17
• Alternatively, keep the
Read
key pressed and, after the first value is displayed, open the lid and
start rotating the cuvette until the read value matches the first cuvette.
• Mark this position on the second cuvette with a water resistant pencil.
• Follow the same procedure for all the cuvettes you need.
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 should you fill the cuvette with sample.
• Pay attention that cold samples do not condense on the sample cell.
REMOVING AIR BUBBLES (TURBIDITY only)
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.
APPLICATION OF VACUUM
Vacuum works by decreasing the atmospheric pressure. In this way the bubbles from the solution
come out to the surface. Application of vacuum is a very simple procedure and can be applied with
any vacuum source at hand. The simplest equipment at hand is a syringe and a rubber stopper for
vacuum degassing.
Notes
: • The vacuum equipment must be clean and oil-free.
• It is not recommended to apply a vacuum to a viscous sample that contains volatile
components. In such cases the vacuum can cause the volatile component of the viscous
sample to increase the bubbles from the sample.