ST-730 Series is designed for monitoring the middle range turbidity in various applications while the ST-73X Series is for very low turbidity applications. It is more robust and tolerant to fouling, particular matters, and air bubbles comparing to other online turbidity meters designed for relatively clean water applications. The turbidity sensor measures two optical densities and two scattering intensities at two wavelengths. This helps measure a large turbidity range and yet maintain a fine resolution. This design also helps the sensor run self-diagnostics.