If an oil-in-water content monitor uses a lamp emitting light in the visible spectra in conjunction with a reference photo-cell and a sampling photocell, what is the operating principle upon which this oil-in-water content monitor works?

Correct Answer: Option B — The monitor works on the absorption/scattering principle where the greater the oil content, the less the amount of visible light detected by the sampling photo-electric cell as compared to the reference photo-electric cell.

Oil-in-water monitors using visible light operate on the principle of light scattering and absorption. As oil concentration increases, the oil droplets scatter and absorb more light, resulting in a lower intensity of light reaching the sampling photocell compared to the reference photocell, which measures the clear water baseline.