3.PRINCIPLE OF OPERATION
The detection of ozone molecules is based on absorption of 254 nm UV light due to an internal electronic resonance of the O3 molecule. The Model 465 uses a mercury lamp constructed so that a large majority of the light emitted is at the 254nm wavelength. Light from the lamp shines down a hollow quartz tube that is alternately filled with sample gas, then filled with gas scrubbed to remove ozone. The ratio of the intensity of light passing through the scrubbed gas to that of the sample forms a ratio I/Io. This ratio forms the basis for the calculation of the ozone concentration.
The
C = − | 109 | × | Τ |
| × | 29.92inHg | × ln | Ι |
| o |
|
|
| ||||
O3 | α × | Κ |
| Ρ |
| Ιo | ||
| 273 |
|
|
Where:
I = Intensity of light passed through the sample
Io = Intensity of light through sample free of ozone
α= absorption coefficient = path length
CO3 = concentration of ozone in ppb
T= sample temperature in degrees Kelvin P = pressure in inches of mercury
As can be seen the concentration of ozone depends on more than the intensity ratio. Temperature and pressure influence the density of the sample. The density changes the number of ozone molecules in the absorption tube which impacts the amount of light removed from the light beam. These effects are addressed by directly measuring temperature and pressure and including their actual values in the calculation. The absorption coefficient is a number that reflects the inherent ability of ozone to absorb 254 nm light. Most current measurements place this value at 308
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