70Xi Cold Flow Properties Laboratory Analyzer
User Guide
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A p p e n d i x D . U n d e r s t a n d i n g a n d
U s i n g P h a s e P l o t s
A phase plot is a graph with the temperature on the horizontal axis and the optics (scattered light) signal
(for freeze point or cloud point) or movement index (for pour point) on the vertical axis.
For freeze and cloud point tests, phase plots depict the optics signal maintaining a low and flat baseline
level when the sample is completely liquid. The presence of wax crystals causes a rise in the signal.
A pour point plot shows a sequence of vertical bars that indicate the relative amount of movement in the
sample when it is perturbed at progressively colder temperatures.
Phase plots are very useful for interpreting and verifying test results, understanding the cold temperature
characteristics of test samples, and diagnosing problems.
The 70Xi analyzer can overlay one or more phase plots by using the OVERLAY GRAPHS button. For
example, overlaying a failed Quality Control (QC) test run on top of a previous good QC phase plot will
immediately show if and how the wax crystal behavior is different between the two tests.
One of the most common causes for a failed test is sample contamination; sample contamination can be
easily identified by observing a dragging “tail” on the phase plot when compared through overlay with a
known good test run.
Cloud Point
Cloud point is the temperature at which the smallest cluster of hydrocarbon (wax) crystals can be
observed when a sample is cooled under conditions prescribed by the ASTM D5773 test method.
Phase Technology’s 70Xi analyzer performs a cloud point test in two stages. First, the sample is cooled at
a fast rate as the optics system looks for crystal formation and signal rise. On a cloud point phase plot,
this can be seen as the blue line moves from left to right. As the crystals form, the baseline rises. The
turning point where the signal rises is called the approximate cloud point.
Next, the analyzer warms the sample until all the crystals disappear. The red portion of the phase plot
shows the warming stage; the plot returns to blue baseline. The analyzer re-tests the sample, cooling the
sample more slowly. The green portion of the phase plot shows the re-cooling.
