CRUDE OIL 101
As none of us on the Dalhousie team had used sensors to measure crude oil in water, we weren't sure what to expect. Here's what we learned.
1. THE PROBLEM WITH FLUORESCENCE
A common method of sensing oil in water is using the principle of fluorescence . Fluorescence sensors shine light at certain wavelengths and measure the wavelengths that are emitted back by objects in the light path. Different objects have different fluorescence characteristics, which is why it's important to check the wavelengths that a fluorescence sensor produces ("excitation" wavelengths) and measures ("emission" wavelengths). Fluorescence sensors for oil usually produce light at wavelengths around 300-400 nm, and identify the presence of oil when they receive wavelengths between 400-600 nm.
Unfortunately, this isn't a perfect method. Other things (or more scientifically, "fluorescent Dissolved Organic Matter", or "fDOM") fluoresce at the same wavelengths as crude oil. This means that you risk overestimating the amount of oil in the water - maybe some of it is actually bacteria, or algae. In controlled experiments, fluorescence measurements are taken before the oil is added, and this baseline is subtracted from the fluorescence measurements of oil. Aside from the fact that we often don't have the opportunity to do this during a real-life oil spill, this method assumes that the baseline is true everywhere. This is not a great assumption since the ocean is a dynamic environment and is therefore constantly moving the very things that are the source of our baseline fluorescence measurement. So, without an additional means of verification, such as taking water samples and measuring the actual oil content in the lab (which kind of defeats the purpose of autonomous mapping), there's no way to know. Thus, for now, we'll just be looking at relative oil concentrations - something along the (admittedly very unscientific) lines of "no oil, a little oil, lots of oil".
2. HOW TO MAKE "CRUDE OIL" AT HOME
Importing samples of crude oil - even for academic research - isn't a very quick or easy process. Fortunately, there's a relatively quick (and dirty) way to approximate crude oil ("sludge") with just two ingredients that are easily sourced: diesel (available at a gas station) and road asphalt (there's usually plenty of loose pieces in ditches). So, I decided to make a batch of sludge that we could use to get an idea of the sensor response before the actual experiment.
Note: this is an extremely messy process that I do not recommend trying. Always take appropriate precautions whenever working with volatile and hazardous materials.
Obtain diesel and asphalt
(fresher = better)
Dissolve small pieces of asphalt in diesel
Repeat process, filtering out solids, until desired density is achieved
... and there you have it: "crude oil".
mass: 707 g
volume: 0.820 L
Wikipedia. Fluorescence. Retrieved from https://en.wikipedia.org/wiki/Fluorescence
Last revised: May 1, 2020