Oil-in-Water Measurement Solutions

Fluorescence occurs when the electrons in a fluorophore release their absorbed energy. Electrons start at ground state (S0) - the lowest energy state available to them. When one collides with a photon, the electron absorbs the photon's energy and jumps up to a higher energy state (S1, S2, etc.). In order to return to ground state, the additional energy absorbed by the electron must be released. This is done via photon emission. The energy of this emitted photon is lower than the energy of the original (colliding) photon, so it has a longer wavelength. This longer wavelength is the cause of a change in color between excitation and emitted lights.

One substance that exhibits fluorescence is Fluorescein. Fluorescein is a powdered orange dye that, when in solution, exhibits a yellow-green glow when excited by light at a wavelength of around 498nm.

Aromatic oils are a typical example materials that fluoresce. When excited with UV light, fluorescence occurs and this emitted light can be used in the measurement of trace oil-in-water and hydrocarbon contamination monitors.

The fluorescent properties of any specific oil are dependent upon a number of factors, including the composition, the proportion of aromatics present, and the oil's carbon structure.

Fluorescence intensity is unique to each oil type. Exciting the oil with ultraviolet (UV) radiation causes it to fluoresce in the UV-visible wavelength range, as shown in the diagram below.

The combined fluorescence from both dissolved and non-soluble oil-in-water can be measured and correlated to oil content. Entrained gas and solids present in the stream do not fluoresce, and thus do not affect the measurement.

Because fluorescence energy is emitted in all directions from poly-aromatic molecules, the simplest measurement method is to use a single optical window arrangement through which both excitation and emitted light passes.

Emitted light intensity is very low in comparison to excitation light intensity. To combat this, the use of a fiber optic cable to deliver the excitation light and a fiber optic bundle to collect emitted light allows for better signal levels and creates a more reliable measurement.

Key features of an oil-in-water fluorescence sensor are:

• Probe style with a single window format
• Large gathering area for emitted energy to improve signal levels and measurement reliability