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3.
4.
5.
9.6
Troubleshooting when no error message is
showing - general
Problem
New temperature during calibration more than 2-3 °C different from the live reading.
Current output is too low.
Alarm relays do not operate when setpoint is exceeded.
Bubbles trapped against membrane.
9.6.1
Difference between transmitter and standard
thermometer is greater than 3 °C.
1.
2.
Instruction Manual
loss in sensitivity. 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
Low readings can be caused by zeroing the sensor before the residual current has
reached a stable minimum value. Residual current is the current the sensor
generates even when no monochloramine is in the sample. Because the residual
current is subtracted from subsequent measured currents, zeroing before the
current is a minimum can lead to low results.
Example: The true residual current for a monochloramine sensor is 20 nA, and the
sensitivity is 400 nA/ppm. Assume the measured current is 600 nA. The true
concentration is (600-20)/400 or 1.45 ppm. If the sensor was zeroed prematurely
when the current was 40 nA, the measured concentration will be (600-40)/400 or
1.40 ppm. The error is 3.5%. Suppose the measured current is 800 nA. The true
concentration is 1.95 ppm, and the measured concentration is 1.90 ppm. The error
is now 2.6%. The absolute difference between the readings remains the same, 0.05
ppm.
Sensor response depends on flow. If the flow is too low, readings will be low and flow
sensitive. 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? If a sample flow becomes too low
(because dirt or slime has built up on 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 cleaning procedures.
Is the reference thermometer, RTD, or thermistor accurate? General purpose
thermometers, particularly ones that have been mistreated, can have surprisingly
large errors.
Review
Section
7.2.2.
8.2.
Troubleshooting
Section
8.3.
See Section
Section 9.6.1
Section 9.6.2
Section 9.6.3
Section 9.6.4
73
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Table of Contents
Troubleshooting
