2002 by ELTRA GmbH Germany – January 2002 – Service Manual OH 900 / ON 900 / ONH 2000
2.15-3
The second circuit checks the temperature of the furnace block ( not the temperature
of the crucible ).
The temperature sensor is attached to the rear of the upper furnace block (inside the
analyser). If the temperature of the furnace block exceeds the threshold value, the
furnace power will stop and a message for too high temperature will appear on the
screen, see also 1.11.
The third circuit on the ON 21 is for detecting the water flow. A part of this circuit is the
plastic wheel on the board ON 21. This wheel is turning when there is enough
secondary water flow, which is created by the water pump.
When the analyser receives the command to start the analysis, the software first
checks the water flow wheel on the ON 21.
If the wheel is turning, and all the other preconditions are fullfilled, then the furnace
power will start. If the wheel is not turning, the analysis will not start, and the message
„No water flow“ will appear on the screen.
The sensing of the wheel is done by a photo diode and a photo transistor.
An adjustment is not necessary, so that P3 which was meant for adjusting, is no longer
installed on the ON 21. Only when the water is very dirty, then there will be a warning,
even if the wheel is turning, see also 1.10.
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