Atmospheric Testing

Atmospheric testing is required before any worker enters a confined space or an excavation where hazardous atmospheres may exist. A calibrated four-gas monitor is the standard instrument, measuring oxygen concentration, lower explosive limit (LEL) for flammable gases, hydrogen sulfide (H2S), and carbon monoxide (CO). Testing must be performed from the top of the excavation downward, as hazardous gases can stratify at different levels.

Hydrovac excavations near gas pipelines, sewer lines, landfills, contaminated sites, and industrial facilities are particularly likely to encounter hazardous atmospheres. Natural gas leaks, sewer gas (containing hydrogen sulfide and methane), and oxygen-depleted conditions can all develop in excavations without visible warning. Even excavations in apparently clean soil can develop hazardous conditions if adjacent underground sources are present.

OSHA requires continuous atmospheric monitoring whenever workers are inside a confined space. The gas monitor should be set to alarm at specific thresholds: oxygen below 19.5 percent or above 23.5 percent, flammable gases above 10 percent of the LEL, hydrogen sulfide above 10 ppm, and carbon monoxide above 25 ppm. If any alarm triggers, workers must immediately evacuate the space.