53
SECTION 5,
continued
5.4.2
Zeroing the Probe
A new DO probe can generate a 0.02 to 0.05 mg/L positive error
in an oxygen-free (anoxic) solution. If this level of error is
unacceptable, zero the meter with the following procedure when:
•
Using a new sensing-membrane
•
Using fresh internal filling solution
•
Measuring dissolved oxygen levels less than 1 mg/L or 10%
saturation
1. Measure about 150 mL of sample or deionized water into a
250-mL beaker. Add a magnetic stir bar.
2. Add 0.25 g sodium sulfite or the contents of one Silica 3
Reagent Powder Pillow to the water. Stir to dissolve
the reagent.
3. Catalyze the reduction of dissolved oxygen by adding 0.1 mL
of a 1000 mg/L Cobalt Standard solution to the water.
4. Place the probe in the stirring sample for at least 10 minutes.
This solution is good for 30 minutes or more.
5. Press the
CAL
key. The Cal icon will appear in the upper left
corner of the display, a flashing question mark will appear in
the upper right corner of the display, and the
keypad
icon will
appear in the lower left corner of the display. The main
display will show
100%
.
6. Press the
0
key on the keypad then press
ENTER
.
7. The meter shows the salinity correction factor. Make sure it is
set to zero and press
ENTER
.
8. The meter shows
Stabilizing...
while readings are taken. When
the meter’s zero DO criteria have been met it will return to
the reading mode. The meter will not exit the zeroing routine
until the meter’s zero criteria have been met.
9. If the meter cannot complete the zeroing procedure it will
begin to beep and show the faulty probe icon. If the meter
does not complete the zeroing procedure and exit to the
