YSI 600DW-B Sonde - Manual (Page 295)

Frequently Asked Questions About YSI Drinking Water Systems Appendix H
Can I just make up a free chlorine standard from commercial bleach and use that to calibrate my
free chlorine sensor?
This procedure is not recommended and is likely to result in significant errors in your subsequent readings
for three reasons: (1) The concentration of hypochlorite on the bleach bottle is for new bleach batches and
may not be correct if the bleach has been on the shelf for varying periods of time; (2) Dilute solutions of
hypochlorite may be unstable and change during the calibration procedure; and (3) Because the free
chlorine sensor is flow dependent, it needs to be calibrated in water flowing at the same rate as will be used
in the continuous monitoring studies. This flow condition is almost impossible to meet by immersing the
sensors in stagnant or agitated solutions of bleach. It is important to understand that YSI may not be able
to provide customer support for users who do not perform their free chlorine sensor calibrations by the
recommended procedure.
Could I measure total chlorine with my colorimeter kit and then calibrate the YSI Free Chlorine
sensor to that value?
Subject to additional field trials as described above, you might be able to do this if you know that there is a
high percentage of combined chlorine in your water and you are willing to accept qualitative versus
quantitative results in terms of accuracy. Remember, however, that, even if the sensor does respond in
some way to combined chlorine, you would not really be measuring total or combined chlorine – only a
qualitative value for the oxidation capability of the water. In addition, it is likely that you would see
calibration errors when using the total chlorine value. Overall, it should be remembered that the YSI
sensor is designed to measure free chlorine, and, until additional research is performed, any attempts to
correlate the sensor output with other parameters is risky.
What accuracy does free chlorine sensor provide?
Immediately after calibration at any value, the sensor should show an accuracy of +/- 15 % of the reading
or 0.05 mg/L, whichever is greater, over the specified range of the sensor (0-3 mg/L). Subsequent
inaccuracy will be determined by the drift of the sensor, the extent of which can vary with the water being
monitored. It is important to remember that the accuracy specification provided for the free chlorine sensor
does not include long term drift even though our empirical studies in several water monitoring sites show
that the specification can be met in extended deployments as shown in the figures below. Inaccuracy of
the sensor from drift (determined from periodic colorimetric analysis) can easily be removed by
recalibration of the sensor at the time of quality checks.
YSI Incorporated Drinking Water Monitoring Systems Operations Manual H-9