Since its introduction in the 1930’s, the Palmer-Bowlus Flume has grown to be the second most common flume for measuring sanitary flows.  There are, however, several distinct disadvantages in using Palmer-Bowlus flumes.

Lack of Standardization

Unlike other styles of flumes that have strictly defined sizes, configurations, and flow characteristics, the Palmer-Bowlus is a class of U-shaped flumes with a raised throat ramp.  

While a standard (ASTM D5390 - 93(2013) Standard Test Method for Open-Channel Flow Measurement of Water with Palmer-Bowlus Flumes) does exist, the standard only lays out the general shape of the flume - and not specifics as with ASTM D1941 for Parshall flumes.

The shape of the ramp can greatly affect the flumes capacity and flow sensitivity and is not standardized.  As a result, each Palmer-Bowlus flume is generally proprietary to each manufacturer and rating tables / equations for one manufacturer’s flume may not be valid for another’s.

Decreased Accuracy at Lower Flows

Under normal flow conditions where the head in a Palmer-Bowlus is large in comparison to the throat length, a Palmer-Bowlus flume is typically accurate to +/-3-5%. 

But when the flows drop and the head is small in comparison to the throat length, the accuracy drops to +/-4-6%.

Upstream Sedimentation

Because the Palmer-Bowlus flume has a raised throat ramp, upstream sedimentation can be a problem.  This is particularly true when the solids are dense / the flows are low.  

Flumes styles with flat bottoms (Parshall, Cutthroat, H, and Trapezoidal) do not suffer from upstream sedimentation in the way a Palmer-Bowlus flume can.

Sensitivity to Upstream Conditions

For greatest accuracy, it is recommended that the channel or pipe upstream of a Palmer-Bowlus flume is straight and without bends, dips, or elbows for at least (25) pipe diameters. 

Other flumes require shorter straight runs (3-5 for HS / H / HL and 15 for Parshall) making them better suited to space constrained applications.