Time for Hydrogen Mixture to Migrate from Leak to Ground Surface
How Long it takes for 5/95 to move from the leak through dirt, asphalt and concrete
The time it takes for a 5% hydrogen and 95% nitrogen tracer gas mixture to reach the surface depends primarily on the depth of the pipe and the density of the covering material. While the gas can reach the surface in a few minutes for shallow pipes in loose material, it can take up to several hours or even overnight for deeper pipes under dense covers like concrete.
Estimated Travel Time by Material and Depth
The following table summarizes estimated wait times for hydrogen tracer gas to percolate to the surface:
| Covering Material | Depth / Thickness | Estimated Wait Time |
|---|---|---|
| Dry Sand | 1 meter (~3.3 ft) | 20 minutes |
| Loose/Dry Soil | 2 feet | A few minutes to 30 minutes |
| Wet Sand / Dry Soil | 1 meter (~3.3 ft) | 1 hour |
| Dry Clay / Wet Soil | 1 meter (~3.3 ft) | 4 hours |
| Wet Clay | 1 meter (~3.3 ft) | 12 hours |
| Asphalt | 5 cm (~2 in) | Up to several hours |
| Concrete | 20 cm (~8 in) | Several hours |
| Deep Pipe (All types) | 8 feet | Overnight (suggested) |
Key Factors Influencing Migration Time
- Pipe Depth: Travel time increases significantly with depth. While a pipe at 2 feet may take only 30 minutes to surface, a pipe at 8 feet might require running the gas overnight before detection begins.
- Material Density and Porosity:
- Hydrogen Advantage: Hydrogen is the smallest and lightest molecule, allowing it to move through dirt, asphalt, and concrete more quickly than other gases like helium.
- Concrete and Asphalt: These dense materials typically require a longer waiting period for the gas to percolate through. However, if there are cracks in the concrete, the gas may escape through them more quickly, though this might not be directly above the actual leak.
- Soil Moisture: High soil moisture content is a dominant influence that reduces lateral diffusion but can elevate gas concentrations near the leak point. Wet clay is particularly resistant, requiring up to 12 hours for gas to penetrate 1 meter.
- Surface Conditions: In some cases, a ground cover of asphalt or water-saturated soil can actually cause gas to migrate laterally 3 to 4 times farther than in dry soil, potentially traveling up to 3.5 times faster.
- Pressure: The injection pressure of the tracer gas is a major factor in the leak rate.
Practical Application Tips
- Direct Path: Hydrogen generally moves straight up and typically escapes the ground within a 3-foot radius of the actual leak location.
- Sensitivity: For best practice start with the leak detector set to the highest sensitivity setting. Once on the leak, set to the lowest sensitivity that will allow you to pinpoint the exact leak location.
- Wait and Probe: Once the gas is injected, operators should wait for the percolating period and then walk the path of the pipe, probing the ground every foot to locate the highest concentration of hydrogen