LaMotte TC-3000 Tri-Meter - Manual (Page 65)

NEGATIVE RESULTS
There are always small variations in readings with analytical instruments. Often these
variations can be observed by taking multiple readings of the same sample. These
variations will fall above and below an average reading. Repeated readings on a 0.00
sample might give readings above and below 0.00. Therefore, negative readings are
possible and expected on samples with concentrations at or near zero. This does not
mean there is a negative concentration in the sample. It means the sample reading
was less that the blank reading. Small negative readings can indicate that the sample
was at or near the detection limit. This is a normal variation that results in a negative
reading. A large negative reading, however, is not normal and indicates a problem.
Some instruments are designed to display negative readings as zero. In this type of
instrument, if the meter displayed zero when the result was actually a large negative
number there would be no indication that a problem existed. For this reason, the
TC-3000 displays negative numbers.
TUBES
The TC-3000 uses one type of tube (Code 0290) for the turbidity, color and chlorine
tests. 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 especially when measuring turbidity. Scratches and
abrasions will affect the accuracy of the readings for all test factors. 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 stains or particulates on the inside of the tubes. Reacted chlorine
samples at high concentrations will stain the tubes. When highly accurate results are
required, reduce error by designating tubes to be used only for chlorine, 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
NEGATIVE RESULTS • TUBES 65