How The D.o. Probe Works; The D.o. Probe - Teledyne 4250 Installation And Operation Manual

4250 Flow Meter
Section 4 Options and Accessories
Figure 4-9 The D.O. Probe
4.11.1 How the D.O. Probe
Works
4-22
For effective use of the D.O. Probe, please read and con-
sider the following before making the installation:
• The D.O. Probe requires constantly moving water.
The probe consumes oxygen during operation; this will
deplete the oxygen available from stagnant or stratified
waters, resulting in an inaccurate reading. Do not
install it in a stilling well or in any location where water
movement is intermittent or very low. As a guideline, a
minimum velocity of one foot per second is suggested.
• Frequent maintenance is necessary when the probe is
installed in flows with grease or solids content.
Tests conducted by Teledyne Isco with probes installed
in various waste streams have indicated that greases
and solids quickly coat the probe's membrane, making it
impossible 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.
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.