14
The reading displayed during the STEL measurement is not the instantaneous reading, but rather the
accumulated STEL calculation. This is done so that the user can abort work or don protective
equipment to avoid further exposure if STEL has been reached before 15 minutes have elapsed.
STEL = Average Concentration x
However, the unit still alarms if the instantaneous concentration exceeds the programmed Low or
High alarm limits. STEL measurement can be aborted by pressing the Right key, and the Right key
again when the unit beeps, and the Right Key a third time to return to benzene tube sampling.
CAUTION!
After a STEL measurement, the pump stops to avoid corrosive liquids from the tube being drawn
into the monitor. The monitor beeps once per second to notify the user that STEL is complete. Press
the Right Key to stop the alarm and a warning appears to remove the tube. Use the Left key to
escape back to Continuous Benzene Screening.
Benzene Tube Replacement
In general it is good practice to replace the benzene filtering tube after each measurement. When used
in a high VOC background, and especially after a STEL measurement, it is likely that the capacity of
the tube to remove interfering chemicals is used up. If the tube has turned from a bright orange to a
pale orange color, or a greenish-brown stain is seen for more than 2/3 of the tube length, the capacity
of the tube is exhausted and a new tube should be used for the next measurement. The image below
shows examples of new versus used tubes. The green-brown-blue colors are formed by reactions
with VOCs while the pale orange color is caused by absorption of humidity, which in turn reduces
the ability of the tube to absorb VOCs. Similarly, do not crack open tubes until immediately before
use, because ambient humidity diffusing in will reduce the capacity for VOCs. Finally, if a tube has
registered a benzene reading but does not show a discoloration due to other VOCs or humidity, it
should still be replaced because residual benzene can carry over to the next reading. An exception to
the tube replacement recommendation can be made for calibration, in that the same tube can be used
for zeroing and subsequent span calibration, because the initial zeroing does not bring any benzene,
other VOCs, or significant amounts of water vapor into the tube.
Time Elapsed
15 min.
New
Used
