YSI 5200 - Manual (Page 112)

YSI Incorporated 110 5200 Recirculating System Monitor
11.4 Sensor Care and Maintenance
Once the sensors have been properly installed, remember that periodic cleaning and DO membrane
changes are required.
11.4.1 DO Sensor
The YSI polarographic dissolved oxygen probe 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 the silver surfaces of the
probe 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.
Our experience in water quality monitoring suggests that DO readings will begin to deteriorate from this
electrochemical action after about 30–90 days under the continuous operation associated with the 5200
system.
For best results, we recommend that the KCl solution and the membrane cap be changed at least once
every 30–90 days.
1. It is important to recognize that oxygen dissolved in the sample is consumed during sensor
operation. It is therefore essential that the sample continuously flow pass the sensor at a rate of at
least 0.5 feet per second. If stagnation occurs, your readings will be artificially low.
2. Membrane life depends on usage. Membranes will last a long time if installed properly and
treated with care. Erratic readings are a result of loose, wrinkled, damaged, or fouled membranes,
or from large (more than 1/8" diameter) bubbles in the electrolyte reservoir. If erratic readings or
evidence of membrane damage occurs, you should replace the membrane and the electrolyte
solution.
3. If the membrane is coated with oxygen consuming (e.g. bacteria) or oxygen producing organisms
(e.g. algae), erroneous readings may occur.
4. Chlorine, sulfur dioxide, nitric oxide, and nitrous oxide can affect readings by behaving like
oxygen at the sensor. If you suspect erroneous readings, it may be necessary to determine if these
gases are the cause.
5. Avoid any environment that contains substances that may attack the probe module and sensor
materials. Some of these substances are concentrated acids, caustics, and strong solvents. The
sensor materials that come in contact with the sample include FEP Teflon, acrylic plastic, EPR
rubber, stainless steel, epoxy, polyetherimide and the PVC cable covering.
6. It is possible for the silver anode, which is the entire silver body of the sensor, to become
contaminated. This will prevent successful calibration. To restore the anode, refer to Section
11.4.1 DO Sensor, Silver Anode Cleaning.
7. For correct sensor operation, the gold cathode must always be bright. If it is tarnished (which can
result from contact with certain gases), or plated with silver (which can result from extended use
with a loose or wrinkled membrane), the gold surface must be restored. To restore the cathode,
refer to Section 11.4.1 DO Sensor, Gold Cathode Cleaning.
Silver Anode Cleaning
After extended use, a thick layer of AgCl builds up on the silver anode reducing the sensitivity of the
sensor. The anode must be cleaned to remove this layer and restore proper performance. The cleaning can
be chemical or mechanical: