Calibrating a Temperature Controller?

[CastAway]

I have an American Marine Pinpoint temperature controller. There's a difference between my controller temp and that indicated on a floating glass hydrometer I have. How does one calibrate the controller? Do I have to get at least one more temperature source, like another thermometer, and just set to the average?

 

[stylaster]

yep when doing my apex i took two glass therometers and took the average of the two