Using the Turbidimeter

The turbidimeter measures the light transmittance of a sample in NTU's (Nephelometric Turbidity Units, a standard measure). It needs no field calibration. Handle the sample vials only by their ends (preferably the lid) so as not to affect the transmittance; wipe any fingerprints, spots, etc. from the outside of the vial; and be sure to close the vial-compartment lid when taking a measurement. The turbidimeter should display ``AUTO RNG'' (for auto-range selection) and ``SIG AVG'' (for take an average reading) when ready for use (Fig. 4.7).

Figure 4.7: Turbidimeter controls and display. Press ``READ'' when ready to take a measurement. After Hach Model 2100P Turbidimeter Instruction Manual, 1997, p. 24.

Use the following procedure when measuring the turbidity of your sample:

1. Fill turbidity vial (has white line around top of glass with downward arrow) to the line (about 15 mL) with unfiltered water. Take care to handle the sample cell by the top. Cap the cell.

2. Wipe the cell with a soft, lint-free cloth to remove water spots and fingerprints.

3. Press I/O - the instrument will turn on. Place the instrument on a flat surface. Do not hold the instrument while making readings.

4. Put the sample vial in the instrument cell compartment so its diamond mark aligns with the raised orientation mark in front of the cell compartment. Close the cover.

5. Set automatic range by pressing the RANGE key. The display will show ``AUTO RNG''.

6. Select signal averaging (reports average of 10 measurements) by pressing ``Signal Average'' key, display should show ``SIG AVG''

7. Press READ - the display will show ``--NTU'' and the light bulb icon will flash 10 times (once for each reading). The final average will be displayed as numbers in NTU after the lamp symbol turns off.