Section 9: Calibration
Instruction Manual
April 2017
LIQ-MAN-1066
80
Calibration
Zeroing the Sensor
The adjacent screen will appear during Zero Cal. Be sure sensor has been
running in zero solution for at least two hours before starting zero step.
The adjacent screen will appear if In Zero Cal is successful. The screen will
return to the Amperometric Cal Menu.
The adjacent screen may appear if In Zero Cal is unsuccessful. The screen
will return to the Amperometric Cal Menu.
In Process Calibration
The adjacent screen will appear prior to In Process Cal
If the In Process Cal is successful, the screen will return to the Cal sub-
menu.
The adjacent screen may appear if In Process Cal is unsuccessful. The
screen will return to the Amperometric Cal Menu.
Calibration – Oxygen
Oxygen sensors generate a current directly proportional to the concentration of dissolved oxygen
in the sample. Calibrating the sensor requires exposing it to a solution containing no oxygen (zero
standard) and to a solution containing a known amount of oxygen (full-scale standard). The Zero
Calibration is necessary because oxygen sensors, even when no oxygen is present in the sample,
generate a small current called the residual current. The transmitter compensates for the residual
current by subtracting it from the measured current before converting the result to a dissolved
oxygen value. New sensors require zeroing before being placed in service, and sensors should be
zeroed whenever the electrolyte solution is replaced. The recommended zero standard is 5%
sodium sulfite in water, although oxygen-free nitrogen can also be used. The 499A TrDO sensor,
used for the determination of trace (ppb) oxygen levels, has very low residual current and does not
normally require zeroing. The residual current in the 499A TrDO sensor is equivalent to less than
0.5 ppb oxygen. The purpose of the In Process Calibration is to establish the slope of the
calibration curve. Because the solubility of atmospheric oxygen in water as a function of
temperature and barometric pressure is well known, the natural choice for a full-scale standard is
air-saturated water. However, air-saturated water is difficult to prepare and use, so the universal
practice is to use air for calibration. From the point of view of the oxygen sensor, air and air-
9.6.4
9.6.5
9.7
1.234 µS/cm 25.0 ºC
S
N
Zero Cal
Zeroing
Wait
1.234 µS/cm 25.0 ºC
S
N
Zero Cal
Sensor zero failed
Press EXIT
1.234 µS/cm 25.0 ºC
S
N
Zero Cal
Sensor zero done
1.234 µS/cm 25.0 ºC
S
N
InProcess Cal
Calibration
Error
Press EXIT
1.234 µS/cm 25.0 ºC
S
N
InProcess Cal
Wait for stable
reading.
