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5.5 Internal Desiccant
A humidity indicator, labeled "INTERNAL CASE HUMIDITY," is
located in the lower left corner of the control panel. It indicates
the amount of moisture present inside the control box. The paper
indicator is blue when the control box is dry.
The control box is a completely sealed unit. You do not need to
open the control box during normal operation. The desiccant
should absorb any moisture that may accumulate in the control
box. Thus, the humidity indicator should remain blue under
normal conditions. If moisture does accumulate, the numbered
areas on the indicator will turn light pink or white, starting with
the area numbered "20." This indicates that the relative
humidity inside the control box exceeds 20%. As more moisture
accumulates, the areas numbered "30" and "40" will turn light
pink or white, indicating relative humidities of 30% and 40%
respectively.
The control box contains electronic circuitry which may be
damaged by static discharge. Open the control box only in a
static free environment, as described in Section 6.2.
If the 30% area of the humidity indicator turns light pink or
white, open the control unit, inspect it for leaks, and renew the
desiccant. To do so, unscrew the 10 screws (indicated in Figure
5-4) around the outer rim of the control box bezel, and carefully
lift the bezel and cover off the control box.
Inspect the Control Box – If th ere is a leak, repair i t by
referring to the previous section, or by returning the control box
to the factory. The factory has specialized equipment to detect
leaks and thoroughly test the units after repair.
Renewing the Desiccant - If you do not detect a leak, renew the
desiccant by removing the bags of desiccant from the control box
(see Figure 5-5).
Place the bags on a piece of heavy paper on a flat metal sheet. You
can use a brown grocery bag and a typical cookie sheet. Do not
stack the bags on top of each other or allow them to touch. Place in
a vented, circulating forced air, conventional oven in a well venti-
lated room. Allow two inches of air space between the top of the
bags and the next metal tray above the bags. Keep the tray a
minimum of 16 inches from the heating element. Heat the bags at
a temperature of 240 to 250•F (116 to 121•C) for 12 to 16 hours.
At the end of the time period, the bags should be immediately
removed and placed in an air tight container for cooling. The des-
iccant will be recharged to approximately 80 to 90% of its previous
capacity. After repeated recharging, the desiccant bag may require
replacement.
Some bags will have the temperature and time for recharging
the desiccant printed on the bag. If they differ, use the temper-
ature and time printed on the bag.
3700 Portable Samplers
Section 5 Routine Maintenance
CAUTION
5-9
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