Question determining meth purity through recrystalization temperature

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I placed some meth, cooking oil and a temperature probe into a vial and heated to 200 °C until all crystals melted. The recrystalization occurred at about 158-160 °C which seems a bit low than what I was lead to expect. What does it say about the quality of my meth? Thank you.

 

[Osmosis Vanderwaal]

165-167°C correlates to sea level, so subtract 3.9°f , 1.8°c for every 1000 ft above sea level. At about 2000 ft.elevation, that's dead on