Model 408 NO Calibration Source Manual Rev. D-3
1
1.
INTRODUCTION
1.1
Description
The Nitric Oxide Calibration Source
is a portable source of nitric oxide that allows
one to calibrate any nitric oxide monitor. The instrument scrubs NO from ambient air
and produces either zero air or air having a mixing ratio of NO in the range 20-1,000
parts per billion by volume (abbreviated in this manual as ppb). The desired NO
concentration is chosen from the easy-to-use menu using a rotary Select Switch. The
instrument can be programmed to output up to 10 individual NO step concentrations
over a chosen time interval. The total output volumetric flow rate is 3.0 L/min, and the
NO mixing ratio is controlled so as to be independent of ambient temperature,
pressure and humidity.
You can attach the NO Calibration Source
output directly to the inlet of any nitric
oxide monitor (providing that its sampling rate is less than 3.0 L/min); the excess flow
must be vented through an external overflow tee. Besides portability, an important
advantage of the Model NO Calibration Source is that it provides a known
concentration of NO in ambient air containing the same level of humidity as the air
sample to be measured.
The NO Calibration Source
is factory calibrated against a NIST-traceable standard.
However, the calibration parameters may be changed in the menu in case the user
wants to recalibrate the NO Calibration Source
against a separately maintained
standard; i.e., the NO Calibration Source
can be used as a transfer standard. The
NO Calibration Source
may be used, for example, for maintaining the calibration of
a large number of NO monitors in the field relative to a highly stable laboratory
instrument. In this case, a huge advantage of the NO Calibration Source
is its
portability.
1.2
Theory of Operation
The Nitric Oxide Calibration Source
makes use of a low-pressure mercury lamp to
photolyze nitrous oxide (N
2
O) and produce NO in a patent-pending process. The
vacuum UV emission lines of mercury near 185 nm are absorbed by N
2
O
to produce
electronically excited oxygen atoms, O
(
1
D
2
). A large fraction of these highly energetic
oxygen atoms react with N
2
O to form NO:
N
2
O + h
→
N
2
+ O(
1
D
2
)
(1)
O(
1
D
2
) + N
2
O
→
2 NO
(2)
_________________________
Net: 2 N
2
O + h
→
N
2
+ 2 NO
(3)
where h
symbolizes a photon of light. Other reactions produce a small amount of
molecular oxygen as well. The concentration of NO produced in a flowing stream of
