Theory Of Operation; Sensor Sensitivity - Hach PAH500 Manual Del Usuario

Figure 3 Sensor unit dimensions

2.3 Theory of operation

The measuring principle is based on the fluorescent properties of PAHs. After excitation caused by a
UV light source, PAHs give light with longer wavelengths after a short time delay. The intensity of this
light is measured and is proportional to the concentration of the PAHs. This measuring principle is
much more sensitive than absorption and scattered light measurement. Thus, it is possible to detect
the smallest trace of PAH contamination in water. PAHs are integral parts of most mineral oil
products and are a very specific indicator of oil contamination in water bodies and process water.

2.3.1 Sensor sensitivity

The sensor unit is factory calibrated to measure phenanthrene concentration in water. Phenanthrene
is one of many polycyclic aromatic hydrocarbons (PAH) to which the sensor unit will respond. In most
areas, there are many different species of PAH in water, and each PAH species will respond at
different wavelengths and intensities to the excitation of the sensor. The different responses of each
PAH species result in varying sensitivity to the different components of a mixed sample. It is not
possible to supply a factory calibration that is applicable to all PAH samples. Other non-PAH
molecules can also fluoresce in the measurement wavelengths, which can cause a positive
interference to the PAH measurement.
The sensor sensitivity to different compounds follows:
• Highest sensitivity: Pure polycyclic aromatic hydrocarbons (PAH)—Phenanthrene,
anthracene, naphthalene, acenaphthene, fluorene, fluoranthrene, pyrene, benzanthracene and
chrysene
• Medium sensitivity: Pure aromatic hydrocarbons—Styrene, biphenyl and phenol
• Lower sensitivity: Oils—Crude oil (sensitivity varies), diesel, gasoline, kerosene, fuel oil,
hydraulic oil and compressor oil
• Low or no sensitivity: Other compounds—BTEX and non-aromatic hydrocarbons
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