LaMotte DO 4000 Dissolved Oxygen Meter - Manual (Page 15)

ZERO CALIBRATION
Dissolved Oxygen meters often include a zero point calibration
procedure. Generally the most common source of zero point drift is
filling solution contamination. There are two methods to correct this
problem. The recommended method is to change the filling solution
(see Membrane Replacement page 8). The second method is to calibrate
out the error caused by the contamination. Calibrating out the error not
only overlooks the source of the problem but leads to errors especially
significant when testing very low oxygen solutions.
Use this zero calibration only when contamination of the filling solution
is suspected.
ZERO CHECK
1. Add 0.1 g spoon (0699) of Sodium Sulfite (3970) and several
crystals of Cobalt Chloride (3971) to about 100 mL of tapwater. Mix
thoroughly. Wait one minute.
2. Select “ppm” on meter to turn the meter on. Immerse probe in
solution and allow reading to stabilize. A reading of 0.1 ppm or more
could indicate significant contamination and require a filling
solution change.
3. With probe still in the Zero Oxygen solution prepared in Step 1,
remove hole plug on side of meter and adjust the “Zero Current
POT” with a slotted screw driver (1/8" or smaller) until meter reads
0.00 ppm.
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