Epsilon Open Archive

The process industry needs fast online measurement for monitoring batch and continuous
processes. Near infrared (NIR) spectroscopy is a fast and reliable technique often used for this
purpose. One can apply NIR spectroscopy in transmission mode (for homogeneous liquids) or in
reflectance mode (for inhomogeneous solids). For turbid liquids, there is a need for a probe that
can measure over larger areas and with flexible penetration depths. Such a probe was constructed
and tested using experimental design and multivariate data analysis. This was done in comparison
with a commercial probe. Dairy waste used for anaerobic fermentation is a typical turbid liquid,
but not very homogeneous. To simulate the turbidity of dairy products, solutions of well-known
amounts of milk powder were used. The factors used in the experimental design were turbidity
(amount milk powder) / temperature / amount of analyte present.
The comparison of the probes shows that the constructed probe has comparable properties to the
commercial one and that it can be used for different at-line and on-line measurements. The
difference is that the commercial probe is meant for immersion in the liquid while the new
construction is a non-contact cell through which the liquids are pumped. The results of the
experimental design give a good insight in which factors are important for a stable and
reproducible measurement.