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The measurement principle behind optical DO sensors is based upon the effect of dynamic luminescence quenching by molecular oxygen. Luminescent quenching is observed through changes in phase angle between excitation and emission light wavelengths correlating to varying oxygen content. The relationship between oxygen and phase angle is described by the Stern-Volmer equation, with the Stern-Volmer coefficient determined empirically from phase angle measurements observed at zero oxygen and ambient oxygen levels. The graph below shows the resulting “lifetime decay” of the luminophore phase angle with respect to oxygen content.

KB DO Accuracy sat

The relationship between phase shift and oxygen content is not linear, with a general decrease in signal sensitivity corresponding to increased oxygen content as luminophore active sites become saturated. Due to this phenomenon, measurement accuracy for VisiFerm mA and VisiFerm RS-485 sensors is divided into three distinct ranges based upon phase angle response to changing oxygen contents, summarized below.

VisiFerm Accuracy Specification @ 25°C

% Volume (Gas Phase)Water Saturated with Air (100%-sat = 21%-vol)
1%-vol ± 0.05%-vol 4.8%-sat ± 0.2%-sat
21%-vol ± 0.2%-vol 100%-sat ± 1.0%-sat
50%-vol ± 0.5%-vol 237.5%-sat ± 2.4%-sat

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