LaMotte 2020wi Portable Turbidity Meters - Manual (Page 46)

13. Open the meter lid. Insert the
tube into the chamber. Align
the index line on the tube with
the index arrow on the meter.
Close the lid.
14. Press
ENTER
to scan the
standard. The screen will
display Reading for about 1
second. The result will appear
on the screen.
Turbidity WB (F)
0.99
NTU
Scan Blank
Scan Sample
12:00:00 001/500
NOTE: The meter will remember the last scanned blank reading. It is not
necessary to scan a blank each time the test is performed. To use the
previous blank reading, instead of scanning a new one, scroll to Scan
Sample and proceed. For the most accurate results, the meter should
be blanked before each test and the same tube should be used for the
blank and the reacted sample.
 DILUTION PROCEDURES
If a sample is encountered that is more than 4000 NTU or FNU, a careful
dilution with 0 NTU/FNU or very low turbidity water will bring the sample
into an acceptable range. However, there is no guarantee that halving
the concentration will exactly halve the NTU or FNU value. Particulates
often react in an unpredictable manner when diluted.
Turbidity-Free Water
The defi nition of low turbidity and turbidity-free water has changed as
fi lter technology has changed and nephelometric instruments have
become more sensitive. At one time turbidity-free water was defi ned as
water that had passed through a 0.6 micron fi lter. Now 0.1 micron fi lters
are available and higher purity water is possible. Water that has been
passed through a 0.1 micron fi lter could be considered particle free and
therefore turbidity free, 0 NTU water. Turbidity is caused by scattered
light. Therefore, low turbidity water is water without any particles that
scatter a measurable amount of light. But water that passed through
a 0.1 micron fi lter may still have detectable light scatter with modern
instruments. This light scattering can be the result of dissolved
molecules or sub-micron sized particles that can not be fi ltered out of
the water. Because there may still be a small amount of scattered light
from dissolved molecules, high purity water is often called low turbidity
water and assigned a value of 0.01 or 0.02 NTU. However, because this
water is used as a baseline to compare to sample water, the difference
between the sample and the low turbidity or turbidity-free water will
46
Analysis