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8.5 Microprocessor Unit Error Messages
The instrument incorporates an automatic self-diagnostic
checking facility for detection of input and output errors. If such a
fault occurs, one of the error messages detailed below is shown
on the dot matrix display.
'CHAN 1 INPUT ERROR'
This error message is usually caused by an open circuit
sensor input allowing the input to drift outside the range of
±400mV. This may be due to any of the following:
a) Low level of internal filling solution in the probe.
b) Open circuit electrode internally, e.g. broken connection,
blocked liquid junction in reference electrode etc.
c) Broken electrical connections between the sensor and the
Microprocessor Unit sensor input.
d) Sensor coaxial plugs not correctly inserted.
'TEMP INPUT ERROR'
'PRT OUT OF LIMITS'
'TEMP REF. ERROR'
'THIRD LEAD ERROR'
The above four error messages refer to electrical problems with
the temperature sensor input, – usually an open or short-circuit
connection.
8.6 Procedure
for
Chloridising
Element
Requirements
a) Plating solution – hydrochloric acid (HCl) 0.1M, 500ml.
Note.
Do not use acid preserved with mercuric ions.
b) Cleaning solutions – Ammonia solution (NH
200ml.
To prepare, dilute 100ml analytical reagent grade
concentrated ammonia solution, s.g. 0.88, with 100ml distilled
water and stir.
c) Nitric acid (HNO
), 25% vol./vol. 200ml.
3
To prepare; cautiously pour 50ml analytical reagent grade,
concentrated nitric acid, s.g. 1.42, into 150ml distilled water,
stirring continuously. Allow to cool before use.
Warning.
STRONG AMMONIA SOLUTIONS AND CONCENTRATED
NITRIC AND HYDROCHLORIC ACIDS ARE IRRITANTS
AND VERY CORROSIVE. TAKE CARE NOT TO INHALE
ANY OF THE FUMES OR TO GET ANY SOLUTION ON
SKIN OR CLOTHING. ANY SLIGHT SPILLAGE ON SKIN,
CLOTHING OR WORKING SURFACE MUST BE WASHED
AWAY WITH PLENTY OF WATER.
d) Constant current supply, 2mA d.c.
e) Silver wire (counter electrode).
f) 1 beaker.
Cleaning –
a) Immerse the lower 5cm of the combination electrode (i.e. so
that the silver reference element is covered) in the ammonia
solution for about one minute. Remove and rinse with distilled
water.
b) Immerse the electrode to the same depth in the 25% nitric
acid until the silver element is a uniform creamy white. This
process usually takes about one minute, but the element must
be inspected frequently as prolonged immersion is
detrimental.
c) If the element is not uniform in colour repeat the ammonia/
nitric acid process.
d) When clean, rinse with distilled water and immediately
transfer to the plating solution such that the reference element
is completely immersed.
Chloridisation
a) Attach the screen of the combination electrode cable to the
negative terminal of the constant current supply and immerse
the lower 5cm of the electrode in the plating solution so that
the reference element is covered. Immerse a silver counter
electrode in the plating solution and connect to the positive
the
Reference
terminal of the supply.
b) Pass a current of 2mA for approximately 30 seconds.
Carefully tap the electrode to remove all bubbles and then
reverse the connections to the supply (i.e. the electrode to be
plated is connected to the positive terminal).
c) Pass a current of 2mA for 30 minutes after which time the
electrode will be a dark brown or grey colour. A more uniform
) 50% vol./vol.,
3
coating may be obtained by employing moderate stirring
during the process.
d) Remove the electrode from the plating solution, rinse with
distilled water and dry with a tissue.
8 MAINTENANCE
39
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