ENMET SPECTRUM SP - Manual (Page 18)

SPECTRUM SP Series ENMET Corporation
16
APPENDIX C
SPECTRUM SP for Hydrazine
SPECTRUM SP 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.