To continuously get a fresh sample of the process liquid, the probe tip must face the flow.

The flow impinging on the measuring window keeps it clean.

The concentration measurement of a refractometer depends on both refractive index and temperature. The optical refractive index measurement reacts immediately, but the temperature has a delay dependent on the probe dimensions.  For a slow temperature reading, a temperature change makes the concentration reading jump, even when the concentration stays constant. Such a jump causes a disturbance in feedback control. Only ThinProbe has comparable reaction times for both measurements.