Model 235 Thermal Conductivity Analyzer
Our old 235 unit is drifting up scale all the time. Power supplies are good, block temperatures are good and stable, zero and span gases have not been changed, but there is a slight imbalance on the bridge resistances. Is the cell block bad?
Old 235 units normally go bad when the thermistor that goes into the cell block degrades from years of being at a high temperature. The insulation drys out and the whole thermistor assembly tends to fall apart.
Also, you can check the stability of the voltage drive directly at the bridge circuit. Make sure this is steady and the correct number.
Also, be certain it isn't a process change, or a leak in the tubing somewhere. What about flow changes -- any possibility of large changes in flow rate or sample pressure?
It is rare for the cells to go bad, except for maybe sealed references.
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What effect does sample and reference flow rate have on the reading when the unit is configured to measure 0-10 N2 in Argon?
The effect of sample and reference flow rate change in the range mentioned below is minimal. This data applies:
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