Zero Current Is Unstable; Sensor Can Be Calibrated, But The Current Is Too Low - Emerson Rosemount MCL-220 Manual

Troubleshooting
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9.5.2

Zero current is unstable.

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9.5.3

Sensor can be calibrated, but the current is too low.

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Has adequate time been allowed for the sensor to reach a minimum stable residual
current? It may take several hours, sometimes as long as overnight, for a new sensor
to stabilize.
Check the membrane for damage and replace it if necessary. Be careful not to touch
the membrane or cathode. Touching the cathode mesh may damage it.
is the sensor properly wired to the transmitter? See
connections are tight.
Readings are often erratic when a new or rebuilt sensor is first placed in service.
Readings usually stabilize after about an hour.
Is the space between the membrane and cathode mesh filled with electrolyte
solution? Often the flow of electrolyte can be started by simply holding the sensor
with the membrane end pointing down and sharply shaking the sensor a few times
as though shaking down a clinical thermometer.
Verify that the sensor is filled with electrolyte solution. Refer to
details.
Is the temperature low? The sensor current decreases about 5% for every °C drop in
temperature.
Sensor current depends on the rate of sample flow past the sensor tip. If the flow is
too low, monochloramine readings will be low. Be sure the liquid level in the
constant head flow controller is level with the central overflow tube and that excess
sample is flowing down the tube. If necessary, disassemble and clean the overflow
sampler. See Section 8.3.
Is a bubble trapped against the membrane? If a sample flow becomes too low
(because dirt or slime has built up in the flow controller), bubbles have a tendency to
collect on the membrane. The bubble reduces the active area of the membrane and
readings drop. The design flow (2 gph) is adequate to push away bubbles. See
Section 8.3 for the cleaning procedure
Low current can be caused by lack of electrolyte flow to the cathode and membrane.
See step 3 in Section 9.5.2.
When was the sensor fill solution last replaced? The monochloramine sensor loses
sensitivity, that is, it generates less current per ppm of monochloramine, as it
operates. Gradual loss of sensitivity can usually be compensated for by calibrating
the sensor weekly. After about two or three months of operation, the sensitivity may
start to drop rapidly. At this point, the electrolyte solution and membrane should be
replaced. Refer to Section 8.2
Is the membrane fouled or coated? A dirty membrane inhibits diffusion of
monochloramine through the membrane, reducing the sensor current and
increasing the response time. Clean the membrane by swirling it vigorously in a
beaker of water or by washing with a stream of water from a washer bottle.
Figure 4-2. Verify that all wiring
Section 8.2 for
Rosemount MCL