Process Wednesday: Seeing through a glass, darkly

I'm going to guess a turbidity probe costs more, too.
Credit: directindustry.com, southernsportsman

Upon looking through a manway and seeing a hazy solution (suspension?), I was reminded of limnology (the study of inland waters) and one of their tools, the Secchi disk:

The Secchi disk, created in 1865 by Pietro Angelo Secchi SJ, is a circular disk used to measure water transparency in oceans and lakes. The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the pattern on the disk is no longer visible is taken as a measure of the transparency of the water. 

Of course, one finds that chemists and chemical engineers have come up with something a little more advanced to measure turbidity inside reactors. From Dow crystallization engineers in the pages of Organic Process Research and Development [1]:

The fiber-optic probe measures backscattered light to generate a real-time turbidity signal indicative of the amount of solid-phase material present in the crystallizing slurry.  

A fiber-optic probe can be used for a broad range of applications because the output signal depends only on the probe’s ability to detect backscattered light; that is, no specific chemical properties such as molecular absorption are required. It also can be quite robust; numerous Dow probes have been in service for more than 10 years with no degradation in performance and without fouling problems. The basic Dow design includes a polished sapphire window, a spring-loaded gasket seal system to resist solvent infiltration, and fiber-retaining inserts (Figure 1). This design allows for expansion and contraction over a wide temperature range without breaching the process seal, yet it maintains precise optical alignment of the fibers and the window.

Well, I guess we'll have to turn that agitator back on and let it stir some more...

[1] Harner, R.S.; Ressler, R.J.; Briggs, R.L.; Hitt, J.E.; Larsen, P.A.; Frank, T.C. Org. Process Res. Dev., 2009, 13 (1), pp 114–124.