Model 405 nm NO
2
/NO/NO
x
Monitor Manual Rev. D
8
2. Vent the EXHAUST port to atmospheric pressure and out of the room or shelter.
The monitor has an internal scrubber that removes ozone (produced to oxidize NO to
NO
2
) from the sample before exiting the instrument though the EXHAUST port.
However, it is recommended that the pump exhaust be vented to a well ventilated
area outside the room or shelter in case the scrubber fails.
If the instrument is being flown on an aircraft, the inlet tube should not point into the
wind, because the resulting pressure fluctuations will cause a noisy signal. Although
the instrument compensates for temperature drift, if strong temperature fluctuations
are expected, as in vertical profiling applications using balloons, the instrument should
be placed in a thermally insulated box in order to slow the rate of temperature change.
3.5. Measurement of the Zero Offset; Auto Zero Function
The electronic zeros for NO
2
and NO may be measured by attaching a NO
x
scrubber
to the air inlet for a period of 5-10 minutes. For an accurate measurement, the
instrument must have been turned on long enough for the internal temperature to
stabilize. The observed offset, which can amount to a few ppb, can be corrected by
changing this calibration parameter from the front panel, as described in Section 4.9
below, or by correcting the data at a later time.
The instrument also has an Auto Zero function. The instrument can be programmed
to periodically measure and apply new zero offsets for NO
2
and NO using an internal
NO
x
scrubber.
3.6. Collecting Data from the Analog Output
The data may be logged in real time using a data logger attached to the BNC analog
output. The range of the analog output is 0-2.5 V. The output is scaled according to a
sensitivity you define in the menu. For example, you may define 2.5V = 250 ppb. In
that case, the maximum output is 250 ppb, and 10 mV is equal to 1 ppb. There is a
small positive offset, typically 2 mV in the analog output, but this offset varies from
instrument to instrument. The offset can be measured by simultaneously observing
the panel display and measuring the analog output with a voltmeter.
3.7. Collecting Data over the Serial Port in Real Time
To transmit data to a computer over the serial port in real time, connect the instrument
to the computer using the 9-pin cable provided (and a serial-to-USB adapter cable if
necessary). Note that the 9-pin cable provided
is a “straight-through” female-female
serial cable. A “cross-over” cable will not work. Start your data acquisition software,
preferably using the 2B Technologies Display and Graphing Software (free download
from
http://twobtech.com/software.htm
; see Appendix A for information on working
with this software). Other terminal emulation software such as HyperTerminal (a
program provided with earlier versions of Windows) or
