[Leak Detection] The Influence of Different Gases and Pressures on Vacuum Leak Rates

Currently, most standard leak standards specify their leakage rates under helium gas conditions at an inlet pressure of 100 kPa. In this study, a constant-volume变压法 was employed to calibrate a standard vacuum leak standard with a rated leakage of 2.3 × 10⁻⁶ Pa·m³/s, measuring its leakage rates for three gases—hydrogen (H₂), helium (He), and deuterium (D₂)—at various inlet pressures. Prior to the measurements, the system was preheated to remove residual gases, allowing us to determine the system's background leakage rate. We also investigated how this background leakage might affect the accuracy of the calibrated leak standard. Furthermore, by integrating insights from both viscous flow and molecular flow theories, we examined how different gases and varying inlet pressures influence the leakage characteristics of the standard leak.

In recent years, vacuum leak holes have been increasingly applied in fields such as aerospace, the electronics industry, the power industry, and refrigeration technology. A vacuum leak hole is a device designed to provide a stable gas flow into the vacuum side (where the outlet pressure is below 1000 Pa). Currently, numerous metrology laboratories and research institutions—both domestically and internationally—have established corresponding calibration facilities. Commonly used methods for leak rate calibration include the constant-volume method, constant-pressure method, mass spectrometry comparison method, fixed conductance method, and分流法 (split-flow method), among others. Among these, the constant-volume变压 method is particularly suitable for calibrating leak holes with larger leak rates, as it features simple equipment and operation while delivering high calibration accuracy.

Currently, the leak rates of most standard vacuum leak standards are calibrated using helium at an inlet pressure of 1 kPa. However, in practical applications, it is often necessary to calibrate vacuum leak standards under conditions involving gases other than helium and pressures exceeding one atmosphere. Both the type of gas and the pressure can significantly influence the leak rate of a leak standard. Therefore, to achieve more accurate measurements of leak rates, it is essential to investigate how these factors—gas type and pressure—affect the performance of vacuum leak standards. In this study, we calibrated the leak rate of the same vacuum leak standard by varying both the type of gas and the pressure at its inlet end, and conducted a preliminary analysis of how these variables impact the overall leak rate.

Leak Rate Calibration Mechanism

By definition, the leak rate Q is the total differential of the gas quantity pV with respect to time t, which means

This is the constant-volume method, in which gas leaking from a vacuum leak port is introduced into a closed system. By measuring the rate of pressure change within the system—specifically, the time derivative of pressure \( \frac{dp}{dt \)—under conditions where both the system's temperature and volume remain constant, we can determine the leak rate \( Q \) of the port. When calibrating leak rates using the constant-volume method, factors such as temperature fluctuations and the size of the constant-volume chamber can significantly influence the calibration system. For instance, connecting the vacuum leak port may cause changes in the system's volume, leading to inaccuracies in the measured volume. To minimize these effects, the system temperature must be maintained at 296 K to avoid unnecessary temperature corrections. Additionally, the system volume should not be too small—ideally, it should exceed \( 2 \times 10^{-3} \) cubic meters. ³ m ³。

Additionally, when measuring the leak rate of a leakage hole using the constant-volume method, it's also necessary to account for the system's inherent outgassing—specifically, the influence of the system's background leak rate. Figure 1 illustrates the relationship between leak rate and time. Here, Line 1 represents the system's intrinsic outgassing, which causes the background pressure to change over time, leading to an increase in the system's baseline pressure. This phenomenon is influenced by factors such as system degassing, permeation through the system's pipe walls, and unknown leaks. Line 2 depicts the ideal pressure-time behavior of the system, where the product of the slope dpL/dt (the rate of change in pressure) and the system volume V directly corresponds to the leak rate of the hole. Finally, Line 3 shows the actual pressure-time relationship observed during the experiment, which is essentially the superposition of Line 1 and Line 2.

Figure 1: Relationship Between Pressure and Time

Considering the influence of system background outgassing, the leak rate of the leakage hole can be calculated as:

When the start times are the same, the true leak rate of the leak hole can be expressed as the difference between the leakage rate within that time period and the background leakage rate.

Calibration Device and Calibration Method

2.1、Calibration Device

The standard leak-orifice calibration device is shown in Figure 2. This calibration setup primarily consists of an air source, the leak orifice being calibrated, a standard volume, a fixed-volume chamber, a thin-film vacuum gauge, a composite vacuum meter, and a pumping system.

Figure 2: Schematic Diagram of the Structure Principle for Constant-Volume Method Standard Leak Calibration Device

Conclusion

(1) After degassing the system by heating and then cooling it down to 23°C, the system is sealed and left undisturbed under a vacuum of 10⁻⁶ Pa. The intrinsic background leakage rate caused by the system's own outgassing is 1.15 × 10⁻⁸ Pa·m³/s. Under these operating conditions, when calibrating for leak rates greater than 10⁻⁶ Pa·m³/s, the influence of the background leakage can be disregarded. However, when the leak rate of the calibrated leak hole falls below 10⁻⁷ Pa·m³/s, the impact of the system's outgassing cannot be ignored.

(2) Calibration results for a standard vacuum leak with a leakage rate of 2.3 × 10⁻⁶ Pa·m³/s, conducted within a pressure range of 100 to 400 kPa, show that the leak rate exhibits a linear relationship with the square of the pressure for all three filling gases—H₂, He, and D₂—indicating that the gas flow through the leak remains in the viscous flow regime.

(3) After comparing the leak rates of pores filled with different gases under the same inlet pressure, it was found that the pore leak rate is inversely proportional to the gas viscosity. At the same pressure, He has the highest viscosity coefficient, resulting in poor gas mobility and the lowest leak rate through the pores. In contrast, H2 exhibits the lowest viscosity coefficient, leading to the highest leak rate—indicating that the gas flow through the pores is in the viscous regime.