Advanced Instruments 4C3, Руководство пользователя

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Careful attention to sample-handling and to simple maintenance proce-
dures (e.g., heat transfer fluid replenishment, and probe and stir/ freeze
wire cleanliness) will ensure continued high performance.
Figure 1 illustrates the temperature of a sample as it progresses through
the freezing cycle, and shows the action of the instrument at each stage.
Although the freezing point plateau in a properly-designed cryoscope
appears flat for a considerable period, extending ideally for many min-
utes, an amplified plot of the plateau temperature would show a very
gradual curve, peaking as the temporary adiabatic condition breaks
down. The “ freezing point ” of a sample has been defined as the warmest
temperature achieved during equilibrium. Unfortunately, the warmest
temperature achieved during equilibrium can only be definitively deter-
mined after the fact, requiring the sample temperature to be monitored
for a substantial period, through and beyond the “ peak ” . In addition, the
measurement is demonstrably more repeatable somewhat earlier on the
plateau, where the freezing and melting processes are more adiabatically
balanced.
Commercial cryoscope users, observing that similar samples have sim-
ilarly sloped freezing point curves under similar conditions, tradition-
ally have solved this dilemma by taking timed plateau measurements
and calibrating accordingly; it simply is not economical to wait for
less-repeatable peak readings. In order to maintain agreement between
different instruments, milk cryoscope manufacturers traditionally have
incorporated plateau timers set for a reasonable compromise between
accuracy and efficiency. Though the length of the plateau varies with
the constituents and size of the sample, we have found freezing point
measurements taken at approximately 30 seconds after the initiation of
crystallization to be most repeatable with 2-mL samples.
Certain milk regulating agencies still mandate or recommend other
times or measurement at the peak of the plateau. The 4C3 is factory-set
for 30-second plateau measurements, but it may be set easily for peak
seeking, if desired, as described in Chapter 2, “ Operating Instructions ”
in this user’s guide.
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Foreword