ENMET SPECTRUM - Manual (Page 28)

ENMET Corporation SPECTRUM Series
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APPENDIX C APPENDIX C APPENDIX C APPENDIX C SPECTRUM for Hydrazine
SPECTRUM For Hydrazine
The detection of hydrazine, N
2
H
4
, is a specialized application. Generally, hydrazine gas blended in air or nitrogen in
known verified amounts is not available to use as calibration gas in the field. Also, the sensor is not responsive to any
other gas which could be used as a correlation gas. The sensor itself has a relatively short operating life, and should be
replaced every six months. Typically, the instrument is returned to the factory every six months for sensor
replacement.
A sensor can be replaced in the field by using a current source to set up the instrument for the particular sensor being
installed. Each sensor has an individual sensitivity to hydrazine, expressed in nanoamps per ppm. This sensitivity is
given on the sensor label and the container in which the sensor is shipped.
A stable voltage of -5 vdc is available on pin 11 of IC4. The instrument is set up for a calibration gas of 0.3 ppm
hydrazine. So the voltage is used with a resistor sized to give the current output of the sensor when exposed to 0.3ppm
hydrazine. This resistor is typically in the 6 to 8 megohm range. The value of the resistor is calculated by:
R = 5.0/ (0.3) (S), where S is the sensitivity of the sensor
For a sensor with a sensitivity of 2250 nanoamps per ppm, for example:
R = 5.0/ (0.3) (0.000002250) = 7 400 000 ohms = 7.4 megohms
Remove the old sensor, and run through the zero portion of the calibration procedure, then stop. With the sensor still
removed, connect the resistor between pin 11 of IC4 and the sensor output, pin 3, on the circuit board. Then resume
the calibration procedure. When it is complete, remove the resistor, and install the new sensor.
For warranty purposes, the hydrazine sensor is considered an expendable item with a warranty life of six months.