Emerson Rosemount TCL - Readings Drift; Readings Are too High; Temperature was more than 3 °C Different from Transmitter; Current Output Is too Low

11.5.5 Readings drift.
1. Is the sample temperature changing? Membrane permeability is a function of
temperature. The time constant for the Rosemount 499ACL-02 sensor is about five
minutes. Therefore, the reading may drift for a while after a sudden temperature
change.
2. Is the membrane clean? For the sensor to work properly, iodine must diffuse freely
through the membrane. A coating on the membrane will interfere with the passage
of iodine, resulting in a gradual downward drift in readings. The coating will also
slow the response on the sensor to step changes. Clean the membrane by rinsing
with a stream of water from a wash bottle. Wipe the membrane with a soft tissue.
3. Is the sensor new or has it been recently serviced? New or rebuilt sensors may
require several hours to stabilize.
4. Is the flow of sample past the sensor about 11 mL/min? See Section 11.5.3 step 9 for
more information.
5. Is the reagent flow about 0.2 mL/min? See Section 11.5.3 step 10 for more
information.
11.5.6 Readings are too high
1. Is the sample conditioning reagent clear and colorless? If the reagent is pale yellow,
results will be high. The pale yellow color is caused by iodine, which comes from the
reaction between atmospheric oxygen and potassium iodide. The reaction is
catalyzed by sunlight. The purpose of the blue carboy is to protect the reagent from
sunlight.
2. Is the sensor fill solution fresh? An old, discolored fill solution may produce a high
reading.
11.5.7 Temperature was more than 3 °C different from
transmitter
1. Is the standard thermometer, RTD, or thermistor accurate? General purpose liquid-
in glass thermometers, particularly ones that have been mistreated, can have
suprisingly large errors.
2. Is the temperature element in the sensor completely submerged in the liquid?
3. Is the standard temperature sensor submerged to the correct level?
11.5.8 Current output is too low.
Load resistance is too high. Maximum load is 600 Ω.
11.5.9 Alarm relays don't work.
1. Verify the relays are properly wired.
2.
Verify that deadband is correctly set. See Section 7.4.
Troubleshooting
94 Rosemount TCL