TM8400B-L-00
1 November 2004
Air pressure for driving the water and for cell flushing is provided by two piston type pumps,
which can be switched on or off. Flushing is initiated by blocking a panel mounted control
orifice, thus diverting the air to the electrode arms and driving the water out of the cell.
3.3. BATH TEMPERATURE CONTROL
A precision control system maintains the bath water temperature at one of six selectable settings
in the range 18°C to 33°C in increments of three degrees. Two incandescent bulbs provide the
bath heating source while a thermoelectric cooling unit provides the cooling. Steadily lighted
during a "heat on" cycle and darkened during a "heat off" cycle, the heater lamps flash on and off
in regular sequence, providing visual indication when bath temperature reaches the selected
setting. The temperature control range enables cooling of the bath to room temperature minus
2°C and heating to room temperature plus 4°C.
3.4. SAMPLE FLOW RATE
It is not possible to obtain a stable reading from a sample which has not achieved bath
temperature due to a combination of a large sample to bath temperature difference and a high
flow rate. If the flow rate is too fast the heat exchanger will be unable to bring the sample to the
correct temperature. Unstable or incorrect readings will result. If the flow rate is too slow an
excessive amount of time will be required to fill the cell and to obtain a reading. If the flow rate
is too fast the indication will be a marked change in the cycling of the heater lamps. A cold
sample will cause an "ON" period. The flow rate should be reduced until the normal cycling of
the heater lamps is restored.
3.5. CONDUCTIVITY MEASURING CIRCUIT
A simplified schematic diagram of the conductivity measuring circuit is shown in Figure 3.2.
The circuit measures the resistance of the cell by maintaining a precise voltage across the
potential arms of the four terminal resistor (cell) and measuring the voltage across a stable
resistor in series with the cell current arms. The current is a square wave alternating current to
prevent polarization of the sample.
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