<p>An apparatus for optically detecting the degree of crystallization in a fluid sample while simultaneously measuring the temperature of the fluid sample during cooling or heating of the sample is disclosed. The apparatus includes: a test vial for containing a fluid sample in an interior of the vial, wherein the test vial is light permeable; a cooling source for cooling the fluid sample; a temperature sensor positioned in the interior of the test vial for measuring the temperature of the fluid sample; an external light source configured to direct light into the test vial; and an external light detector configured to measure the amount of light that traverses both the test vial and the fluid sample.</p>
