YSI Incorporated
Model 5000/5100 Service Manual
14
5.4 ZERO
CALIBRATION
All oxygen probes have a small background current, even in the absence of oxygen. The Model
5000/5100’s default zero value is based on the average probe’s background current. This default
value may cause errors when measuring low concentrations of oxygen.
For highest accuracy measurements, a zero calibration should be performed to compensate for
the specific background current of the probe in use.
To calibrate to a true zero, place the probe in a zero oxygen environment and adjust the
calibration value to zero.
A standard method for creating such an environment is to dissolve in water excess sodium sulfite
(Na
2
SO
3
), and a trace of cobalt chloride (CoCl
2
). These chemicals will remove all oxygen from
the sample (See Standard Methods for the Examination of Water & Wastewater, method 4500-O
G, 19th edition). Alternatively, you may place the probe in 100% nitrogen gas.
1. Place the probe in the zero oxygen sample and allow at least 20 minutes for the probe to
come to equilibrium.
2. Press the [CALIBRATE] soft-key to change to Calibration mode. The following screen will
be displayed.
98.0
*
% 8.15
mg/L
Calibration
0.0
ppt
740
mmHg
24.38°C 11:33
AM
AUTO CAL
DO CAL
SETUP
DIAGNOSTICS
3. Press the [DO CAL] soft-key to enter the manual DO calibration menu.
000.0
*
% 0.00
mg/L
Cal %
0.0
ppt
740
mmHg
24.38°C 11:33
AM
UP
DOWN
DIGIT
NEXT
4. Make sure that the display readings are stable, then use the [UP], [DOWN] and [DIGIT] soft-
keys to enter the calibration value of 0.0% (or 0.00 mg/L).
NOTE: The instrument will not except any value other than zero for the first point of a two-point
calibration.
5. Press [ENTER] to confirm your calibration. The screen will momentarily display “D.O.
Calibration saved”.
6. You must now recalibrate in an oxygen environment (see
Auto Cal
or
DO Cal
).
NOTE:
When you change the zero calibration point, you offset the other value, so that after
zeroing the probe for zero oxygen, you must recalibrate in an oxygen environment.
