Teledyne 2240 - Portable Handheld Hydrogen Leak Detector, 3rd generation (revision 2/29/08) - Manual (Page 33)

Handheld Hydrogen Leak Detector Hydrogen Sensing
Teledyne Analytical Instruments 23
Hydrogen Sensing Considerations
From any given source, hydrogen gas disperses rapidly and
generally upward due to the very low density of hydrogen compared to
air. Understanding this behavior allows the operator to more effectively
search for hydrogen leaks.
If the sensor element is near (and above) the leak, the concentration
will likely be higher but the leak may be difficult to locate. As hydrogen
dissipates (generally above the leak) the concentration decreases.
Generally, greater distances will increase the chance of intercepting the
leak stream, but if the sensor is too far away, the response may be too
weak to detect.
When drafts or air currents are present, hydrogen will tend to be
dispersed. Testing for hydrogen leaks downwind of the leak area may
increase the chance of detecting the leak.
If hydrogen is rising in an enclosed building (where there may be a
layer of hot air near the ceiling) the hot air near the ceiling may have a
lower density and act to retard the hydrogen from continuing to rise as
rapidly as it did in lower layers of air. Thus, sensing hydrogen near
ceiling areas with high temperatures present may not be as effective.
Low temperatures can also affect the behavior of hydrogen.
Hydrogen stored in a liquid state is at an extremely low temperature.
The low temperature of any escaping hydrogen will be of a higher than
normal density and may initially move downward. As the hydrogen
warms, it will begin to rise upward. When checking for a leak in areas
where liquid hydrogen is stored, check both above and below the area of
concern.