Where False Positives Come From
Most trace gas false positives are not sensor malfunctions. They are setup and technique failures. The gas found something real - just not what the technician thought it was finding.
Understanding the most common sources of false readings is the first step toward avoiding them.
Gas escaping at the equipment pad
If the line is not properly isolated at the equipment end - a loose plug, a valve that is not fully closed, or gas backing up through the pump - the gas may escape at the pad rather than travel down the line to the leak point. The technician then picks up a reading near the equipment and mistakes it for the underground leak location.
Carrying gas between scan positions
If the sensor picks up a reading at one location and the technician moves quickly to the next position, residual gas on or around the sensor can register as a reading at the new position. The sensor needs adequate time to clear - following manufacturer guidance on purge time - before the next position reading is treated as meaningful.
Wind and surface ventilation
Wind can carry gas horizontally across the surface before it registers on the sensor, making the reading appear at a different location than the emission point. Scanning in windy conditions requires extra caution and comparison with multiple passes.
Old pipe trenches and utility corridors
Gas from a leak may travel along a historic pipe trench, a utility corridor, or a void and surface somewhere other than above the active leak. If the technician is not aware of other buried utilities or old plumbing paths, a reading in that area can be misinterpreted.
Moving too fast
Scanning too quickly across a surface does not allow the sensor to settle at each position. The technician misses the gradual rise and fall of signal strength that characterizes a real migration pattern and instead chases individual spikes.
The Pre-Scan Protocol
- Check sensor function before the job - confirm it responds to a known trace gas source and reads clean air as zero or baseline
- Follow manufacturer instructions for warm-up time, sensitivity settings, and purge procedures
- Confirm the correct line is isolated at both ends before introducing gas
- Allow adequate time for gas to migrate after introduction - do not start scanning immediately
- Start in clean air away from the suspected area to confirm baseline before approaching the scan zone
During the Scan
- Move slowly - the pattern of reading rise and fall as you cross the area is more reliable than any single peak
- Allow the sensor to clear between positions per manufacturer guidance
- Mark readings systematically rather than chasing single spikes
- Compare the reading pattern against the known plumbing route - does it make sense?
- If the strongest reading is in an unexpected location, cross-check against pressure test results and acoustic data before marking
Document what you did. Record the line tested, gas used, pressure applied, migration time allowed, scan pattern, reading locations, and any conditions that may have affected the result. That documentation is what separates a defensible finding from an educated guess. If the excavation finds the pipe failure near where the reading was strongest - that is a confirmed result. If it does not - the documentation shows what conditions existed and why the reading may have been offset.
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