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Description page-8-1-31
Operation page-8-1-31
Response Time page-8-1-32
Alarm Function page-8-1-32
Battery Power Supply page-8-1-32
Flow Verification page-8-1-32
Power Down page-8-1-32
Equipment
Thermal Conductivity Gas Analyzers
8-1-2 Treatment Manual 06/2004-01
PPQ
Thermal Conductivity Gas Analyzers
Fumiscope and Gow-Mac
The thermal conductivity gas analyzer (T/C units) is a scientific
instrument specifically designed for determining the concentration of
gases within a chamber or other enclosure while the actual fumigation
is being conducted. These fumigation gases include methyl bromide,
ethylene oxide-carbon dioxide mixtures, and sulfuryl fluoride. The
following discussions relating to the T/C unit are under these
subheadings:
◆ Description
◆ Standardizing the instrument
◆ Operational procedures
◆ Repair and calibration
◆ Maintenance
Description
The Fumiscope® or Gow-Mac® is light in weight, portable, completely
contained in a compact metal cabinet. It contains a thermal
conductivity cell, scale, gas pump, range switch, and gas flow meter. A
gas drying tube is also included. For large enclosures, an auxiliary
pump may be needed.
Inlet This tube connector is the gas inlet for the instrument. The sampling
tubes are connected directly to the inlet or through the drying tube.
Flow Rate Meter Indicates the gas flow rate in “simulated cubic feet per hour (SCFH).”
Note: The flow rate should always be read at the middle of the ball.
Flow Rate
Adjustment
This dial controls the air or gas flow rate by adjusting the pump. After
connecting to the gas sampling tube, the flow rate should be adjusted
upward until it reads exactly 1.0. The gas concentration reading
should be taken only after the meter that registers “ounces per
thousand cubic feet” stabilizes, which may take a minute or more
(depending upon the length of the tubing and whether or not an
auxiliary pump is being used).
Scale Indicates the concentration of the MB fumigant in ounces per 1,000
cubic feet (milligrams per liter or grams per cubic meter). For some
fumigants, the concentration is calculated as the reading times a
specific factor.
Equipment
Fumiscope and Gow-Mac
06/2004-01 Treatment Manual 8-1-3
PPQ
Zero Adjustment This dial controls the scale needle which is brought to zero as an air
sample is being drawn through the instrument.
Line Switches Control electrical supply to pump and scale.
Range Switch Regulates the scale indicating the concentrations of fumigant
measured, for example, 0-100 ounces per 1,000 cubic foot or as
0-400 ounces per 1,000 cubic foot (some models). Digital models can
indicate a range from 0-999 ounces per 1,000 cubic feet.)
Exhaust Outlet Always connect a tube to exhaust outlet to carry exhaust gas away
from the instrument and operator. When using the T/C unit in
confined or poorly ventilated areas, recirculate the exhaust gas back to
the fumigation space or exhaust it to the outside.
Drying Tube The drying tube (filter tube) is for use with a prepared chemical for
removal of certain contaminant gases or vapors which interfere with
correct readings of fumigant concentration. For most fumigations, the
tube will contain a desiccant such as Drierite® (granules of anhydrous
calcium sulfate), or Ascarite® (soda asbestos). Both are available from
scientific supply houses. The tube is inserted in the gas sampling line
just before the inlet connection. Drierite®, blue in color when dry,
turns pink when moisture is absorbed. When most of the desiccant
has turned pink, it should be replaced. In extremely high moisture
conditions, two tubes can be connected in tandem. Drying tube
openings should be closed when not in use.
When a drying tube is used, a thin layer of glass wool or aquarium
filter wool should be placed at the bottom and top of the tube to
prevent small particles from sifting into the Fumiscope®. The use of
absorbent cotton or similar materials is not recommended. Cotton
tends to pick up moisture and to become matted, and once matted,
the cotton may restrict normal air flow, thus adversely affecting the T/
C unit’s operation. (Matting may also cause the flow of air to bypass,
rather than flow through the Drierite®.)
Mount the drying tube vertically so that the gas mixture moves
through the drying material and does not pass over the top. This
happens when the tube is mounted horizontally (lengthwise).
Commodities that are actively respiring produce carbon dioxide gas
that may interfere with the correct readings of fumigant concentration.
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