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Mechanical components which are involved with the liquid
handling, for example, pumps, valves, tubing and tubing
connections etc., should be systematically checked for correct
operation, and for leaks or blockages which change the chemical
conditions around the sensor. The majority of all problems are
found to be associated with the chemistry and the liquid handling
section.
8.4.2 Monitor Diagnostic Information
Out of Service Alarm
This alarm relay output is a normally energised relay which de-
energises in the following circumstances:
a) Loss of mains supply.
b) Calibration Failed – the calculated electrode slope value is
outside the acceptable limits or the response of the electrode
is too slow. The 'CAL FAIL' l.e.d. on the front panel is
illuminated with the appropriate text on the 20-character
display.
c) Block temperature out of limits – the measured temperature of
the Block is not within 5 °C of the Control Temperature.
'TEMP. CONTROL ERROR' is indicated on the 20-character
display.
d) 'OUT OF SAMPLE' alarm – a float switch in the constant head
unit detects the loss of sample. 'OUT OF SAMPLE' is
indicated on the 20-character display.
e) 'OUT OF SERVICE' alarm – displayed when the pump is
switched off.
Note. The heater is switched off by the software when an
'OUT OF SAMPLE' condition is detected.
Calibration Fail Alarm
A calibration failed condition occurs after a two point calibration if
the calculated slope value is less than 80% or if the sensor output
is unstable. The failure could be caused by a number of factors
which should be investigated.
In the 'CAL. FAILED (SLOPE)' case some indication of the
problem may be obtained from the slope value.
Slope Value Just Below 80%
a) Check for surface deposits on membrane of nitrate electrode.
b) Refurbish the nitrate electrode – see Section 8.5.
Slope value grossly abnormal –
a) Check for air bubbles trapped on nitrate electrode membrane.
b) Check for surface deposits on membrane of nitrate electrode.
c) Check, and top up as necessary, the ion–exchange solution in
the nitrate electrode. Maintain the level in the electrode at a
depth of approximately 15 mm. Use the syringe in the
refurbishing kit and clean with methanol.
d) Check level of salt bridge solution in reference electrode.
e) Check that the fill hole stopper has been moved away from
the salt bridge filling solution hole in the reference electrode.
f) Refurbish nitrate electrode – see Section 8.5.
g) Reference electrode liquid junction partially or completely
blocked, replace if required.
Noisy, unstable response or slow response –
a) Check for air bubbles trapped on nitrate electrode membrane.
b) Check for surface deposits on membrane of nitrate electrode.
c) Check level of salt bridge solution in reference electrode.
d) Check that the fill hole stopper has been moved away from
the salt bridge refill aperture in the reference electrode.
e) Nitrate electrode fouled or poisoned, refurbish the nitrate
electrode – see Section 8.5.
f) Reference electrode liquid junction partially or completely
blocked, replace if required.
Note. Electrodes exposed to high levels of interfering ions
e.g. chloride, may drift for some time on returning to a nitrate
standardising solution.
In the 'CAL. FAILED (SLOW)' case the failure is usually due to
slow sensor response, but may be due to an unstable (noisy or
drifting) sensor output:
a) Check sensor connections in the coaxial plugs and sockets,
and inside the microprocessor unit.
b) Refurbish the nitrate electrode – see Section 8.5.
8.5 Refurbishing the Electrode
a) Withdraw the inner electrode, wipe the tip with a tissue and
wash out the gelled filling solution with distilled water.
Warning.
• Wear protective gloves and perform the following steps
under a fume hood. Avoid skin contact with the ion
exchanger solution; if contact occurs, wash off
immediately with soapy water.
• Methanol is highly flammable. Avoid contact with skin and
eyes. Do not inhale the vapour.
b) Hold the electrode body over a small beaker of methanol;
unscrew the end cap and allow the end cap assembly and
ion–exchanger solution to fall into the methanol.
c) Wash the end cap and washer in more methanol; allow to air
dry. Discard the membrane.
d) Wash the electrode body, inside and out, with methanol and
allow to air dry.
e) Follow procedure in Section 8.2.7.
8 MAINTENANCE...
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