When I asked what it was contaminated with, he could not answer. He mentioned a specific gravity of nine when it needed to be an eleven.
Ask them to show you what they used to measure the specific gravity with. They will probably stall and eventually show you a battery hydrometer. Now you have them dead to rights. Here's a link that explains how specific gravity works for petroleum, API gravity.
https://en.wikipedia.org/wiki/API_gravity
I'll condense things a bit. API gravity uses different hydrometer scale than water. But there is a relation. Above 10 means a petroleum liquid is lighter than water and all fuels are. Less than 10 means a petroleum liquid is heavier than water. This is only good for water contamination in fuel. It's worthless for confirming gasoline contamination in diesel fuel.
If they show you a battery hydrometer or hydrometer used for water, it doesn't matter if it's the old school manual type or digital, ask them what they used for the conversion factor to convert specific gravity to API gravity. There are tables for this but in my 45 years of driving I've never seen one in any dealership or shop. Calculating API gravity from specific gravity is easy when you know the formula.
API gravity formulas: The formula to calculate API gravity from Specific Gravity (SG) is
And the opposite, going from API gravity to specific gravity.
This is where they threw those numbers at you. Heavy oil with a specific gravity of 1.0 (same density as pure water at 60°F) has an API gravity of
After you ask them what conversion factor they used, ask them if they adjusted their number for temperature. Specific gravity to API gravity calculations have to be adjusted to 60F to be correct.
For example, let's use the 9 number the guy at the dealership threw at you. Starting on the left side follow the 9 line across to 60F. That's the ideal but not the actual. Lets assume that the temperature was higher than 60F when the dealer took this reading, lets say 75F considering the time of year. Notice how fast the API gravity changes. Follow the API 9 line down to where 75F would be on the bottom scale, and back across to the left, now the API gravity is what, around 12.5 to 13! A 15 degree difference make a huge difference is the API reading, and 12.5 over the 11 number they claimed the fuel has to be. As you can see, temperature changes API gravity, which changes fuel density, which also changes BTU/gal. That's why the BTU scale on on the right.
Here is where it gets really stupid when someone starts throwing around specific gravity readings. Looking at the BTU side of the graph, oils in the 9 to 11 range are in the asphaltine range, heavy fuel oils, roofing cement, etc. Diesel fuel is in the range of 32 to 40 API gravity, depending on the crude oil it was refined from, and/or if it's straight D2 or some kind of winter blend.
The worst trick in the book is if they say they poured a fuel sample from your car into a styrofoam cup and it dissolved the cup. All diesel fuel fuel will do this within a few minutes.
Do your homework and use knowledge against them and make them prove their numbers. Be polite but firm. Knowledge is power!
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