Laboratory catalogue

milk does not meet this criterion. Then a second sample
of the milk has to be measured. If this error suddenly
starts to make a frequent appearance, although the
device has otherwise been working properly, the problem
lies either with the thermistor, or is the result of disturbance
caused by external interference.
Uncalibrated or defective thermistor:
The device tests the actual thermistor value when commencing
a measurement of calibration. As is well known,
its electrical resistance is a function of the temperature.
This electrical resistance is translated into a number by
an analogue-digital converter (ADC) and this value is
further processed by the device. Now if the thermistor
has a short circuit or is interrupted, its resistance is zero
or infinite, both of which conditions are impossible for a
properly working thermistor. In this case, the thermistor
will not commence with the measurement.
The device will also fail to commence measuring if the
actual thermistor value, together with the calibration
constants stored in the device, produce a result that is
lower than +1 °C (which cannot happen if the thermistor
is positioned in a new, i.e. still warm, sample).
Identifying operational errors
Most of the errors that are made when using the device
result from faulty calibrations. The calibration of a
cryoscope is an essential condition for its use. For technical
reasons relating to measurements, it is necessary
to use a thermistor for measuring the temperature of the
sample. Thermistors are sensitive to a wide range of
temperatures and this sensitivity is necessary for a resolution
of more than 1 m°K. Unfortunately, fluctuations
in the resistance values of these components are so
great that the zero temperature point (0 °C) usually has
to be determined by pre-calibration before the device
can be calibrated with a new thermistor.
It must be assumed that an A-calibration cannot be performed
successfully after a thermistor has been
exchanged. The reason for this is that the device must
first of all reach the set knocking temperature and then,
after knocking, has to identify an increase in temperature
(as an indication that freezing has begun). But this
is not the case, because the new thermistor values
result in the wrong temperatures being given when calculated
according to the calibration constants of the old
thermistor. This is why a so-called pre-calibration is necessary,
in which the device ignores the temperatures
and follows a purely time-controlled measuring procedure.
After this, the calibration constants must be
adapted to the new thermistor characteristics so that
both the A-calibration and the B-calibration can be performed
successfully.
Unfortunately, it often so happens that sample flasks
filled calibration solutions are mixed up, or that the
wrong menu item is selected.
Mix-up: confusing solution A with solution B:
To begin with, the A-calibration goes as expected. But
when it comes to the B-calibration, the device reports
the error “uncalibrated or defective thermistor” and it
remains in the uncalibrated state. With older versions of
firmware, the device retains the wrong values and is
henceforth not prepared to perform a measurement. It
is advisable to carry out a new pre-calibration, followed
by a proper calibration, in any case.
Mix-up: taking the A-calibration instead
of the B-calibration
This results in the displacement of the entire temperature
scale of the device. Re-measuring of the calibration
solutions gives reversed values and a reversed sign. For
example:
Calibration A with 0.000
Calibration A with 0.000
Calibration B with –0.557
Calibration A with –0.557 (faulty operation)
Re-measuring solution B: results in 0.000
Re-measuring solution A: results in 0.557
Defective thermistor
This is the most common source of errors. There are two
possibilities here:
1. The thermistor is (was) broken. This can be recognised
because the display constantly shows a negative
value that does not change.
2. The thermistor bonding is porous. This results in
extremely unstable measurements. The reproducibility
is very poor, e.g. there are variations of about
+0.1 °C.
The thermistor must be exchanged in either case.
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