Model 405 nm NO
2
/NO/NO
x
Monitor Manual Rev. D
20
desired units. Pressure units may be selected as either
torr
or
mbar
. A click on
returns the display to the
Unt
menu.
4.14.2. To Change the Units for NO
2
and NO
From the
Unt
menu, select
NOx
to display the following:
NOx Units
NOx: ppb
Select the units (ppb in this case) and rotate the Select switch to cycle the cursor
between the choices of units. NO
2
(and NO) units may be selected as ppb, pphm and
ppm. A click on
returns the display to the
Unt
menu.
4.15. To Test the LED Voltage and Precision
A diagnostic LED test is provided in the
Cfg
submenu of the main menu. During the
LED test, the instrument measures “effective” NO
2
and NO concentrations without
switching the solenoid valve on and off. These are electronic zeros and should after a
few readings settle down to ± a few ppb. If the value is outside the range -9 to +9, the
instrument may not be operating correctly. The display also gives a standard deviation
of the electronic zero. For best results, the standard deviations should be not greater
than ±2.0. Note that because less averaging is involved, this is not equal to the overall
precision of the instrument; it is a diagnostic test of LED fluctuations. The LED test also
measures and outputs the voltage levels of the photodiode detector (Vs).
For best performance the photodiode voltage (Vs) should be in the range 0.5-2.0
volts. For detector voltages less than about 0.5 volts, the data may be noisy due to
insufficient light intensity to make precise measurements. If the voltage is zero, the
LED light source may have burned out. For voltages above 2.5 volts, the A/D
converter is saturated and the measured NO
2
and NO values will always be zero.
This could happen if the instrument is very hot so that the LED output is too bright.
To exit the
LED
test mode, hold in the Select switch until the screen is returned to the
Cfg
menu.
4.16. Measurement Modes
The Model 405 nm has the capability to measure nitrogen dioxide and nitric oxide
each individually or simultaneously (5 seconds apart). There are 3 measurement
modes available: NO
2
mode, NO mode, and NO
2
and NO mode. The measurement
mode can be changed by entering the
MOD
menu and selecting either “NO2”,
“NO2/NO”, or “NO”. If using a serial connection, the mode can be changed using
serial commands “G” for NO
2
mode, “B” for NO
2
and NO mode, or “N” for NO mode.
