32
SECTION 2, continued
Use secondary standards for periodic calibration checks. Please note
that Gelex
®
standards must be assigned values after StablCal Stabilized
Formazin calibration or formazin calibration and before use as
secondary standards. Gelex standards must be recalibrated (values
assigned) each time the instrument is calibrated with StablCal
Stabilized Formazin or formazin. See Section 3.6 on page 37 for
detailed information on the use of StablCal Stabilized Formazin,
formazin, and Gelex standards.
2.3.9 Representative Sampling
A representative sample accurately reflects the true 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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