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SECTION 3, continued
3.2.5 Measuring Overrange Samples
Nephelometric turbidity measurement depends on detection of light
scattered from particles suspended in the liquid. If the turbidity is very
high, a significant amount of light is blocked or absorbed by the
particles and only a small amount of light reaches the detector. The
measured turbidity is then lower than the actual turbidity (negative
interference). This condition is called “going blind”. The 2100P ISO
Turbidimeter minimizes this effect and extends the instrument range.
Dilution of highly turbid samples should be avoided since it may alter
the characteristics of the suspended particles and produce
erroneous results.
Light-absorbing particles such as activated carbon and highly colored
samples may also cause an instrument to “go blind”. Dilution may not
correct for these interferences. The 2100P ISO will correct for the
presence of light absorbing particles and color.
3.2.6 Condensation (Fogging)
Condensation may occur on the outside of the sample cell
when measuring a cold sample in a warm, humid environment.
Condensation interferes with turbidity measurement, so wipe moisture
off of the sample cell before measurement. If fogging recurs, let the
sample warm slightly at room temperature, or immerse in a warm bath
for a short period. After warming, mix the sample thoroughly before
measurement. Warming samples can alter sample turbidity, so avoid
warming before measurement if possible.
3.2.7 Representative Sampling
A representative sample accurately reflects the condition of the water
source from which the sample was taken. To ensure a representative
sample, gently but thoroughly mix every sample before aliquots are
taken. Do not allow the sample to settle.
When sampling from a tap in a distribution system or treatment plant,
allow the water to run for at least five minutes before sampling.
When sampling from a stream, reservoir, clarifier, or storage tank,
collect at least one liter (l quart) and thoroughly mix before
measurement. If the water source is not uniform, it may be necessary
to sample several locations at varying depths and combine the samples
into a single, well-mixed composite sample before measurement.
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