Hach 2100P Instrument And Procedure Manual page 41

SECTION 3,
continued
the dilution water turbidity is greater than 0.5 NTU. In this case, prepare
the water as directed below.
The value of the dilution water can be arbitrarily forced to zero (see
calibration procedure). This is not recommended for most applications
and, if done, should be done only if the dilution water turbidity is less
than 0.2 NTU.
3.6.2.3 Preparing Dilution Water
Note: Use the same dilution water for all dilutions and the sample blank.
Collect at least 1000 mL of high quality dilution water (distilled or
deionized water). The 2100P Turbidimeter, as received from the factory,
is precalibrated and may be used to check the dilution water turbidity. If
the turbidity is greater than 0.5 NTU, filter the water with the Sample
Filtration and Degassing Kit (Cat. No. 43975-10) or equivalent. When
measuring low range turbidity, clean all glassware with 1:1
hydrochloric acid and rinse several times with dilution water. If the
glassware is not used immediately, use stoppers to prevent
contamination from small particles.
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