It used to be that NTUs were the unit for all turbidity sensors. In 2007, an ASTM committee formed and established different units for each category of turbidity sensor technology, and there are many. Backscatter sensors such as the OBS-3+ and the OBS501 have units of FBU (Formazin Backscatter Units), whereas ISO 7027 side-scatter sensors have units of FNU (Formazin Nephelometric Units). The side-scatter OBS501 and the ClariiVUE™ sensors are part of this category. While sensors of different technical designs can be calibrated and measure Formazin the same, they will not make the same measurement in natural waters where size, shape, color, reflectivity, etc., of the particles vary. The ClariiVUE™ sensor measurements will not match the OBS-3+ measurements in natural waters.