YSI 556 MPS - Manual (Page 97)

Maintenance Section 11
YSI Incorporated YSI 556 MPS Page 91
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
11.1.1 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 11.1.1 DO Sensor, Gold Cathode Cleaning.
8. To keep the electrolyte from drying out, store the sensor in the
transport/calibration cup with at least 1/8″ of water.
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:
Chemical Cleaning: Remove the membrane cap and soak the entire anode
section in a 14% ammonium hydroxide solution for 2 to 3 minutes, followed
by a thorough rinsing with distilled or deionized water. The anode should
then be thoroughly wiped with a wet paper towel to remove the residual
layer from the anode.
Mechanical Cleaning: Sand off the dark layer from the silver anode with
400 grit wet/dry sandpaper. Wrap the sandpaper around the anode and twist
the sensor. Rinse the anode with clean water after sanding, followed by
wiping thoroughly with a wet paper towel.
NOTE: After cleaning, a new membrane cap must be installed. Refer to
Section 3.4.3 Membrane Cap Installation.
Turn the instrument on and allow the system to stabilize for at least 30
minutes. If, after several hours, you are still unable to calibrate, contact your
dealer or YSI Customer Service. Refer to Appendix E Customer Service.