So, what did I learn?
I regained respect for an old lesson: what isn't known is often more important than what is known. Figuring out what you don't know is a very difficult task.
Preliminary experimental conclusions:
I heated Canola oil at 75-85C for 5 days, the viscosity increased by approximately 5% -- not terribly much, all things considered. The presence of metal tubes had no immediately noticeable effect on vegetable oil viscosity. All heated samples experienced an increased viscosity of about 5%.
I was surprised to note that the the oil exposed to stainless steel and aluminum took on a slightly darker color after 5 days. It should be noted, however, that the oil exposed to copper retained the same color as the control samples. I was surprised by the stainless steel, but the aluminum was something I expected. I also expected copper to cause a color change.
I haven't completed the complete numerical analysis of the data, so it's more than possible that I will be able to observe more trends as I start crunching numbers. I also have a few more samples to analyze - I'm especially interested to see what happens with the oil from my fuel tank.
Update:
After doing all the number crunching, I've found that copper doesn't appear to cause any significant changes in vegetable oil viscosity alone. All my heated samples increased viscosity by about 5%.
When I sampled oil from my actual tank over several days, I discovered that the effect of the backflowed diesel (in my two-tank system) overwhelmed any evaporative effect from heating. My tank samples were all significantly less viscous than any of my experimental samples - 10-20% less viscous.
The takeaway here is that I wasn't able to find a significant change in viscosity or detect polymerization. Additionally, in real-world applications, even a little bit of diesel has a significantly greater effect on viscosity than evaporation or polymerization.
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