TM8400B-L-00
1 November 2004
3.2. SAMPLE WATER FLOW ARRANGEMENT
The tank configuration is shown in Figure 3.1. The tank is insulated with polyethylene foam and
has a gasket seal around the top. It has a rectangular cross section with a divider in the middle
and a smooth curved bottom to reduce turbulence and avoid dead zones. Bath circulation is
achieved by an impeller driven by a small motor mounted above the tank. Bath volume is about
17 litres and cycle time for the water is about 15 seconds. The sequence of water flow is from
the impeller to the heat exchanger coil which is mounted near the tank divider then to the cell. In
the space between the heat exchanger and the side of the bath are two 40-watt tubular
incandescent lamps located so that any water heated passes the impeller for mixing before
striking any critical component. On the side of the bath, by the lamps, is the cold element of a
thermoelectric cooling unit which runs continuously. Two thermistors located near the
conductivity cell act as the temperature sensors. A bulb behind the front panel illuminates the
conductivity cell.
The sample bottle is placed in the holder with its neck pressing against an air seal and the thin
Teflon pick-up tube projecting almost to the bottom. Making the seal allows low pressure air
from a pump to build up pressure above the sample, forcing a slow flow (30 ml/min) through the
pick-up tube into a coil of stainless steel heat exchange tubing which is mounted in the precisely
thermostatted water bath. The flow of the sample, now at bath temperature, passes through a
Teflon tube to the Pyrex conductivity cell. Once the cell is filled the sample exits from the other
end of the cell through a plastic tube and out of the tank to a larger tube which leads to a drain
spigot.
The four electrodes of the cell are platinum-rhodium coils mounted in side arms spaced along the
upper side of the cell as shown. The two outer arms are used as potential leads and the inner
arms as current leads. In addition to the electrical lead each side arm accommodates a very fine
Teflon tube which serves as an air vent so that the side arm can fill with water. The cell is
mounted at an angle of about 15° allowing the cell to be emptied by forcing air down the Teflon
tubes into the side arms. Back flow of water in the heat exchanger is prevented by the higher
filling pressure. After flushing, the cell refills via the inlet tube with the first water running
along the bottom of the cell to clean out the previous water. As soon as the cell has been filled
above the electrodes, readings can be taken, or, alternatively, a new flush can be started.
The end of the pick-up tube in the sample bottle is located slightly lower than the conductivity
cell; consequently, if the cell empties just before removing the bottle the water in the tube will
siphon slowly away from the cell thus causing backflow of old sample into the new one. To
prevent the backflow of sample water, a Mylar pickup tube holder has been provided. Insert the
pickup tube into the holder to initiate siphoning towards the cell. Once the sample water reaches
the bend in the pickup tube, it will continue to siphon towards the cell. This also creates a plug
of air to separate the old and new water in the heat exchanger. The thermostatted bath has a
window facing the operator to allow full viewing of the cell.
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