quote:
Dark oils indicate that the oil is dark
There is one of the very few definitive statements I can support.
A color change is a good reason to be alert, but a poor reason to run around with your hair on fire (until you know why it's dark, or even IF it's dark, as the original poster appears uncertain as of yet).
There are two additional potential causes of oil darkening, both rather benign:
Amine antioxidants will darken significantly under some circumstances. Antioxidants will react as they do their job, frequently producing reaction products that give you colors ranging from a brilliant yellow through a deep red to an inky black, depending on a number of factors that include formulation, operating conditions and any contamination that may be present. While it may look scary, these reacted species still retain considerable antioxidant potential, since they go through a series of these reactions before becoming truly exhausted.
The other cause is perception. A certain amount of color change is inevitable in service, but some people are more easily freaked out. I had a frantic phone call from a sales guy because a customer had seen a color change among drums from the same shipment. He had provided me label info, so I knew the shipment included drums from at least two batches. I walked over to the lab to get a look when they arrived. I asked where they were and was pointed at two samples of pale yellow oil. On the lab bench, they looked identical. When I held them up to the light, one was just barely darker (or was it the other one?) Run basic analyses and cut-and-paste my boilerplate statement about normal batch-to-batch variation. Oh yeah, a phone call to the sales guy telling him we'd rather not see any more "cardiac" samples from this customer unless he's actually visited the location and confirmed that there really is something going on.
The first question to ask when you observe a physical phenomenon related to the oil:
Are there any operating changes? These can be filter-plugging, oil temperature or pressure changes, foaming, a REAL viscosity change (not measured by the Eyeball Viscometer) or other abnormal behavior. If the only thing that has changed is the color, it bears watching, it may merit analysis, but I'd avoid shutting the operation down on the basis of a color change alone.
