Model 408 NO Calibration Source Manual Rev. D-3
3
cc/min. A pressure relief valve inside the instrument housing prevents over
pressurization by opening to the atmosphere if the pressure exceeds 25 psig. A
Voltage Sensitive Orifice (VSO) valve is used to provide fine control of the N
2
O flow
rate to ± 1 cc/min. Within the photolysis chamber, a very small fraction of the N
2
O is
converted to NO and N
2
by absorption of light at wavelengths near 185 nm.
Measurements of the output of the photolysis chamber show that no insignificant
concentration of NO
2
is produced. The lamp intensity at 254 nm is monitored by a
photodiode and controlled by the microprocessor. The photodiode voltage is
calibrated against the output NO concentration. Pressure within the gas stream is
measured but not controlled. Instead, the lamp intensity is varied to compensate for
changes in pressure and flow rate using a proprietary algorithm. Air containing nitric
oxide at a selected concentration in the range 20-1,000 parts-per-billion by volume
exits the instrument.
