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should be stabilized at a temperature of 0°C (32°F). This
is accomplished by using one of the two following
methods, whichever is more convenient.
Unit Running:
Place set point at 0°C (32°F). After unit has pulled down
to this temperature, allow the compressor to cycle
ON-OFF three to five times to be certain temperature
has stabilized at 0°C (32°F). If the temperature
indicated by the recorder differs from 0°C (32°F) by
more than 0.6°C (1°F) when compressor cycles off,
rezeroing must be performed.
Unit Off:
Place the recording thermometer sensing bulb in 0°C
(32°F) ice-water bath. Ice-water bath is prepared by
filling an insulated container (of sufficient size to
completely immerse bulb) with ice cubes or chipped ice,
filling voids between ice with water, and agitating until
mixture reaches 0°C (32°F) as shown by a laboratory
thermometer.
When the ice bath temperature reaches 0°C (32°F), as
shown by the laboratory thermometer reading, compare
the temperature indicated by stylus with temperature
shown by the thermometer. Allow sufficient time to
ensure bulb has cooled to the ice bath temperature. If
the two readings do not agree within +/-- 0.3°C (1/2°F),
the recording thermometer should be rezeroed.
c. Rezeroing the Temperature Recorder
1 Be certain that the element sensing bulb tempera-
ture has stabilized at 0°C (32°F). Note the amount of
temperature difference between the test meter or
thermometer reading and the stylus indicated tem-
perature.
If the difference noted between the known element
temperature and indicated temperature is within
acceptable limits (+/-- 0.3 of 0°C = 1/2° of 32°F), do
not attempt to rezero. If more than +/-- 0.3°C (1/2°F)
in variation, carefully note the number of degrees.
2 Loosen set screw (item 6, Figure 6-32) and zero
thermometer by turning pinion shaft (item 7). Length-
ening pinion shaft (counterclockwise) raises stylus
indicated temperature reading; shortening shaft
T-285
(clockwise) lowers stylus reading. Then retighten set
screw.
3 Reset control at 0°C (32°F), start the refrigeration
unit and repeat accuracy check. After temperature
stabilization, temperature recorder should be within
0.3°C (1/2°F) limits.
d. Replacing Temperature Recorder Element
(Bulb and Capillary)
The element and bulb are mercury-filled. The element
flange contains three O-rings. Care should be taken to
install the new element flange without damaging the
O-rings. It is possible for a mercury leak to develop at
the flange if O-ring damage occurs.
The stylus reading will continue to fall below actual
container temperature if a leak develops in the flange,
capillary or bulb. To replace the temperature recorder
element:
1 Turn unit OFF and disconnect power source.
2 Remove upper back panel. Remove bulb clamps
securing bulb to unit.
3 Remove two flange screws from recording thermom-
eter. If a thermistor is installed, remove the fitting
from the recorder platen. Feed capillary and element
through the unit.
Recorder element capillary tubing may be
bent, but never sharper than 1/2" radius;
extra care should be taken when bending
adjacent to welds. The bulb should never
be bent, as this will affect calibration.
4 Push replacement bulb and capillary through the
unit.
5 Fill slots with silastic (RTV432, Dow Corning).
6 Attach bulb clamps tightly to sensor.
7 Connect element flange to recorder making sure hub
of flange faces out to fit into the hole in recorder box.
Install thermistor fitting if required.
8 Rezero the recorder. (Refer to sections 6.24.a.
through 6.24.d.)
9 Install inlet air grille and upper panel. Start unit and
check recorder calibration.
6-26
CAUTION
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