All very logical when you know what's going on but it threw me until I'd worked out what was happening.
Picked up some oil the other day from a regular but infrequent supplier. The call to collect came a lot sooner than expected, only a month or two after the last collection when normally it's at least six months between collections.
Got it home and it contained copious amounts of water where they had punched the top of the tin, hence the early call. Knowing it was wet and usually not the best quality, I stuck it all in my fat melting tank (the one that's defiantly not made from a gas bottle, and if it is, it's of indeterminate supplier), switched the heat on and left it to separate.
Came back half an hour later and the tank hadn't warmed up (immersion stat is set at 70°C). Thoughts turned to replacing the heater, but then further investigation revealed the very bottom section of the tank was hot. Drawing on my many phisics qualifications I deducted that the heat should have risen to the top ... this completely threw me until I sused that the free water level must be above the heater and it was only the free water which was getting hot, no heat passing across the oil/water interface. All very logical, but a weird feeling of disbelief before I'd worked out what was going on.
While quite amusing in this circumstance, it's a situation worth remembering if you use a Berco or similar tank to dewater and take the temperature anywhere close to boiling. It could result in a sudden release of steam from under the oil layer.
