Thermo Fisher Scientific TN100 Turbidimeter - Manual (Page 26)

Instruction Manual TN-100/ T-100
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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.
9.3
Degassing
Air or other trapped gases should be removed before measurement. Degassing is
recommended even if no bubbles are visible. There are three methods commonly used
for degassing:
 Addition of a surfactant: This involves adding a surfactant to the water
samples to lower the surface tension of the water, thereby releasing trapped
gasses.
 Application of a partial vacuum: Partial vacuum can be created by using
simple syringe or vacuum pump. (This is only recommended for ultra-low
turbidity measurement).