Instruction manual

MODEL XMT-A-FF/FI SECTION 13.0
CALIBRATION - OZONE
SECTION 13.0
CALIBRATION — OZONE
13.1 INTRODUCTION
As Figure 13-1 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 or zero current. The transmitter 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. The best zero
standard is deionized water.
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 13-1. Sensor Current as a Function of Ozone Concentration
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