Consider toxic gas density when positioning a gas monitor
Answered by Werner R. Haag, Ph.D., senior applications chemist, RAE Systems Inc., San Jose, CA.
When selecting a portable gas meter to measure toxic vapors, many people ask whether the gas they are measuring is "heavier" or "lighter" than air. The assumption is that a "heavier" gas tends to sink to the floor; therefore, the floor is the place with the greatest exposure hazard and the best place to monitor. Conversely, "lighter" gases tend to rise and accumulate near the ceiling. This view may be useful when measuring for explosion hazards of combustible vapors and gases, but it is not correct for measuring toxic gases near their 8-hour exposure limits, which are typically in the parts-per-million range.
To understand the difference, consider what happens when pouring honey into tea. The pure honey first sinks to the bottom of the glass and then gradually distributes into the liquid. Stirring accelerates this process. Once diluted, the honey stays distributed throughout the liquid and all the tea tastes sweet. Pure honey temporarily forms a layer at the bottom; diluted honey remains uniformly distributed.
It is the same with gases and vapors. If a propane tank leaks, the gas can accumulate near the floor because it is more dense than air. If a natural gas pipe leaks, the methane can accumulate at the ceiling because it is less dense than air. However, once the leak is stopped and the air is mixed, the gases distribute uniformly throughout the room. If the gas source is small or leaking slowly, resulting in ppm levels of contaminants, the air never becomes stratified in the first place. This is understandable, considering that even a high level of a toxic gas at 100 ppm is only 0.01 percent by volume. Thus, the gas is still 99.99 percent air and has essentially the same density. Gases tend to diffuse and mix quickly, so even if the gas starts out stratified, it cannot stay stratified for a long time in a small, confined space.
With explosive gas measurements using combustible gas (LEL) or oxygen sensors, we are more concerned with much higher concentrations (in the volume percent range) than we are with toxic gas meters such as photoionization detectors or electrochemical sensors that measure in the ppm range. Therefore, with LEL and O2 measurements, the gas can be more stratified and it is important to measure at various levels before entering a confined space. It is important to sample at a distance from the opening because air intrusion near the entrance can give a false sense of adequate oxygen presence. But low-level contaminants such as carbon monoxide, hydrogen sulfide, chlorine and volatile organic compounds are likely to be detected evenly throughout the space.