Process Readings Are Erratic; Readings Drift - Emerson Rosemount MCL-220 Manual

7.
9.5.4

Process readings are erratic.

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3.
9.5.5

Readings drift.

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4.
Instruction Manual
CAUTION!
EQUIPMENT DAMAGE
Do not use a tissue to wipe the membrane.
If cleaning the membrane does not remove the sensor response, replace the
membrane and electrolyte solution. See the sensor instruction sheet for details.
Readings are often erratic when a new sensor or rebuilt sensor is first placed in
service. The current usually stabilizes after a few hours.
Verify that wiring is correct. Pay particular attention to shield and ground
connections.
Is the membrane in good condition, and is the sensor filled with electrolyte solution?
Replace the fill solution and electrolyte. Refer to
Is the sample temperature changing? Membrane permeability is a function of
temperature. The time constant for response to a temperature change is about five
minutes. Therefore, the reading may drift for a while after a sudden temperature
change.
Is the membrane clean? For the sensor to work properly, monochloramine must
diffuse freely through the membrane. A coating on the membrane will interfere with
the passage of monochloramine , resulting in a slow response. Clean the membrane
by rinsing with a stream of water from a wash bottle or by swirling it vigorously in a
beaker of water.
CAUTION!
EQUIPMENT DAMAGE
Do not use a tissue to wipe the membrane.
Is the sample flow within the recommended range? Gradual loss of flow will cause
downward drift. 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, dissassemble and clean the overflow sampler. See
Is a bubble trapped against the membrane? For the sensor to work properly,
monochloramine must continuously diffuse through the membrane. Bubbles block
monochloramine in the sample from reaching the membrane, so readings drift
downward as bubbles form and grow. The nozzle at the bottom of the flow cell
pushes bubbles to the edges of the membrane where they do no harm. In cold
samples, the nozzle may not be as effective.
a. If bubbles are visible, confirm that they are blocking the membrane by removing
the sensor from the flow cell and replacing it. Removing the sensor breaks the
bubbles, so when the sensor is replaced, readings return to normal.
Troubleshooting
Section 8.2.
Section 8.3.
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