Model 408 NO Calibration Source Manual Rev. D-3
2
air depends on the intensity of the photolysis lamp, the concentration of N
2
O
(determined by pressure and temperature), and the residence time in the photolysis
cell (determined by volumetric flow rate and cell volume). By holding these
parameters constant, it is possible to produce a flow of air containing a constant
concentration of NO, and the concentration of NO produced can be varied most
conveniently by varying the lamp intensity. Figure 1 is a schematic diagram of the NO
Calibration Source.
Figure 1. Schematic diagram of the NO Calibration Source
.
An air pump draws air in through NO and ozone scrubbers to remove any ambient
NO. The air flow rate is controlled by use of restrictors (not shown) and a needle
valve that vents part of the flow and is measured by a flow meter. A constant fraction
of the flow bypasses the flow meter (not shown) so that the flow through the meter is
in the range 0-1 L/min. The needle valve is adjusted to produce a total output
volumetric flow rate of ~3.0 L/min. Nitrous oxide is supplied by a cartridge containing
liquid N
2
O with a headspace pressure of ~50 atmospheres. The internal pressure is
dropped by a combined cracker/regulator and additional flow restrictor, which also is
used to provide coarse adjustment of the flow rate of N
2
O to be in the range 60
10
