General Monitors 2180A Instruction Manual page 38

Model 2180A
or 0-100 ppm H S. Therefore, the change produces a signal approximately proportional
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to the logarithm of the H S concentration. This signal is processed, linearized, and
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displayed as a digital number on the front of the controller. The controller's analog current
output signal tracks the digital reading as well. The adsorption process is reversible so
that when the air containing no H S diffuses into the sensor, the H S gas desorbs. The
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semiconductor then resumes its original "clean air" resistance value.
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