Hanna-instruments HI 98703 User Manual download pdf (Page 16)

• Open the instrument lid, slightly rotate the cuvette and take a new reading.

• Repeat the last step for the second cuvette until the reading is within 0.01 NTU of the value

obtained for the first cuvette.

• Alternatively, keep READ u pressed and, after the first value is displayed, open the lid and start

rotating the cuvette until the displayed value matches the first cuvette.

• Mark this position on the second cuvette with a water resistant pencil.

• Follow the same procedure for all the necessary cuvettes.

Note: If the cuvette is indexed, use the index to position it in the instrument.

SAMPLING TECHNIQUE

When taking turbidity measurements it is very important to select a representative sample. For

consistent results, follow the next tips when sampling:

• Gently mix the water before taking the sample.

• If the sample is taken from a pipe, discard the first few liters.

• If measuring a non uniform source, collect samples from different places and mix them.

When measuring the collected sample, keep in mind the following:

• Samples should be analyzed immediately after collection because the turbidity can change in

time.

• To avoid dilution of the sample it is better to rinse the cuvette with a quantity of sample and then

discard. Only after this you can fill the cuvette with sample.

• Pay attention that cold samples do not condense on the sample cell.

REMOVING AIR BUBBLES

Any air bubbles present in the sample will cause high turbidity readings. To obtain accurate

measurements, remove the air bubbles using one of these methods:

• Application of a partial vacuum;

• Addition of a surfactant, such as Triton X-100;

• Use of an ultrasonic bath;

• Heating the sample.

Sometimes it is necessary to combine two or more methods for efficient air bubble removal.

Note: Each method can alter the sample turbidity, if misused, so they have to be used with caution.

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Hanna-instruments HI 98703 User Manual Page 16