YSI 6500 Environmental Process Monitor - Manual (Page 54)

Proper Use and Care of the 6500
Section 5
shown on the 6500 display with those taken by recently-calibrated single parameter instruments.
For dissolved oxygen, the probe of a handheld DO instrument similar to the YSI Model 55 can be
placed in the stream near the 6-Series Sonde and the reading recorded. If a comparison of the
readings from the recently-calibrated DO meter and the 6500 indicates a drift of the deployed
instrument, the 6500/6-Series system can be recalibrated to reflect the DO meter reading using the
6500 interface at the site. Note, however, that this comparison should only be used in a reset of
the calibration if the 6-Series Sonde has been cleaned of all algae and other debris prior to the
determination. If a gross variation is observed between the two instruments (> 1 mg/L), it is
usually a sign that maintenance is required on the 6-Series DO sensor.
For pH, a similar field calibration adjustment can be made using a YSI Model 60 or 63 field pH
meter. Alternatively, a sample of the stream can be taken to the laboratory for evaluation of the
true pH. After this value is determined, you should immediately return to the deployment site
and, leaving the 6-Series Sonde in the water, perform a single point pH calibration using the 6500
interface and inputting the laboratory pH as the calibration value. It is important to remember,
however, that this “adjustment” of DO and pH values to those determined by recently calibrated
single parameter instruments should only be used to compensate for minor drifts. It is not a
substitute for the detailed quality assurance procedure outlined above which confirms proper
sensor function and which should always be performed on a regular basis.
5.2.4 RECOMMENDED MONTHLY MAINTENANCE OF THE DO
PROBE
The YSI polargraphic Rapid Pulse dissolved oxygen probe installed in the
6-Series Sonde is a Clark-type sensor in which the reduction of oxygen at
the gold cathode is accompanied by a corresponding oxidation of a silver
anode to silver chloride (AgCl). This natural deposition of AgCl will be
seen initially as a slight darkening of one of the silver surfaces on the probe
face with no compromise in sensor accuracy. Eventually, however, the
AgCl coating will become so thick that it will affect the function of the
sensor, usually resulting in erroneously low and noisy DO readings.
gold cathode
silver anodes
Face of DO Probe
IMPORTANT!
Periodic resurfacing of the DO sensor is required in order to obtain accurate sensor output.
Our experience in water quality monitoring suggests that DO readings will begin to deteriorate
from this electrochemical action after about 30 days under the continuous operation associated
with the 6500 system. With this in mind, we recommend that users of the 6500/6-Series system
establish a regular monthly schedule of sensor resurfacing as described in Section 5, Proper Use
and Care of the 6500 Monitor System, even if no problems are evident. The resurfacing
procedure is easy to carry out and takes only about 20 minutes, including the recommended
break-in prior to recalibration of the sensor.
YSI Incorporated 6500 Environmental Process Monitor 51