MODEL SOLU COMP II SECTION 8.0
TROUBLESHOOTING
8.3.7 Ozone readings are too low.
A. Was the sample tested as soon as it was taken? Ozone solutions are unstable. Test the sample immediately
after collecting it.
B. Low readings can be caused by zeroing the sensor before the residual current has reached a stable minimum
value. Residual current is the current the sensor generates even when no ozone is in the sample. Because
the residual current is subtracted from subsequent measured currents, zeroing before the current is a mini-
mum can lead to low results.
Example: The true residual current for an ozone sensor is 4 nA, and the sensitivity is 350 nA/ppm. Assume the
measured current is 200 nA. The true concentration is (200-4)/350 or 0.560 ppm. If the sensor was zeroed pre-
maturely when the current was 10 nA, the measured concentration will be (200-10)/350 or 0.543 ppm. The
error is 3.6%. Suppose the measured current is 100 nA. The true concentration is 0.274 ppm, and the meas-
ured concentration is 0.257 ppm. The error is now 6.2%. The absolute difference between the reading remains
the same, 0.017 ppm.
C. Sensor response depends on flow. If the flow is too low, readings will be low and flow sensitive. Verify that the
flow past the sensor equals or exceeds the minimum value. See the sensor instruction manual for recom-
mended flows.
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