The reading shown by SIGRIST fluorescence instruments is based on a quinine sulphate calibration (quinine sulphate) in the 0..10 ppm range. The associated measurement unit is referred to within the SIGRIST organization as FLU (Fluorescence Unit).
For certain applications, it is desirable to directly display the concentration of the substance being measured. Once the respective product is known, there is no reason why the instrument cannot be calibrated to do so.
The range of interest for oil trace measurement is the linear portion of the curve (concentrations up to about 100 mg/l). The shape of the above curve is typical for many petroleum products, but the absolute figures differ sharply from case to case.
Fig. 6: Fluorescence of various petroleum products
An important practical aspect of the calibration procedure is the mixing of the insoluble oil with water. What is needed is a reproducible, homogeneous suspension of finely dispersed oil droplets.