Ambient SO₂ Monitor

The APSA-370 uses an innovative detector and a new optical system for low background, high sensitivity (0.05 ppm F.S.), and greatly improved stability.

The fluorescent chamber design gives measurements with minimum influence from moisture.

The unit has built-in aromatic hydrocarbon cutter with a selective transmission membrane. This reduces the influence of interference components. Coupled with HORIBA's unique flow-path, it also makes it possible to extend the working life of the cutter and to take measurements effects of sample flow variations.

In comparison with the FPD method, the APSA-370 design is (1) highly selective for SO₂, (2) requires no supplemental gas, and (3) gives linear output.

Compensation for the lamp's luminous energy decline guarantees prolonged calibration stability.

The sample inlet has a built-in Teflon filter.

UV fluorescence
The UV fluorescence method operates on the principle that when the SO₂ molecules contained in the sample gas are excited by ultraviolet radiation they emit a characteristic fluorescence in the range of 220-420 nm. This fluorescence is measured and the SO₂ concentration is obtained from changes in the intensity of the fluorescence.
The reactive mechanism is
(1) SO₂+h ν₁ → SO₂*
(2) SO₂* → SO₂+h ν₂
(3) SO₂* → SO+(O)
(4) SO₂*+M → SO₂+M
Here, (1) shows the excited state of the SO₂ mole-cules that have absorbed the amount of energy h ν₁ by ultraviolet radiation. (2) shows the amount of energy, h ν₂ emitted by the excited molecules as they return to the ground state. (3) shows the decomposi-tion by the light emitted from the excited molecules. (4) shows the quenching, i.e., the energy lost by the excited molecules colliding with other molecules. The APSA-370 uses an Xe lamp as the light source, and the fluorescent chamber design minimizes scattered light. The optical system has been care-fully designed with low background light, making it possible to take measurements with a highly stable zero point. In addition, a reference detector monitors any fluctuation in the intensity of the light source. This allows the unit to calibrate itself automatically for sensitivity, resulting in greater span stability.