4230 Flow Meter
Section 4 Optional Equipment
4-21
4.9.3 Membrane
Thicknesses
Isco supplies a 2 mil (.002") thick membrane for use with the
D.O. probe. This membrane is recommended for long-term moni-
toring situations only, typical of our users' applications. Use only
this thickness of membrane with D.O. probes connected to Isco
flow meters. Do not use other thicknesses of membranes as the
Parameter Module used with the probe is calibrated only for the
2-mil membrane, and cannot be recalibrated in the field. Use of
other membrane thicknesses will result in inaccuracy.
4.9.4 Probe Installation The D.O. probe attaches to a sensor carrier bracket that snaps
into an Isco mounting ring. Use the specific size mounting ring
for pipes less than 15" in diameter, and the Scissors Ring for pipe
sizes greater than 15" diameter. Refer to the instruction sheet
supplied with the mounting ring.
Note
When installing the D.O. probe and its sensor carrier bracket,
make sure the mounting slots on the sensor carrier are com-
pletely pressed into the mating tabs on the ring. The probe
relies on a full engagement between tabs and slots for secure
mounting. If the slots are loose against the tabs, the probe may
be swept away by the force of the stream.
4.9.5 Probe Operation and
Precautions
The following factors determine the life of the D.O. probe and the
frequency of service.
• Membrane life depends on use. Membranes will last
longer if installed properly and treated with care during
use. Contents of the flow stream are also important, as
some substances will foul the membrane very quickly.
Erratic readings will result from loose, wrinkled or
fouled membranes, or from large bubbles in the
electrolyte reservoir. If erratic readings or evidence of
membrane damage occur, you should replace the
membrane and the KCl electrolyte.
• In environments where the membrane becomes rapidly
coated with oxygen-consuming or oxygen-evolving
organisms, erroneous readings may occur. Chlorine,
sulfur dioxide, nitric oxide, and nitrous oxide can affect
readings by reacting like oxygen at the probe. If your
readings seem unreasonable, you may need to perform
analysis to determine whether these gases are the
cause. Long-term use, as for monitoring, can magnify
the effect of these factors in some applications.
• Avoid any environment containing substances that may
attack any of the probe's materials. Some of these
substances are concentrated acids, caustics, and strong
solvents. The probe materials that come into contact
with the sample include FEP Teflon, acrylic plastic, ABS
plastic, EPR rubber, stainless steel, epoxy, polyether-
imide, and the polyurethane cable covering.
• For correct probe operation, the gold cathode must
always be bright. If it is tarnished (which can result
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