4220 Flow Meter
Section 5 Options and Accessories
5-19
Tests conducted by Isco with probes installed in various
waste streams have indicated that greases and solids
quickly coat the probe's membrane, making it impos-
sible for oxygen to enter the reaction chamber. This will
result in an abnormally low reading, or no reading at all.
• Cleaning the membrane is generally not effective, as it
tends to drive the solids further into the membrane
pores. You must usually replace both the electrolyte and
the membrane to get an accurate reading. In severe
cases of fouling it may be necessary to change the
membrane very frequently, even as often as every other
day.
5.10.1 How the D.O. Probe
Works
A thin, permeable membrane stretched over the sensor isolates
the electrodes from the environment, but allows gases to enter.
When a polarizing voltage is applied across the sensor, oxygen
that has passed through the membrane reacts at the cathode,
causing a current to flow. The membrane passes oxygen at a rate
proportional to the pressure difference across it. Since oxygen is
rapidly consumed at the cathode, it can be assumed that the
oxygen pressure inside the membrane is zero. Hence, the force
causing the oxygen to diffuse through the membrane is propor-
tional to the absolute pressure of oxygen outside the membrane.
As the oxygen partial pressure varies, both the oxygen diffusion
through the membrane and the probe current change propor-
tionally.
5.10.2 Probe Preparation The following describes how to prepare a new probe for use, or
how to change membranes on an existing probe. First, unscrew
the sensor guard; remove the O-ring and membrane, then thor-
oughly rinse the sensor with distilled water. Prepare the KCl
electrolyte according to the directions on the bottle. Use distilled
water only. You install the membranes by hand. The probe is
shipped with a small folder containing several replacement mem-
branes.
Steps for installing a new membrane:
1. Secure the probe body so it is in an upright position. You
can use a vise, adjustable wrench, or locking pliers, etc.,
whatever is satisfactory. Do not apply too much force to the
probe body, or you will crack it.
2. Fill the cavity around the silver anode with electrolyte to
the point where any more electrolyte would spill over. The
liquid should be free of bubbles and should completely
cover the tip of the sensor.
3. Lightly lay the membrane across the top of the probe. If
you do this carefully, there will be no bubbles under the
membrane.
Note
Handle the membrane material with care, touching it at the
ends only.