http://skunkpharmresearch.com/diy-vacuum-purging-chamber/
http://skunkpharmresearch.com/vacuum-purging-and-processing-tips/
http://skunkpharmresearch.com/spraying-butane-directly-on-silicone-mats/
I thought I was doing a pretty good job purging my oil but these guys have taught me I wasn't doing it as good as I could and that there is a better way! According to skunkpharm 115f is an ideal temp for your heating mats controlled by a light dimmer under your vacuum chamber. They specifically say a griddle is not able to reach and hold the required low temps you would want to preserve the terpenes when purging.
If you don't want to invest in a vacuum chamber a toaster oven works if you don't mind destroying terpenes in the name of purging. Griddle could work never tried though. Put your oil on parchment paper and turn your oven or griddle to lowest and let it melt into a puddle. The higher the temp, the darker it will be, the less terpenes are left in your oil. Butane makes large bubbles and terpenes make small bubbles. Initially when heat purging your oil should be quite frothy from the butane escaping, when the bubbling calms to no visible large bubbles and just tiny ones you can remove from heat as your only destroying terpenes now.
Here's how I make a small run quickly
-never whip your oil...
-preheat toaster oven with ceramic tile and pyrex dish
-fill fry pan with hot water and place outside
-grab pyrex dish,butane,glass extractor..put heated pyrex in fry pan full of hot water (double boiler)
-blast into pyrex stop soon as the liquid coming out turns clear
-seconds later most butane has evaporated, throw pyrex in oven, place tile on top to keep it warm
-remove and scrap pyrex onto awaiting square of parchment paper and place on preheated ceramic tile
-throw tile in vacuum chamber until large bubbles stop, small bubbles are terpenes..preserve these
-if not using vac place tile in toaster oven until only small bubbles then remove