Chapter 6: Maintaining the Ductless Enclosure
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inserted between the stacked filters to sample the lower filter or inserted
through the holes in the blower exhaust cover to sample both filters. Connect
the syringe to the detector tube while the system is running and pull the air
through the tube with the syringe. Each stroke of the syringe represents a
100-ml sample and corresponds to the number of strokes necessary to give
the indicated color changes. Due to the wide variety of organics and varying
exposure limit/TWA’s, it is recommended that specific detector tubes be
purchased directly from Sensidyne, Dräger or your laboratory supply dealer.
Alternate detector pumps can also be purchased from your laboratory supply
dealers.
The vast majority of detector tubes available start measuring at the exposure
limit/TWA and the concentration value measured is not important. When a
user observes any color change in the tube, they should replace the filter
immediately. If no detector tube for your specific chemical is available, other
means of detection must be used.
Time
- For applications that have very consistent inlet concentrations and
operating time, “Time” can be used to anticipate saturation or exposure
limit/TWA levels based on prior experience. However, this does not replace
the need for sampling. Consult Labconco Specialist for an estimate of carbon
filter life based on chemical usage. Refer to
Chapter 3
on how to “Set the
Filter Life and Filter Check Timers.” Detector tubes, or analytical
instrumentation should
always
be used to determine concentrations in the
carbon filter. It is recommended that the carbon filters be checked with
detector tubes or other means at intervals of 20% of the total estimated filter
life. The exception to the 20% recommendation is formaldehyde and any
carcinogen or suspected carcinogen. These more hazardous chemicals must
be checked at least every 10% of the total estimated time.
Analytical Instrumentation
- This is the most accurate means of measuring
concentrations of any chemical. It is the method of choice when no detector
tubes are available or the tubes are not sensitive enough to measure at the
exposure limit/TWA concentration for the chemical. This method is also to
be used to determine saturation when the chemical concentration is below the
measurement range of detector tubes. One relatively low cost method of
analytical instrumentation includes the use of a PID (photoionization
detector) that measures gas concentrations and is available from RAE
Systems or PID Analyzers, LLC.
Determination of When to Replace HEPA Filters
The HEPA filters in ductless enclosures gradually accumulate airborne
particulate matter and powders from the enclosure and room. The rate of
accumulation will depend upon the cleanliness of the room air, the amount of
time the enclosure is operating, and the nature of work being done in the
enclosure. Replace HEPA filters if the face velocity drops below the
recommended velocity. Replace HEPA filters if it fails the HEPA Filter Leak
Test in
Chapter 6
.
