Hach-Lange CL17 USER MANUAL - Manual (Page 10)

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General information
2.2.3 Theory of operation
The analyzer is designed to capture and analyze a portion of the sample every
2.5 minutes. The sample portion is captured in the colorimeter measuring cell where the
blank absorbance is measured. Measurement of sample blank absorbance allows
compensation for any turbidity or natural color in the sample, and provides an automatic
zero reference point. Reagents are added at this point to develop the magenta color,
which is measured and compared to the reference.
A linear peristaltic pump/valve module controls the flow of incoming sample and injects
metered volumes of the buffer and indicator reagents in a 2.5 minute cycle. The
pump/valve module uses a motor-driven cam to operate pinch blocks that squeeze
special thick-walled tubing against a fixed plate. The cycle operates as follows:
1. The sample inlet line is opened, allowing sample under pressure to flush sample
tubing and the colorimeter sample cell where the measurement is made.
2. The sample inlet line is closed, leaving fresh sample in the cell. Cell volume is
controlled by an overflow weir.
3. As the sample inlet line closes, reagent lines open, allowing buffer and indicator
solutions to fill tubing in pump/valve module.
4. A measurement of untreated sample is taken to determine an average reference
measurement prior to reagent addition.
5. The reagent outlet block opens, allowing buffer and indicator to blend and enter the
colorimeter cell to mix with the sample.
6. After a delay for the development of color, a measurement of treated sample is taken
to determine the chlorine concentration.
This sequence is repeated every 2.5 minutes.