Personal Ozone Monitor Manual Rev. E-2
17
6.
LAMP TEST
If the instrument is excessively noisy (standard deviation greater than 2 ppb) when
measuring clean or zero air, or always reads near zero in the presence of ozone, it is
useful to perform the lamp test to make sure that the lamp is turning on and does not
fluctuate too rapidly. Before performing the lamp test, allow the instrument to warm up
for at least twenty minutes.
Choose
Lmp
from the main
Menu
. The display wil
l momentarily read “
Lamp Test
.
”
The photodiode voltage will then be displayed, and after a few lamp measurements
have been made, the electronic offset and standard deviation also will be displayed
as, for example:
PDV= 0.89801 V
1.2+/-1.85
The photodiode voltage (PDV) is a measure of the lamp intensity and should be in the
range 0.6-2.2 volts. Since absorbance is a ratio measurement, the absolute value of
the voltage is not particularly important. However, above 2.5 volts, which could occur
if the instrument is allowed to become too hot, the photodiode is saturated and the
calculated ozone concentration will be zero. Photodiode voltage less than 0.6 volts is
indicative of either a weak lamp or a dirty detection cell and may result in a noisy
measurement. The photodiode voltage will typically increase as the instrument warms
up. Lamp drift is continuously monitored and corrected for in the firmware and thus
has very little effect on the measured ozone concentration.
Once the instrument is warmed up, fluctuations in photodiode voltage should be
limited primarily to the last digit displayed. The lamp test also calculates an electronic
offset and standard deviation of the measurement itself, displayed in the above
example as 1.2 ppb for the electronic offset and +/-1.85 for the standard deviation.
The standard deviation is a quantitative measure of the lamp and associated
electronic noise. Electronic offsets should normally be
–5 to 5 ppb equivalent. After
running the lamp test for a few minutes, values above 2.50 for the standard deviation
usually indicate an excessively noisy lamp. Lamps seldom
“burn out” but may
become noisy with time and need to be replaced. Some lamps become noisy after
only a short period, while others will be extremely stable for years. If your lamp fails
the lamp test during the first year of operation, contact us for a new lamp under the
instrument warranty. Contamination of the detection cell may also cause a high
standard deviation, in which case the flow path should be cleaned with methanol and
the internal ozone scrubber replaced. Please contact us to return the instrument to 2B
Tech for cleaning and/or lamp replacement.
To exit the Lamp Test, press and hold the Select button.
