Thermal Conductivity Analyzer | Model 212R |
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6.2.1Dynamic Balance Procedure.
The measuring bridge circuit (see schematic) incorporates two 10 turn potentiometersthatareutilizedtocompensateforminutedifferencesinthe resistive and reactive properties of the hot wire elements and the center tapped secondary of the bridge transformer in the reference and zero gas related to the application.
The potentiometers are of the screw driver adjustment type and are equipped with shaft locking assemblies that have been factory adjusted to create enoughfrictionsothattheshaftwillnotchangepositioninshipment.Thepotenti- ometers are located on the amplifier chassis, and are controlled through two holesidentified“RESISTIVE”and“REACTIVE”ontheinnercontrolpanel.
DO NOT OPEN THE INNER PANEL OF THE ANALYZER TO GAIN ACCESS TO THESE CONTROLS. The balance adjustments should not be disturbed if the analyzer is not up to operating temperature. Correct bridge balance can only be achieved at operating temperature be- cause of the inherent thermal junctions within the potentiometers themselves. The minute voltages produced by contact between the dissimilar materials within the potentiometers are effectively balanced out in the procedure when it is accomplished at operating temperature. Because the voltage generated by these junctions varies with temperature, and because the analyzer will resolve a 0.25 microvolt change in bridge voltage, temperature stability is a mandatory prerequisite to the dynamic balance procedure.
To restore zero control within the limits of Range # 1, employ the following Procedure:
1)With zero gas flowing, adjust the sample and reference gas flowme- ters to the same value between 0.1 and 0.3 scfh.
2)Set the ZERO potentiometer to 500.
3)Connect a voltmeter to the instrument signal output. Connect an oscilloscope to the probe point an d set the sweep rate to 5 milliseconds. Set the voltage level so ± 15 volts can be read.
4)If a reading of +15 or
Teledyne Analytical Instruments | 19 |
