Hach 2100AN - Measure the Gelex Stray Light Standard; Calibration Verification; Optical System Check; Prepare a Sample Cell

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SECTION 2, continued
2.3 Measurement Techniques
Accurate and repeatable turbidity measurements depend on good,
consistent measurement techniques. Measurements are more accurate and
repeatable if close attention is paid to proper measurement techniques.
Four important considerations are:
• Use clean sample cells.
• Use sample cells in good condition.
• Remove air bubbles (degassing).
• Apply silicone oil to the sample cell.
Measure samples immediately to prevent changes in sample characteristics due to
temperature shifts and settling. Avoid dilution whenever possible; particles
suspended in the original sample may dissolve or otherwise change characteristics
when the temperature changes or the sample is diluted. Thus, the measurement
may not be representative of the original sample.
2.3.1 Cleaning Sample Cells
Cells must be meticulously clean and free from significant scratches. Glass
imperfections and superficial scratches from manufacturing are effectively
masked by the silicone oiling procedure outlined in Section 2.3.2. Clean the inside
and outside of the cells by washing thoroughly with a nonabrasive laboratory
detergent. Then continue cleaning with a 1:1 HCl bath followed by multiple rinses
with distilled or deionized water. Air dry the cells. Handle sample cells by the top
only to minimize dirt and fingerprints.
2.3.2 Applying Silicone Oil
Treat the outside of the cells with a thin coating of silicone oil to mask minor
imperfections and scratches that may contribute to light scattering. Use only Hach
silicone oil (Cat. No. 1269-36); it has the same refractive index as the sample cell
glass.
Apply a thin bead of silicone oil from the top to bottom of the cell- just enough
to coat the cell with a thin layer of oil. Using the oiling cloth provided, spread
the oil uniformly. Then, wipe off the excess so that only a thin coat of oil is left.
The cell should appear nearly dry with little or no visible oil. Avoid application
of excess oil that may attract dirt, and contaminate the sample compartment of
the instrument.