LaMotte 2020i Turbidimeter - Manual (Page 36)

Tubes
The 2020 uses one type of tube (Code 0290) for the turbidity test. There is no need
for a special turbidity tube.
The handling of the tubes is of utmost importance. Tubes must be clean and free from
lint, fingerprints, dried spills and significant scratches, especially the central zone
between the bottom and the sample line.
Scratches, fingerprints and water droplets on the tube can cause stray light interference
leading to inaccurate results when measuring turbidity. Scratches and abrasions will
affect the accuracy of the readings. Tubes that have been scratched in the light zone
through excessive use should be discarded and replaced with new ones.
Tubes should always be washed on the inside and outside with mild detergent prior to
use to remove dirt or fingerprints. The tubes should be allowed to air-dry in an inverted
position to prevent dust from entering the tubes. To prevent introducing moisture into
the meter chamber, tube positioning rings should be removed before washing tubes. If
tubes are washed with tube positioning rings in place, the rings should be removed
and thoroughly dried before replacing them on the dry tubes. Dry tubes should be
stored with the caps on to prevent contamination.
After a tube has been filled and capped, it should be held by the cap and the outside
surface should be wiped with a clean, lint-free absorbent cloth until it is dry and
smudge-free. Handling the tube only by the cap will avoid problems from fingerprints.
Always set the clean tube aside on a clean surface that will not contaminate the tube. It
is imperative that the tubes and light chamber be clean and dry. The outside of the
tubes should be dried with a clean, lint-free cloth or disposable wipe before they are
placed in the meter chamber.
Tubes should be emptied and cleaned as soon as possible after reading a sample to
prevent deposition of particulates on the inside of the tubes. When highly accurate
results are required, reduce error by designating tubes to be used only for very low
turbidity and very high turbidity testing.
Variability in the geometry of the glassware and technique are the predominate causes
of variability in results. Slight variations in wall thickness and the diameter of the tubes
may lead to slight variations in the test results. To eliminate this error the tubes should
be placed in the chamber with the same orientation each time. The orientation of the
tubes in the chamber is controlled by use of a tube positioning ring. For improved
accuracy and precision, especially at low concentrations, the tubes should always be
used with a positioning ring. (See page 32)
36 TUBES