Advanced Instruments 4250, User Manual

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xvii
Foreword
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 adiabatical-
ly balanced.
Commercial cryoscope users, observing that similar samples have simi-
larly sloped freezing point curves under similar conditions, traditionally
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 dif-
ferent 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 4250 is factory-set
for 30-second plateau measurements, but it may be set easily at 60 or
90 seconds or for plateau seeking, if desired, as described in Chapter 2,
“ Operating Instructions ” in this user’s guide.
Figure 1: Standard Freezing Curve