Thermo Fisher Scientific CyberScan DO 110 - Manual (Page 55)

Instruction Manual CyberScan DO 110
50
then there is probably
moisture somewhere. See
Problem 2.
With membrane cap
removed and probe
internals thoroughly dry, the
reading from the probe is
not zero and/or is erratic. Moisture has entered the
system – either into the probe
itself or at junctions or points in
the cable. This moisture creates
a secondary galvanic action in
addition to that produced by the
probe and results in non-zero or
erratic readings. Locate the source of
moisture by process of
elimination. If moisture has
entered a junction box or a
cable joint, thoroughly dry out
the area and take measures
to prevent reoccurrence.
It is not possible to calibrate
the probe in air – the
display will not read high
enough after fully adjusting
the offset. Probe has dried out – no
electrolyte inside.
Probe is overdue for servicing –
excessive build-up of anode
oxide.
A deposit has built-up on the
silver cathode, which is
inhibiting the reduction of
oxygen at its surface.
Service probe and change
membrane. Use a stiff nylon
brush to remove the oxide
built-up from the anode. Do
not use a wire brush. It is
only necessary to remove the
loose oxide layer. If it is
suspected that the anode is
badly corroded, replace with
a new DO probe. Remember
to tighten the nut under the
anode before fitting a new
anode.
If it is suspected that a
deposit is coating the silver
cathode, clean the cathode
with 400 grit wet/dry emery
paper or with some scouring
powder. The deposit is
sometimes visible as a
brownish stain on the surface
of the cathode.
Display values are erratic
when membrane is lightly
touched. Membrane has
bulged outwards. The membrane has been
damaged. If the membrane has been
damaged change it and
service the probe.
NOTE: The cathode must not be polished – the surface must remain dull
(do not use a wire brush).