As explained in Helium Consumption Reduction there are a couple of key factors that make helium an ideal tracer gas for leak testing, namely its low concentration in the environment and its small atomic mass. However, due to helium shortages and consequent expense, we have also discussed ways of reducing helium consumption. In this article we will examine alternate leak test methods that might be considered to replace helium.
What are my Leak Testing Options?
The following table compares general leak testing categories to the helium leak testing method. The comparison is for typical automated chamber or global leak testing applications, not for manual sniffing.
Table 1. Comparing Leak Testing Methods
|Helium Tracer Gas Hard Vacuum Method||Air Test Methods||Air Under Water|
|Other Tracer Gas Methods|
|Vacuum Level Required (Typical)||Below 0.1 Torr||N/A||N/A||Atm to below 0.001 Torr|
|Sensor Technology||Magnetic Sector Mass Spectrometer||Pressure Transducer, Mass Flow Meter||N/A||Mass Spectrometer, other|
|1 x 10-9 atmcc/sec||1 x 10-3 atmcc/sec||1 x 10-4atmcc/sec||1 x 10-6 atmcc/sec (tracer gas dependent)|
|Sensitivity to Product Contamination||Medium||Low-Medium||Low||High|
|Measurement Technique (Typical)||Dynamic||Rate of rise, Direct Flow||Visual Inspection||Dynamic, Accumulation|
|Test Volume Dependency||Low||High||Low||Low-Medium|
|Relative Cycle Time|
(for a nominal 1×10-4 leak limit)
Air and Air Under Water (Bubble Test) Methods
While a thorough understanding of the application is necessary to make an evaluation and recommendation for alternate test methods, some general conclusions can be made regarding alternatives for helium leak testing. As shown in the above table, air test methods are, in most cases, not candidates to replace helium primarily because they are not as sensitive. While air under water (bubble testing) methods may have the necessary sensitivity in non-demanding applications, it is not often considered as an alternate due to its high operator dependence: Your bubble counting guy has to be sharp-eyed and not easily distracted.
Use of Alternate Tracer Gases
Attention has turned to hydrogen and argon as potential substitutes for helium. In considering alternate tracer gases it is helpful to consider the following characteristics:
- Natural Ambient Concentration
- Environmental Concerns
- Atomic Mass
- Gas Viscosity
- Detect ability (sensors or instruments available, and detection limits)
- Suitability of Sensor (rugged for production environments)
- Applicable Test Methods (influenced by sensors)
Argon vs. Helium
Let’s first consider Argon as an alternate and assume the test method will be with a mass spectrometer using the chamber hard vacuum method similar to what is used in the helium hard vacuum method. Here are a few characteristics of Argon compared to helium:
- Natural Ambient Concentration: 9,340 ppm (about 1,800 times higher than helium)
- Atomic Mass: 40 (10 times larger than helium, about 1/3 of the flow rate of helium in molecular flow through a given leak)
- Gas Viscosity: 0.00021 Poise (about 10% more viscous than helium, resulting in a 10% lower flow rate in viscous flow compared to helium)
- Mass Spectrometer Detection Peaks: 40 and 20 mass to charge ratio (compared to helium who’s peak is at 4)
One significant advantage to helium is that when detected using a mass spectrometer, its signal is isolated at a mass to charge ratio of 4, which is 50% away from the next species of hydrogen and far away from other species up the scale. This means that there is very little interference of stray gas species with helium. In contrast, Argon’s primary peak at mass 40 is potentially near, or even shares its peak with other gas fragments, including hydrocarbon species. In order to prevent interference of the potential interfering peaks the cleanliness of the test part and the system is paramount.
The Challenges of Using Argon
Another significant factor is the fact that Argon has a much higher ambient concentration compared to helium. The following calculations compare a typical helium leak rate and the required vacuum levels to achieve a low background noise of the tracer gas (due to atmospheric concentrations). The calculations also account for the differences between the gases’ atomic masses and the impact on the gas flow through a molecular flow leak.
- Helium Leak Rate: 1.2 x 10-7 atmcc/sec at 100 psig internal fill pressure
- Vacuum Level Required for Helium: 6.1 x 10-3 Torr*
- Flow Regime: Molecular to Transitional (assumed molecular for the calculations)
- Theoretical Leak Geometry: 0.55 um diameter x 1 mm long hole
- Equivalent Argon Leak Rate: 3.9 x 10-8 atmcc/sec (reduced due to molecular flow differences)
- Vacuum Level Required for Argon: 1.1 x 10-6 Torr*
* Vacuum level to get 5 ppm helium and 9,340 ppm argon to a background level equal to the leak rate.
As shown in the data above, achieving an equivalent leak rate sensitivity to helium without any special argon purging or electronic zeroing would require over 5000 times deeper vacuum in the test chamber for argon. This results in a challenging production environment that would require deeper vacuum levels and longer test cycle times compared to helium. While one manufacturer claims a leak rate sensitivity of 1 x 10-8 atmcc/sec in published literature, such are likely not to be achieved outside of a laboratory. Practical test sensitivities in actual production environments will be closer to -6 or 10-5 atmcc/sec. When considering argon, buyers must be cautioned to the practical test sensitivities of their production environments.
In our next article we will look at hydrogen as a tracer gas.
For more information on leak detection, visit www.lacotech.com to see a variety leak detection instruments that are compatible with your application needs.