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8.3 Shutdown Procedure
8.3.1 Short Term
The monitor can be left with the power switched off for up to 24
hours without any detrimental effects. When returned to normal
monitoring mode, the monitor should be run on sample for 30
minutes, followed by a calibration – see Section 7.
If the probe is left in the monitor with no liquid flowing, the internal
filling solution becomes more concentrated by evaporation of
water through the membrane. The probe output drifts when
normal operation is resumed and many hours may be required to
re-establish stability. In such cases normal performance can
sometimes be restored by unscrewing the glass electrode two to
three turns and then returning it to its original position, thus
allowing fresh filling solution to flow between the electrode and
the membrane. If this procedure is not successful replace the
membrane and filling solution.
If the probe is likely to be out of use for more than one day, remove
the probe from the monitor and store it with the lower part of the
body immersed in a small quantity of liquid in a beaker or other
suitable vessel. Probe filling solution is the best storage solution.
It is essential that all traces of the storage solution are rinsed away
with clean water before use.
8.3.2 Long Term
When the monitor is required to be shut down for more than 24
hours, proceed as follows:
a) Switch off the monitor.
b) Close the sample valve upstream of the monitor.
c) Remove and dismantle the probe, and return it to its box. To
dismantle the probe, unscrew the glass electrode and empty
the rest of the probe. Rinse with clean water and drain. Store
the glass electrode with its end in a teat containing a neutral
buffer solution, taking care not to cover the reference element.
d) Clean sample pipework both inside and outside the monitor,
constant head unit, and filtration system (if used).
e) Remove pressure plate from peristaltic pump.
8.4 Unscheduled Servicing
The monitor indicates abnormal operation by means of signals on
the 20 character dot matrix display and the l.e.d.s The indications
are listed in Table 8.1.
8.4.1 Malfunction of the Monitor
It should always be remembered that any unpredictable
problems may be due to the standard or reagent solutions, or their
flow through the flowcell. If any doubts exist regarding the integrity
of the solutions, they should be replaced with freshly prepared
solutions in the early stages of the fault finding investigations.
Note.
The accuracy of the monitor is governed by the condition of
these solutions which may be incorrectly made or
contaminated.
In general, any problems are likely to be due to the probe, which
may require refurbishment (see Section 8.4.3), but may be due to
other parts of the liquid handling section of the monitor.
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 probe. 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 70% 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 70%
a) Clean the membrane by flushing the flow-through cap with
clean water from a wash-bottle or syringe.
b) Disassemble probe and reassemble – see Section 8.2.5.
c) Check glass electrode – see Section 8.4.3.
Slope value grossly abnormal
a) Check, and top up as necessary, the filling solution in the
probe. There should be a 50 to 60mm depth of solution inside
the probe.
8 MAINTENANCE...
37
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