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wiring length should be avoided. When T/C unit readings in multiple
locations are necessary, care should be taken to see that each location
is the best available.
The T/C unit should be supported on a sturdy, level surface, outside
the traffic pattern and protected from wind, rain, excessive cold, and
sun in hot weather. Temporary shelter such as a tarpaulin cover may
be adequate in some cases. The gas concentration readings indicated
by the T/C unit may be inaccurate unless the unit is placed in an area
that is approximately the same temperature as the gas mixture in the
enclosure being fumigated. Temperature differential may cause
moisture to condense inside the gas sampling line.
Most T/C units operate on 110-120 volts alternating current (AC). T/C
units operating on 210-220 volts AC on DC are available for overseas
or other assignments as necessary. A converter is required to use
direct current. To reduce the possibility of electric shock, T/C units
repaired at the Center for Plant Health Science & Technology (CPHST)
have been converted from the standard two-prong plug to a polarized
Equipment
Fumiscope and Gow-Mac
8-1-6 Treatment Manual 06/2004-01
PPQ
plug. Extension wiring and gas sampling line length should be kept to
a practical minimum and should be raised above floor level when
feasible.
Measuring Gas
Concentrations
With the
Standardized
Unit
As a protection for the cell and the pump of T/C units, a drying tube
should be used at all times.
In making gas concentration checks, the unit is first warmed up for 15
to 30 minutes depending on ambient temperatures. The pump is then
turned on and the gas flow meter adjusted to a 1 cubic foot per hour
flow. Should the scale needle seem unstable and wander for a short
period of time, the flow rate may be reduced slightly below the 1 cubic
foot per hour level for zeroing and maintained at that level for
measuring. The unit is now ready to measure gas samples drawn
through position tagged tubes from the area being treated. The needle
will indicate gas concentrations in ounces per 1,000 cubic feet (grams
per cubic meter).
Sufficient time to draw a true sample must be allowed. With 150 to
200 feet of 1/4 inch OD tubing and a temperature of 70°F, this will be
approximately 7 minutes. Stations equipped with small, auxiliary
pumps can draw a sample through the same length of tubing in 12 to
15 seconds.
Readings should be constant for at least 30 seconds before the line is
disconnected. As each sampling tube is disconnected from the T/C set
and the needle begins to recede toward zero, a new sampling tube may
be attached. It is not necessary to wait for the needle to return to zero
after each reading.
Re-zeroing the instrument is generally not needed during the course of
a fumigation, except under the following circumstances:
◆ The pump has been turned off for an hour or more since the last
reading. (Be sure to warm up the unit for 15-30 minutes before
taking the next reading.) During fumigations of 6 hours or less, it
is recommended that the unit be kept running constantly.
◆ Fresh Drierite® has been added to the gas drying tube.
◆ The weather during the fumigation has drastically changed since
the last reading.
The gas concentration readings indicated by the T/C unit’s meter will
generally be more accurate if the temperature of the gas mixture
within the fumigated enclosure is approximately equal to that of the
ambient air outside the enclosure. If there are great differences
between the two temperatures, water vapor may condense inside the
gas sampling leads. Such condensation, if desiccant is saturated, can
result in a lower than normal T/C meter reading, thus leading to the
unnecessary addition of fumigant to compensate for the apparent
Equipment
Fumiscope and Gow-Mac
06/2004-01 Treatment Manual 8-1-7
PPQ
shortage. Therefore, if vapor condensation appears inside the gas
sampling leads, purge the line and move the T/C unit to a new
location where the ambient temperature approximates that of the
enclosure.
T/C gas analyzers are sensitive to a number of gases other than MB.
For example CO2 may be troublesome when fumigating fruit where
kerosene heaters are placed under the tarpaulin to raise pulp
temperatures, or with plant material packed in peat moss or subsoil.
Correct MB gas concentration readings may be obtained if a CO2
absorbent is used in the gas sampling line before the air-gas mixture
enters the T/C unit. A CO2 absorbent which may be used is Ascarite®.
Inspectors using Ascarite® should observe the poison warning labels
　
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