Teledyne 452 3 Theory of Operation

Teledyne API M452 Ozone Sensor Instruction Manual, 02852, Rev. E

3-1

3 Theory 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 452 uses a mercury lamp constructed so that a large majority of the

light emitted is at the 254 nm wavelength. Light from the lamp shines

through an absorption cell through which the sample gas being measured is

passed. The ratio of the intensity of light passing through the gas to a

reference measurement, which does not pass through the gas, forms the

ratio I/Io. This ratio forms the basis for the calculation of the ozone

concentration.

The Beer-Lambert equation, shown below, calculates the concentration of

ozone from the ratio of light intensities.

Opsi

CΙ

−= ln

695.14

273

Where:

I = Intensity of light passed through the sample

Io= Intensity of light through sample free of ozone

= Absorption coefficient

l = Path length

O3 = Concentration of ozone

T = Sample temperature in degrees Kelvin

P = Pressure in pounds per square inch (absolute)

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 cell, 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. Lastly, the absorption path length determines how

many molecules are present in the column of gas in the absorption cell.