is in a vacuum state, and a leak-indicating substance is applied to the outside of the tested part. If there is a leak, the leak-indicating substance will enter the tested part and the sensitive element through the leak hole. The leaking material is detected by the sensitive element, so that the existence of the leak and the size of the leak rate can be determined. This is the vacuum leak detection method.

Other leak detection methods: the tested parts are neither pressurized nor evacuated, or their external pressure is classified into other leak detection methods. Back pressure method is one of the main methods.

The so-called “back pressure leak detection method” is to use the back pressure chamber to first fill the leak-indicating gas into the test piece through the leak hole, and then make the leak-indicating gas leak from the test piece in a vacuum state. A method of detecting the leaked leaking gas by a certain method (or leak detector) to determine the total leak rate of the tested part.

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