Calibration method of Vacuum Leak

In the leak detection work, to quantitatively determine the leak rate of the leak detection hole, it is necessary to correctly transfer the value of the leak rate from the gas microflow standard or the leak rate standard to the mass spectrometer leak detector. In this transfer process, the calibration of the reference leak is a very important link. Based on the relevant standards and literatures at home and abroad, the common calibration methods of vacuum leaks – constant volume and variable pressure method, constant pressure and variable capacity method, variable volume and variable pressure method and comparison method are respectively analyzed and introduced.

In the leak detection work, it is more practical to quantitatively determine the leak rate of the leak detection hole. Quantitatively determining the leak rate of the tested leak requires three basic conditions: (1) a reference leak with a stable leak rate and reliable value; (2) correct calibration of the leak detector with the reference leak; (3) detection The leak detection conditions of the leak detector are the same as the calibration conditions.

According to the pressure state of both ends when the reference leak is used, it can be divided into two types: vacuum leak and positive pressure leak. Vacuum leaks are calibrated and used when the population pressure is one atmosphere and the outlet pressure is less than 1kPa. Positive pressure leaks are leaks that are used when the population pressure is greater than one atmosphere and the outlet pressure is one atmosphere. Due to the different operating conditions of vacuum leaks and positive pressure leaks, their calibration methods are not exactly the same. For the calibration of vacuum leaks in the world, calibration devices with various measurement principles have been developed, and comparative studies have been carried out among different standard devices. The calibration of positive pressure leaks was limited by the poor quantitativeness of positive pressure leak detection and the difficult to meet the calibration conditions of positive pressure leaks. Application and popularization, the calibration of positive pressure leaks has begun to attract attention. This article mainly introduces the calibration method of vacuum leak, and the calibration method of positive pressure leak will be introduced in another article.

Calibration method of vacuum leak

 Constant pressure varactor method:

 The gas flowing from the vacuum leak is introduced into a container. Under the condition of constant temperature and pressure, the leak rate of the vacuum leak can be measured by measuring the volume change rate △V/△t of the gas in the container, and the pressure P temperature T is obtained.

QL=(p/RT)(△V/△t) (2)

If during the measurement process, the gas temperature in the container changes by ΔT and the pressure changes in elevation; p9T, ΔV and Δt are δp, δT, δ(ΔV), δ(Δt) respectively, then the leak rate of the reference leak hole The measurement error δQL is [5]

In formula (3), the error caused by temperature change ΔT and pressure change Δp is the largest, so strict constant temperature and constant pressure are required. To reduce this error, the volume change ΔV should be increased as much as possible and the initial volume V should be reduced. However, increasing △V will prolong the measurement time and increase the difficulty of constant temperature and constant pressure. Currently, the lower limit that this method can measure is 10 -12 mol/s.

The lower limit factor of the constant pressure variable volume method is the suction and discharge of the system, and its working pressure cannot be lower than 1Pa.

Figure 1 shows the schematic diagram of a gas microflow standard device developed by the Lanzhou Institute of Physics (LIP) in China. The calibration chamber is a variable volume chamber made of bellows, and the outside of the bellows is a sealed oil chamber. Using the characteristic that the volume of hydraulic oil changes very little with pressure, the volume of the variable volume chamber is changed by controlling the volume of the piston in the oil chamber. The differential pressure capacitive film gauge is used to measure the pressure difference between the varactor chamber and the reference chamber. During the measurement process, the gas pressure in the reference chamber is adjusted between 10 and 10^3Pa. The measurement and control system uses a slow-speed constant-torque motor to drive the piston to move in the closed oil chamber, and continuously increases the volume of the variable volume chamber to make the variable volume chamber The pressure difference between the reference chamber and the reference chamber is maintained near zero, and the gas pressure in the variable volume chamber is constant. The product of the cross-sectional area of  the piston and the moving speed is equal to the rate of change of the gas volume in the variable volume chamber. The variable volume chamber is a bellows structure with good sealing performance. It can calibrate reference leaks in the range of 10^-7～10^-11 mol/s with a standard uncertainty of less than 2% [6～8]

Sandia National Laboratory (SNL) has developed a piston displacement vacuum leak calibration system. It uses a stepping motor to drive the piston to move in the variable volume chamber, continuously changes the volume of the variable volume chamber, and maintains a constant pressure in the variable volume chamber. Since the “O” ring is used to seal between the piston and the variable volume chamber, the leakage and deflation are relatively serious. This system can calibrate vacuum leaks in the range of 10^-6 to 10^-11 mol/s.

In formula (3), OP refers to the pressure change at two measurement points, as long as the pressure change does not change the leak rate of the vacuum leak between these two measurements. SNL has also developed a leak calibration device (see Figure 2), which adopts the method of increasing the fixed volume, that is, opening the valve, 1 or valve 2 or valve 3 at the time of t1, so that the pressure P2 in V1 decreases, and as the gas passes through The leakage hole flows into V1, and until P2 at time t2 returns to the pressure before time t1, both ΔV and Δt can be determined. This device has a simple structure and is easy to operate, but it is only an approximate constant pressure varactor method, and the constant pressure effect is poor. SNL can use it to calibrate vacuum leaks in the range of 5 × 10^-10 to 5 × 10^-8 mol/s with an uncertainty of 5%.

Figure 3 shows a schematic diagram of a glass capillary variable volume chamber, the capillary provides a gas source for the vacuum leak, and the liquid column in it plays the role of sealing, variable volume, constant pressure, and marking volume changes. This method is simple and can be used to calibrate reference leaks with leak rates greater than 5 × 10^-10 mol/s, but has its own drawbacks. First, the capillary should not be too thick (diameter less than 1 mm; secondly, the inner diameter of the capillary should be uniform, otherwise the error will be too large; also, the infiltration of the liquid and the tube wall will bring greater errors to the measurement, changes in ambient atmospheric pressure, liquid The liquid volume change caused by the evaporation and temperature change will bring certain errors to the leak rate measurement. In addition, the liquid vapor will flow back to the calibrated leak along the capillary, which may easily cause the leak to be blocked or corroded.

The vacuum leak of the helium mass spectrometer leak detector of Shanghai EVP Vacuum Technology Co., Ltd. is detected by the China National Defense Science and Technology Industry Vacuum First-class Metrology Station. The method is relatively accurate. The calibrated leak rate obtained by the vacuum leak detection is Q =10^-8Pam3s^-1.

For more information, please feel free to contact the EVP team.