wtw pHotoFlex, Operating Manual

Page: 62 / 111

Operation pHotoFlex / pHotoFlex Turb
62 ba75491e05 04/2009
4.7 Turbidity
4.7.1 General information
Venting the sample Air bubbles in the sample affect the measuring result to a massive ex-
tent because they have a large scattering effect on the incident light.
Larger air bubbles cause sudden changes in the measured values
whereas smaller air bubbles are recorded by the instrument as turbidi-
ty. Therefore, avoid or remove air bubbles:
Avoiding or removing
air bubbles
z
During sampling, ensure all movement is kept to a minimum
z
If necessary, vent the sample (ultrasonic baths, heating or adding a
surface-active substance to reduce the surface tension)
Note
For the measurement of turbidity values under 1 FNU/NTU, please also
note the Appendix 2 on page 110.
4.7.2 Aligning and marking a cell
Even completely clean quality cells exhibit tiny directional differences
in their light transmittance. Therefore, if you want to achieve accurate
and reproducible measurement results, it is necessary to always align
the sample cells and cells for calibration standards in the same way
(see section 2130 of the "Standard Methods for the Examination of Wa-
ter and Wastewater", 19th edition).
To do so, the optimum alignment of the cell is determined.
Note
Never apply oily liquids (or so-called "special silicone oils") in order to
"smooth" possible scratches. They would unnecessarily soil the meter
and your working environment. The measurement accuracy is ensured
by aligning the cells. Scratched cells have to be replaced.
Aligning the cell
1 Press the <M> key (long pressure) repeatedly until the Turbid-
ity measuring mode is selected.
2 Clean the cell (see section 5.2.2).
3 Insert the cell (see section 4.2).