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