Hach-Lange ORBISPHERE 31xxx Installation and Maintenance - Manual (Page 33)

31
Maintenance
5.3.3 Anode and cathode nitric acid cleaning
5.3.4 Sensor face polishing
Check for the absence of silver deposit on the central guard
ring electrode walls, as such deposits can make contact with
the counter electrode.
To eliminate any silver residue inside the sensor cell, it is
sometimes required to repeat the chemical cleaning using
nitric acid (HNO
3
, not over 70% by weight).
Also, the 32301 electrochemical cleaning does not remove
deposit on the cell’s plastic parts, so nitric acid cleaning may
be required.
Note: This procedure is not recommended for normal maintenance,
and should not be used more often than twice a year, as the acid
degrades the metal of the counter electrode, thus reducing the
sensor's life.
Place concentrated nitric acid into the sensor electrolyte
reservoir, and add 1 drop on the center electrode.
Leave for no longer than 3 seconds.
Quickly empty the acid and rinse thoroughly under tap water
for one minute.
CAUTION
Nitric acid is dangerous! Please refer to the safety
information from your chemical supplier.
Once the sensor has been cleaned, the face of the center
electrode must be polished together with the membrane
support.
Note: Install the membrane support with the groove on the upper
side. The membrane support is individually machined and paired with
the sensor. Therefore make sure that the correct membrane support
is used on the correct sensor.
Insert the prongs of the
membrane support removal
tool into the membrane
support holes.
Tighten the membrane
support finger tight.
CAUTION
Too much torque will damage the sensor electrodes