Model 306 Ozone Calibration Source Manual Rev. G-1
20
8.
CALIBRATING THE MODEL 306 OZONE CALIBRATION
SOURCE
The absorption of 185-nm light by oxygen is very weak (
2 × 10
-22
cm
2
/molec). As
a result, the absorbance within the photolysis chamber is optically thin, meaning that
only a very small fraction of the emitted light is absorbed by oxygen to produce ozone.
Thus, for a constant flow residence time within the photolysis chamber (constant flow
rate), we expect the concentration of ozone produced to be linear with lamp intensity.
In practice, however, a slight curvature in the calibration plot is observed when the
calibration plot is extended to 1,000 ppb. This could be due to a number of factors,
one of which is a lack of a constant ratio of the intensities of the emission lines at 185
nm and 254 nm as the lamp duty cycle is varied. Since 185 nm falls in the vacuum
UV where light intensity is difficult to measure, the output of the Ozone Calibration
Source is actually calibrated against the measured intensity of the 254-nm emission
line. In order to remove the small curvature observed in the plot of ozone
concentration produced and lamp intensity at 254 nm, four parameters are used to
calibrate the instrument, consisting of two offsets Z_L and Z_H, and two slopes S_L
and S_H. The offset Z_L and slope S_L are determined for the range of output ozone
concentrations of 0 and 30-300 ppb. The offset Z_H and slope S_H are used to
produce the most accurate ozone concentration in the range 301-1,000 ppbv.
We recommend returning the Ozone Calibration Source to us annually for a new
calibration. Alternatively, you can calibrate the instrument against your own reference
ozone monitor by plotting the observed ozone concentration against the setpoint
ozone concentration and obtaining the linear regression slope m and intercept b. For
example, for the low range of ozone outputs (0, 30-300 ppbv):
[𝑂
3
]
𝐿_𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑
=
m
[
O
3
]
L_setpoint
+
b
The calibration parameters that should be entered into the instrument are S_L = 1/m
and Z_L= -b (in units of ppbv).
If you want to extend the calibration to higher concentrations, plot measured ozone vs
setpoint ozone in the range 301-1,000 ppbv and obtain the linear regression slope m
and b as above:
[𝑂
3
]
𝐻_𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑
=
m
[
O
3
]
H_setpoint
+
b
Then, S_H = 1/m and Z_H = -b (in units of ppbv). Use of two sets of slope/intercept
calibration factors allows expansion of the calibration range while maintaining the
accuracy within the specifications of 2 ppb or 2% of the ozone setpoint.
If you don’t plan to output concentrations higher than 300 ppbv, simply set S_H = S_L
and Z_H = Z_L.
