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the <UP> or <DOWN ARROW> key
until the Mark 25 displays PEr.
To return to the monitor mode, press the
<RUN> key.
3.10 Sensor Design & Maintenance
The dissolved oxygen sensor is a
galvanic-style sensor using a platinum
cathode, lead anode, potassium iodide
electrolyte and a gas-permeable Teflon
membrane.
The dissolved oxygen sensor consists of
five principle parts:
• Spindle
• Outer body
• Filler Valve
• Membrane Ring
• Teflon Membrane
Refer to the photos at the back of this
manual for additional information.
The sensor is normally shipped dry from
the factory without electrolyte or a
Teflon membrane installed. The
following procedure should be used
when changing the membrane or
replacing the electrolyte.
1. Turn the Filler Valve clockwise until it
stops.
2. Unscrew the black Outer Body
counter-clockwise and remove it
from the gray Spindle. Empty out
any electrolyte still in the Outer
Body.
3. Unscrew the stainless steel
Membrane ring from the Outer Body
and remove the old membrane.
4. Place a new membrane in the
bottom of the Membrane Ring and
place several drops of electrolyte
onto the Membrane.
5. Screw the Membrane Ring onto the
Outer Body and gently tighten.
6. Using the dropper from the
electrolyte bottle, carefully fill the
four small flow-through holes located
around the platinum tip of the
Spindle.
7. Fill the Outer Body with electrolyte
just above the interior ledge of the
Outer Body.
8. With the Filler Valve facing up at a
o
45
angle, screw the Outer Body
back onto the Spindle and gently
tighten (some electrolyte should flow
out of the Filler Valve).
9. Gently screw out the Filler Valve
until it stops (two or three threads
should be visible).
10. Follow the normal calibration
procedure.
If the dissolved oxygen sensor is not
going to be immediately used, fill the red
vinyl wetting cap with water and place
over the Membrane Ring squeezing as
much air from the cap as possible.
If the sensor is not to be used within two
weeks, empty the electrolyte out of the
sensor, dry all of the parts and store the
probe in a cool place.
Normally, very little maintenance is
required for the dissolved oxygen
sensor and it can operate for weeks,
even months, without the need to
change the electrolyte or membrane,
especially if the sample stream has little
or no dissolved oxygen present.
However, over time or in a sample
stream with a high dissolved oxygen
content, lead oxide will form on the lead
12
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