Here is some info from a trusted site that may provide some insight. You have this link already Ken ;)
Purging:
"Butane is relatively easy to purge from cannabis oleoresins or concretes, as it has a boiling point of around -.5C/31.5F, or right about the freezing point of water. Given enough time just sitting around, it will purge below our 6.16 ppm sensory threshold, which is .0009% of the 658, 000 ppm, that the MSDS LD-50 tells us it took to asphyxiate 50% of the test rats in 4 hours.
We can speed up that purge, by using a dish with a large surface area, relative to the depth of the pool of oil. Usually small extractions, so as to keep the pool depth thin, are the easiest to purge.
Typically, the thin film is scrapped and stirred periodically, to speed up the process, and a flame may periodically be lightly run over the surface, to warm it and determine if the bubbles exiting are butane and terpenes, or CO2. Some care needed here, as THC, CBD, and CBN are di-terpene alcohols, and are flammable themselves, as are the other terpenes present.
Air movement over the pool speeds up evaporation, by whisking away the saturated boundary layer and providing the extra energy for the molecules of butane to escape the surface of the oil, as it is ricocheting about in the pool. Care must be exercised here, as any dust or lint in the air will end up in the oil, so usually a cheese cloth or similar porous cover is placed over it, before blowing over the top with a fan.
We can also speed it up with the application of heat. Any heat will speed up the evaporation, and one line of thought is to keep the heat low and around 60C/140F, using a hot pad after the hot water bath and scraping and popping any bubbles with a razor blade. A typical purge might take an hour and provides maximum terpene retention.
When adding bottom heat, you can also add a loose fitting lid, which will speed up the purging and keep out lint and dust.
For a faster purge, the temperature can be raised to above the melting point of the cannabis essential oils, or around 82C/180F, to give the butane molecules maximum mobility.
Instead of heat, vacuum may be applied to speed up the purge process. That is the process that we use when we wish to maintain the cannabinoids in their carboxylic acid forms.
In thin film vacuum purging, we place about an 3/16″ of the oil in a 6″ Pyrex Petri dish, and place that in a vacuum chamber, which also contains a hot plate. That allows us to manipulate both the temperature and atmospheric pressure, so that we can achieve boiling at very low, or even ambient temperatures.
While we use 180F to vacuum purge a raw oleoresin, adding heat isn’t necessary when thin film vacuum purging raw oleoresins redissolved in ethanol. The alcohol will boil away under 28.5″Hg at ambient temperatures, as will the water that is left behind, even without adding any heat."