Re: WSN: On-line turbidimeters vs benchtop (fwd)

• To: Water Science Network <water@gibbs.oit.unc.edu>
• Subject: Re: WSN: On-line turbidimeters vs benchtop (fwd)
• From: Water Science Network Administrator <wsn-adm@mmlds1.pha.unc.edu>
• Date: Tue, 20 Jun 1995 10:51:05 -0400 (EDT)

Sender: hsanchez@ns.usma.pa (Prof. Humberto Sanchez ... (Quimica)...Internet)
Subject: Re: WSN: On-line turbidimeters vs benchtop

> 
> We recently began using some on-line turbidimeters (Great Lakes Instruments)
> and noticed that they routinely show higher readings (by ca. 0.5-0.6 NTU) than
> concurrent manual grab samples read on a Hach 2100A turbidimeter. Has anyone
> encountered a similar bias, or have any thoughts as to the reason? We suspected
> air bubbles as a possible cause: concurrent particle counts showed particle
> sizes < 10 microns, and we have been told that air does not show up at size
> levels below 10 microns. Can anyone confirm this size characteristic for air
> bubbles? Thanks.
> 
> 
> 
Yes, there are several conditions such as: light source emission decay, 
coatings on turbidimeter housing, response time, air bubbles as you 
mentioned. On line turbidimeters are in continuous operation so 
calibration of this units is not achieved until units are serviced. 
Deviations found by you are normal. Deviations of 0.5 or 0.6 are normally 
encountered in on-line systems. Size characteristics for air bubbles will 
depend on the type of instrument sample inlet device. ok.
truly, hsanchez@ns.usma.pa

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