MODEL SOLU COMP II
SECTION 6.0
CALIBRATION
6.3 CALIBRATION — OZONE
As Figure 6-1 shows, an ozone sensor generates a current directly proportional to the concentration of ozone in
the sample. Calibrating the sensor requires exposing it to a solution containing no ozone (zero standard) and to a
solution containing a known amount of ozone (full-scale standard).
The zero standard is necessary because ozone sensors, even when no ozone is in the sample, generate a small
current called the residual current. The analyzer compensates for the residual current by subtracting it from the
measured current before converting the result to an ozone value. New sensors require zeroing before being
placed in service, and sensors should be zeroed whenever the electrolyte solution is replaced. Either of the fol-
lowing makes a good zero standard:
•
Deionized water.
•
Tap water known to contain no ozone. Expose tap water to ozone-free air for several hours.
The purpose of the full-scale standard is to establish the slope of the calibration curve. Because stable ozone stan-
dards do not exist,
the sensor must be calibrated against a test run on a grab sample of the process liquid.
Several manufacturers offer portable test kits for this purpose. Observe the following precautions when taking and
testing the grab sample.
•
Take the grab sample from a point as close to the sensor as possible. Be sure that taking the sample does not
alter the flow of the sample to the sensor. It is best to install the sample tap just downstream from the sensor.
•
Ozone solutions are unstable. Run the test immediately after taking the sample. Try to calibrate the sensor
when the ozone concentration is at the upper end of the normal operating range.
FIGURE 6-1. Sensor Current as a Function of Ozone Concentration
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