h)
communication error
The connection to the measurement board is broken. This could caused by a
defective cable or a defective measurement board. Both should be checked.
9.2. Detection of errors during use
Most of the errors made during use of freezing point determination equipment are
miscalibrations. The calibration of a cryoscope is a necessary precondition for every use. It
is necessary to use a thermistor for measuring the temperature of a sample for measuring
technology reasons. Thermistors have a very large temperature effect which is necessary
for resolutions of more than 1 m°K. Unfortunately, the fluctuation range of the resistance
values of these components is so large for production technology reasons that the
temperature zero point (0°C)in the cryoscope must usually be determined by a pre-
calibration before the machine can be calibrated with a new thermistor. It must be
assumed that the A-calibration cannot be performed successfully after a thermistor
replacement. The reason for this is that the machine must first reach the specified beat
temperature and then recognise an increase in the temperature after the beat (as an
indication that the freezing has initiated). However, this is not guaranteed because the
values of the new thermistor after calculation with the calibration constants of the old
thermistor result in wrong temperatures. Therefore, a so-called pre-calibration is necessary
where the machine does not pay attention to the temperatures but instead carries out a
purely time-controlled measuring procedure. Afterwards, the calibration constants are
adapted to the characteristics of the new thermistor so that both the A as well as the B
calibration can be carried out successfully. Unfortunately, it is often the case that the
sample vials filled with the calibration liquids are mixed up during the calibration.
Attention: Please do NOT mix up probe vials with calibration vials.
a)
Mixing up the A-solution with the B-solution:
Initially the A-calibration is made without any anomalies. During the B-calibration, the
machine reports the “not calibrated or defective thermistor” error and remains in an
uncalibrated state. In every case, a new pre-calibration and then a correct calibration
should be performed.
b)
Mixing up A-calibration instead of B-calibration
Afterwards, the complete temperature scale of the machine is shifted. During
remeasurement of the calibration solutions, transposed values and a transposed leading
sign are obtained.
Example:
A-calibration with 0.000°C
A-calibration with -0.557°C (Operating error)
Remeasurement B-solution: results in 0.000°C
Remeasurement A-solution: results in + 0.557°C
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