Using Your DW Sonde with a 6500 Process Monitor
Appendix G
YSI Incorporated Drinking Water Monitoring Systems Operations Manual
G-6
Activate the “Sample and Hold Function” and Select Your Sample Interval.
The new DW sonde
software which is supplied in your instrument includes a new feature designated “Sample and Hold”.
Basically, when “Sample and Hold” is active, the 6500 display and the SCADA output of the 6500
becomes equivalent to the internal memory of the sonde while an Unattended study is running. In the
Unattended mode, the sonde “sleeps” between periodic samples with the interval between samples defined
by the user. This method of operation results the extension of both the sonde battery life (not important in
6500 applications) and the time between required sensor maintenance procedures (which is important to
6500 drinking water applications).
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 until 10:10, when the next readings would be
sent to the 6500.
The “Sample and Hold” feature can be activated in the
Advanced|Setup
menu of the sonde software while
attached to a PC or a 650 Display/Logger or can be activated directly from the
Setup sonde
menu of the
6500.
NOTE CAREFULLY: If the Sample and Hold feature is activated from a PC or 650 and
then the sonde is attached to a 6500, no interface will occur until the first point is taken using
the Sample and Hold mode. At the time the first point is taken, correct interface with the
6500 will take place and readings will be visible on the 6500 display. For example, if a sonde
is put into Sample and Hold mode with a 15 minute interval at 15:05 and then immediately
attached to a 6500, readings will not be visible on the 6500 display until 15:15, i.e. the time of
the next reading in the Sample and Hold mode.
If activating the feature directly from the 6500, press the
Esc
and
Enter
keys simultaneously to activate the
Main menu of the 6500 and highlight the
Setup sonde
selection and press
Enter
. Then highlight the
Sample and Hold
entry and press
Enter
to activate the feature as evidenced by a checkmark in the box.
After activating the
Sample and Hold
feature, you will also be required to enter a time interval between
samples during which time the 6500 display and 4-20 mA and Modbus outputs will be frozen. Use the
right/left arrow keys on the 6500 to move to the proper digits and then the up/down arrow keys to input the
proper value. Then press the
Enter
key to confirm your interval selection. Our empirical experience is
that most parameter change events in drinking water systems are observed with a 15 minute sample interval
(the default value), but, to be sure, you may want to select a 5 minute sample interval in the 6500 System
Status menu. Sample intervals of less than 5 minutes are not recommended when using your DW sonde
with a 6500 in long term monitoring studies.
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