Xylem TURB 430 PORTABLE TURBIDITY METER - Manual (Page 33)

Turb 430 IR/T Operation
33 ba75507e03 07/2006
4.5 Turbidity
4.5.1 General information
Venting the sample Air bubbles in the sample affect the measuring result to a massive
extent 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
turbidity. Therefore, avoid or remove air bubbles:
Avoiding or removing
air bubbles
! During sampling, ensure all movement is kept to a minimum
! If necessary, vent the sample (ultrasonic baths, heating or adding a
surface-active substance to reduce the surface tension)
Note
For turbidity values under 1 FNU/NTU, follow the instructions given in
the appendix (see A
PPENDIX : T URBIDITY VALUES UNDER 1 FNU/NTU).
4.5.2 Aligning and marking a cell
Even perfectly 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 index the
sample cells and cells for calibration standards in the same way (see
section 2130 of "Standard Methods for the Examination of Water and
Wastewater", 19th edition).
To do so, the optimum alignment of the cell is determined.
Aligning the cell
Note
To keep the drift as low as possible, the time for aligning the cell while
pressing and holding the <START/ENTER> key is limited to 30
seconds. After this time, the meter starts measuring automatically.
1 Clean the cell (see section 5.2.2).
2 Insert the cell (see section 4.2).
3 Align the cell:
! Press and hold the <START/ENTER> key.
! Slowly and in small steps rotate the cell by one complete
rotation (by 360 °).
After each step wait for a short time until the displayed
measured value is stable.
! Turn the cell back to the position with the lowest measured
value.