One of the key elements in using a flume for accurate flow measurement is ensuring that sub-critical flow is present. 

Flumes work by using a change in floor elevation, sidewall contraction, or both to accelerate sub-critical flow to super-critical flow in the throat of the flume.  In doing this, the result is that as a single defined point of measurement (Ha) upstream of the throat (where the flow has been accelerated) there’d a know level-to-flow relationship.

When flow enters a flume with critical or super-critical velocity, the throat of the flume no longer serves it purpose and the level at Ha no longer accurately reflects the flow rate through the flume.

In cases where the flow stream is clean and clear of fibrous / sanitary / large solids, Tranquilizer Racks can be used to slow the flow stream prior to it entering the flume.  Typically used in rows of two or more, Tranquilizer Racks serve to break up the flow stream – creating backwater – and straighten it out so that it flows parallel to the longitudinal axis of the flume.

Tranquilizer Racks can also serve to create well distributed and laminar flow in a flume when approaching velocity profile is poor and / or waves/surface disturbances are present.

 

Tranquilizer Racks for sanitary (screened with no fibrous solids) applications are fabricated from 316 stainless steel, while those for surface or stormwater applications are fabricated from 304 stainless steel. 

In specialty applications – such as Acidic Mine Discharge and Landfill Leachate – where sampling for metals occurs, Tranquilizer Racks may be fabricated from PVC or pultruded fiberglass.

In applications where the approach velocity is in excess of what can be handled by Tranquilizer Racks alone, Energy Absorbers (as inserts, Energy Absorbing Enclosures, or Energy Absorbing Manholes) may be placed upstream flume / channel containing the Tranquilizer Racks.