Instruction Manual TN-100/ T-100
21
9. Addendum 2:
Guide to Good Measurement Technique
Turbidity is a very complex analytical measurement which can be affected by many
factors. Some are inherent in the instrument’s design such as angle of detection,
light beam aperture, incident beam wavelength and color sensitivity of the photocell.
However, there are other factors such as stray light, air bubbles and care of vial,
which can be prevented through proper care of equipment and accessories, and in
the operating procedure for measurement. Here are some points you may want to
note:
9.1 Maintain sample vials in good condition
Sample vials must be meticulously clean and free from significant scratches. It
should be treated on the outside with a thin coat of silicone oil. This is to mask
minor imperfections and scratches that may contribute to stray light. Sample vials
should be handled only by the top to avoid dirt accumulation (or deposits) and
fingerprints that might interfere with the light path.
More information is found in Section 5.1 - Vials – Handling, Cleaning and Care on
page 16.
9.2 Match Sample Vials
Best accuracy and repeatability of turbidity measurement are achieved using a
single, indexed vial. However, for more convenience, different vials can be used for
measurement provided their readings with the same solution are matched. That is,
the meter gives identical readings or within the specified repeatability and accuracy
of the meter.
Select a few vials. After the sample vials are cleaned, fill them with ultra-low
turbidity water. Allow the sample vials to stand and for air bubbles to rise. Polish
sample vials with silicone oil and take turbidity measurement at several points while
rotating it in the sample well. Find the position where turbidity reading is the lowest
and index it for each vial. Whenever these sample vials are used, use the indexed
mark to position each vial into the sample well. Choose those vials that match the
readings.
NOTE: Not all vials can be matched because of some manufacturing
variations.
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