YSI 600DW-B Sonde - Manual (Page 221)

Principles of Operation Section 5
operation is NOT RECOMMENDED by YSI due to potential sensor maintenance issues as
described in detail in Appendix G of this manual.
4. Applications where the sonde is attached to a PC and data is being displayed to the
computer monitor using the Real-Time mode of Ecowatch for Windows. For this mode of
operation, your must calibrate the free chlorine sensor before activating the Real-Time EcoWatch
mode of EcoWatch. Access the sonde menu after connection of the sonde to a PC or 650 Display
logger and make certain that the Autosleep RS232 feature is activated in the Advanced|Setup
menu and that the Chlorine warm-up time is set to 60 seconds in the Advanced|Sensor menu.
With these software entries in place, the user will input the free chlorine value from the DPD
analysis of the flow cell effluent, and the unit will automatically calibrate after the selected warm
up time. After calibration, select the Run|Discrete Sample entry from the sonde menu and set the
Discrete Sample interval to the value corresponding to the frequency at which you wish data to be
logged to the hard drive of your PC. Then click on the Real-Time entry in the top EcoWatch
toolbar (Do NOT click on the sonde icon in the lower toolbar). The Real-Time study will start
automatically with data displayed graphically on the PC monitor and logged to the hard drive of
the PC under the file name which you have selected. See Section 4.3.5 of this manual for detailed
instruction in using the EcoWatch Real-Time mode.
The free chlorine readings are affected by temperature (approximately 2.5 % per degree Celsius) due to the
effect of temperature on the diffusion of free chlorine through the membrane. Note, however, that the sonde
software automatically carries out this compensation using the Chlorine tempco factor found in the
Advanced|Sensor menu. The default value of this factor should NOT be changed except under unusual
conditions and only then after contacting YSI Technical Support for advice.
FLOW DEPENDENCE
As noted above, the Rapid Pulse free chlorine sensor exhibits significant flow dependence (20-30 %) in
stagnant water. This dependence is overcome by a water flow rate through the flow cell of approximately
500 mL/minute and therefore lower flow rates are not recommended. Note also that this characteristic
means that you must use the free chlorine sensor in flowing water utilizing YSI flow cells in order to
realize the stated accuracy specification.
EFFECT OF PH
The YSI free chlorine sensor appears to be less sensitive to OCl
-
than it is to HOCl and as the pH rises,
there is no question that the amount of HOCl relative to OCl - is reduced significantly. This means that the
sensitivity of the sensor to the mixture of HOCl and OCl - is reduced at high pH, but the sensor has been
shown empirically to still have enough overall sensitivity to be functional at pH values up to ca. 9.3 and
data to support this claim is available on request from YSI. (Note, however, that at pH values above 8.8,
there is a possibility that calibration errors might occur. If you see these errors, contact YSI Technical
Support for advice on whether to override them.)
The YSI philosophy relative to the use of the free chlorine sensor at high pH is to calibrate the sensor under
these conditions of reduced sensitivity in the high pH water and then assume that minor variations in pH
will have only a minimal effect on accuracy during the monitoring study. This philosophy has been
adopted rather than compensating for changes in pH using the on-board pH sensor in the sonde. It was
decided not to do pH compensation because chlorine is usually the most important parameter to the
drinking water industry and the accuracy of this parameter would be grossly compromised if a pH sensor
failed while providing compensation data during long term monitoring studies. In other words, we feel
that the risk of pH probe failure or significant drift is more likely to be a problem to users than the smaller
inaccuracies in the uncompensated pH data which might result from small changes in pH. Note that the
YSI Incorporated Drinking Water Monitoring Systems Operation Manual 5-8