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...4 LIQUID HANDLING SECTION
4.2 General Operation – Fig. 4.2
The sequence of events is:
a) The sample enters the constant head unit from below and any
excess is allowed to overflow to drain.
The constant head unit is fitted with a float switch to signal an
'Out of Sample' condition. This switch is used by the monitor
to initiate the 'Out of Sample' alarm.
b) From the constant head unit the sample is drawn through the
normally open ports of the solenoid valves SV1 and SV2 by
one channel of the peristaltic pump.
c) The reagent is drawn through another channel of the
peristaltic pump, and is then mixed with the sample. The tube
diameters are arranged so as to obtain the correct ratio of
sample and reagent.
d) The electrodes are housed in a temperature controlled
flowcell which includes a heat exchanger to remove the
effects of sample and ambient temperature variations. The
electrode pair generates an electrical potential when exposed
to the reacted sample which changes in proportion to
changes in concentration of the ion being measured. The
probe is connected to the electronic section where, after
digital conversion, the signal is processed by the
microprocessor.
e) After measurement the sample flows to waste via the
contaminated drain connection.
f) During calibration the monitor introduces two calibration
solutions sequentially in place of the sample by means of the
solenoid valves SV1 and SV2.
Microprocessor Unit
External
Serial
Interface
Connections
Reference
Electrode
Heater
Control
User Junction
Box
Interconnection
Cables
Out of Sample
Pump and
Float Switch
Stirrer Motors
Ion-Selective
Sample/Calibrate
Electrode
Soleniod Valves
Liquid Handling
Section
Fig. 5.2 Location of Controls, Displays and L.E.D.s
5 ELECTRONICS SECTION
5.1 Electronic Layout – Fig. 5.1
The electronic section comprises two separate sections:
•
The User Junction Box at the top right hand side.
•
The Microprocessor Unit at the top left hand side.
5.2 User Junction Box
The User Junction Box contains the relays for the heater, solenoid
valves and alarms, and all the user external connection terminals,
with the exception of the serial interface (if fitted).
Once installed there should be no need to remove the junction
box cover on a regular basis. However, to assist in any fault
finding procedure, there are l.e.d's on the p.c.b. to indicate if the
relays and heater are being energised.
Switches for the mains and pump/heater are situated on the right
hand side of the junction box, together with a mains indication
lamp and mains fuse – see Fig. 2.3.
Two additional fuses (F2 and F3) are located within the junction
box. These are connected in the 24V AC circuits.
5.3 Microprocessor Unit
The Microprocessor Unit contains the analogue input processing,
microprocessor, alarm and current output generation, and (if
fitted) the serial interface output.
The programme controls, digital and dot-matrix displays, alarm
indication and status l.e.d's are all mounted on the front panel of
the microprocessor unit.
Mains Supply – see Fig. 2.3
Concentration Alarm Relays
Calibration Mode Relay
Out of Service Relay
Out of Sample Relay
Current Outputs
Protective Earth
Flow Cell
Heater Block
External
User
Connections
9
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