Model S4000TH
54
9.0 Appendix
9.1 Warranty
General Monitors warrants the S4000TH to be free from defects in workmanship or material
under normal use and service within two years from the date of shipment.
General Monitors will repair or replace without charge any such equipment found to be
defective during the warranty period. Full determination of the nature of, and responsibility for,
defective or damaged equipment will be made by General Monitors’ personnel.
Defective or damaged equipment must be shipped to the General Monitors’ plant or
representative from which the original shipment was made. In all cases this warranty is limited
to the cost of the equipment supplied by General Monitors. The customer will assume all
liability for the misuse of this equipment by its employees or other personnel.
All warranties are contingent upon proper use in the application for which the product was
intended and does not cover products which have been modified or repaired without General
Monitors’ approval, or which have been subjected to neglect, accident, improper installation or
application, or on which the original identification marks have been removed or altered.
Except for the express warranty stated above, General Monitors disclaims all warranties with
regard to the products sold, including all implied warranties of merchantability and fitness and
the express warranties stated herein are in lieu of all obligations or liabilities on the part of
General Monitors for damages including, but not limited to, consequential damages arising out
of, or in connection with, the performance of the product.
9.2 Principle of Operation
General Monitors uses a proprietary Metal Oxide Semiconductor (MOS) film on the sensor for
detecting hydrogen sulfide (H
2
S) gas. The MOS film is deposited onto a substrate between two
electrodes. With no gas present, the measured resistance between these two electrodes is very
high (in the mega-Ohms). As H
2
S adsorbs onto the film, the resistance between the two
electrodes decreases (to kilo-Ohms). This decrease in resistance is logarithmically proportional
to the concentration of H
2
S that is present.
The change in resistance is converted to a change in voltage, and amplified by the input circuit.
This amplified signal is fed to an Analog to Digital (A/D) Converter, converted to a digital signal
and sent to the microprocessor to be processed. The process of H
2
S adsorbing onto the MOS
film is most effective at an elevated temperature. On the outer edge of the substrate is a heater
ring. The temperature of this heater ring is measured with a thermistor, and kept constant by a
circuit located inside the body of the sensor.
As H
2
S adsorbs onto the film, electrons move more freely from one electrode to the other. This
is represented as a decrease in resistance. The process of H
2
S adsorbing onto the MOS film is
completely reversible. As the concentration of H
2
S decreases (H
2
S desorbs), the resistance
between the electrodes will increase.
