The Model 49i operates on the principle that ozone (O3) molecules absorb UV light at a wavelength of 254 nm. The degree to which the UV light is absorbed is directly related to the ozone concentration as described by the Beer-Lambert Law:
where: K = molecular absorption coefficient, 308 cm-1 (at 0°C and 1 atmosphere)
L = length of cell, 38 cm
C = ozone concentration in parts per million (ppm)
I = UV light intensity of sample with ozone (sample gas)
Io = UV light intensity of sample without ozone (reference gas) The sample is drawn into the Model 49i through the sample bulkhead and is split into two gas streams, as shown in Figure 1-1. One gas stream flows through an ozone scrubber to become the reference gas (Io). The reference gas then flows to the reference solenoid valve. The sample gas (I) flows directly to the sample solenoid valve. The solenoid valves alternate the reference and sample gas streams between cells A and B every 10 seconds. When cell A contains reference gas, cell B contains sample gas and vice versa.
The UV light intensities of each cell are measured by detectors A and B. When the solenoid valves switch the reference and sample gas streams to opposite cells, the light intensities are ignored for several seconds to allow the cells to be flushed. The Model 49i calculates the ozone concentration for
KLC−
=e
oI I
Introduction Specifications
Thermo Fisher Scientific Model 49i Instruction Manual 1-3
each cell and outputs the average concentration to the front panel display, the analog outputs, and also makes the data available over the serial or ethernet connection.