YSI 600DW-B Sonde - Manual (Page 290)

Frequently Asked Questions About YSI Drinking Water Systems Appendix H
greater than 500 mL per minute and is relatively constant. However, the varying flow conditions which
exist directly in water storage tanks (from completely stagnant to significant flow during filling and
emptying operations) will result in significant inaccuracies in the free chlorine data. These errors could be
up to 30 % of the reading depending on the time of calibration, in addition to the +/- 0.15 mg/L accuracy
specification associated with the sensor when used in flowing water. In addition, if the chlorine content of
the tank is stratified, obtaining a representative sample for calibration my also be difficult and would result
in an additional error component. Users with tank applications should be prepared for levels of error
significantly higher than those specified for flow cell applications if they suspend their drinking water
sondes directly in storage tanks.
INTERFACING TO YSI DRINKING WATER SONDES
What modes of operation can be used with the YSI Drinking Water Sondes?
As described below, the YSI DW sondes are designed to operate in a variety of modes while acquiring long
term continuous data.
• Stand-alone – In this mode the sonde is set up and calibrated off line (except for free chlorine, see
below) and then installed at the site in the appropriate flow cell with the sonde powered by internal
batteries. An Unattended study is then begun at a suitable interval (usually 5-15 minutes) and
data is logged to internal memory for the length of the study. The data can be uploaded
periodically for after-the-fact viewing using either a PC or a YSI 650 Display/Logger.
• Attached to a PC for Real-Time Graphic Data Display – In this mode, after calibration the
sonde is attached to a PC which is powered externally and has had the YSI software EcoWatch for
Windows installed. Using EcoWatch, a real-time study is begun with data logged automatically
to the hard drive of the PC at the desired interval as well as displayed graphically on the monitor
of the computer in real-time.
• Attached to a YSI Process Monitor for Use with SCADA Systems – In this mode, the sonde is
placed in the flow cell and then attached to the 6500 which is AC powered at the site. The data
from the sonde sensors are automatically displayed on the screen of the 6500 for real-time viewing
in non-graphical form. The sensor signals can also be converted by the 6500 hardware/software
into either 4-20 mA or Modbus protocols for interface to a SCADA system. While data can be
output continuously to the 6500 (and to the SCADA), YSI strongly recommends the use of
the “Sample and Hold” feature of the system which outputs data to the 6500 (and the SCADA)
at a user selected interval with the sensors “turned off” between readings. There are two reasons
for this strong recommendation:
1. Because of the hydrophilic nature of the chlorine sensor membrane, cross-talk between
the chlorine sensor electrodes and the pH sensor can occur. If cross-talk is present, it is
likely to reduce the life of the pH sensor if it is run in the continuous mode.
2. The oxidation of the anode of the free chlorine sensor is the factor which normally
dictates maintenance on the sensor and the extent of the oxidation is proportional to the
time the sensor is polarized. Thus, if the sensor is only polarized for 20 % of the time,
the time between sensor reconditioning will be extended by approximately a factor of
five.
The only disadvantage of the “Sample and Hold” feature is that the data output to the
6500/SCADA from all sensors is “frozen” for the sample interval. Thus, if a study were begun at
10:00 at a 5 minute interval, the sensor readings shown by the 6500 (and output to the SCADA)
which were taken at 10:00 would remain the same (even though they might be changing slightly)
until 10:05, when new sensor readings would be output. These readings in turn would be frozen
YSI Incorporated Drinking Water Monitoring Systems Operations Manual H-4