The Advanced
®
Model 3250/4250 Service Manual
Place a tube with saline or distilled water in
the sample well. Turn on the instrument At
“Press START to Continue”, press START.
The instrument performs a check of the cool-
ing assembly and the probes. If a Block
Probe Failure, Sample Probe Failure or
Cooling System Error message displays, use
the A/D tests to perform further testing.
A/D Tests
Note: You must set the correct sample and
block bin numbers before performing
A/D tests.
1. Turn on the instrument. If there is a
Supervisor/Operator keyswitch, turn it to
the Supervisor position (if required).
2. Press TEST.
3. Press < or > to select “A/D Tests”. Place
a sample tube containing either isopropyl
alcohol or a high value standard into the
freezing chamber, and press START.
4. The display will change to:
off Blk NNNN.NN ohm
readout
(resistance or temperature)
channel setting
(sample or block)
target temperature
(off, +1 to -8)
5. Set the target temperature from +1°C to
-8°C by pressing < sequentially.
6. Press > sequentially to read:
Block probe resistance in ohms,
Sample probe resistance in ohms,
Block probe in degrees Celsius,
Sample probe in degrees Celsius, or
Cooling block duty cycle (a % value).
Block probe resistance will indicate current
temperature. The probe begins to cool after
the target temperature is set, and the resist-
ance increases until it reaches the target tem-
perature.
Sample probe resistance will indicate cur-
rent temperature. The probe begins to cool
after the target temperature is set, and the
resistance increases until it reaches the target
temperature. Due to thermal mass considera-
tions between the positions of the block and
sample probes, the sample probe may be ±
0.5ºC of the block probe reading.
Duty cycle displays the percentage of time
that the thermoelectrics power is on. When
the thermoelectrics begin to cool, the duty
cycle is usually at 100%. As the instrument
stabilizes, the duty cycle decreases to around
60%.
Increased ambient operating temperatures do
not affect the sample probe reading, but will
increase the observed duty cycle, up to 85%
at the maximum temperature of 35ºC.
Variations in ambient non-condensing relative
humidity have no effect on the sample probe
reading or duty cycle.
The following situations indicate a problem
with either the probes or the cooling assem-
bly:
• The resistance reading is replaced by the
number 0. Scrolling from right to left indi-
cates an open circuit.
• The target temperature cannot be reached,
but probe readings that vary around ambi-
ent indicate a cooling system problem.
49
Power On Diagnostics
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