A 0.6-Foot H Flume with probe and stilling wells.  Other than differing in diameter, probe wells and stilling wells identical in construction and configuration.  

Probe wells are typically 1-inch to 3-inches in diameter and extend 3-inches below the floor of the flume.  Stilling wells are typically 6-inches to 18-inches in diameters and also extend 3-inches below the floor of the flume.  Both are connected to the flume through a 1-inch or 2-inch diameter opening in the flume's sidewall.

Probe wells are normally used to house either pressure transducers or single parameter probes.  Stilling wells are used with ultrasonic flow meters, float meters, and larger, multi-parameter probes.