Open Channel Flow Elements-non-pressurized

Halmi-Parshall flume and nozzle: greater accuracy for wastewater discharge measurement


We developed the Halmi-Parshall flume to overcome the inaccuracy of the traditional pipe-to-box-to-flume-to-box-to-pipe installation. Flow calibrations performed at the Alden Research Laboratory reveal that the customary round-pipe-to-rectangular-flume-and-back-again design introduces the flow metering errors shown below and greater ones, depending on the geometry of the metering section.



Our patented Halmi-Parshall flume offers these significant advantages:

  • +/- 2.0% accuracy, +/- 0.75 with flow calibration
  • Accuracy traceable to NIST (formerly NSB)
  • Accuracy can be verified visually in the field
  • Low head loss
  • Adjustable laying length
  • Any pipeline size
  • Removable restrictors for adjustable flow meter capacity
  • Our design service will properly locate the meter
  • Meter can be built of any material required by the application
  • Any signal sensor can be used, including C/R, staff


We also offer our patented Halmi-Nozzle for free discharge into tanks or lagoons from partially filled pipes. It provides essentially the same features as the Halmi-Parshall flume.



For many of our clients the most important benefit of working with us is our expert design service, which is part of the metering package we offer. Knowledgeable design is important because placement of a flume in the pipeline and the geometry and slope of the upstream and downstream piping play a major role in securing accurate flow measurement.


This level of design assistance is unavailable from the “pipe-to-box-to-flume-to-box-to-pipe” design manufacturers. When these manufacturers are approached with placement questions, they disclaim responsibility for the accuracy of the installed meter. Instead, they refer customers to standard published Parshall flume literature, implying that the flumes described therein are identical to those they are selling. This implication is far from the truth, as proved by the errors uncovered through our own research (see PFS Publication No: 820-A). In contrast, Primary Flow Signal. takes full responsibility for the performance of PFS-Halmi Flume and PFS-Halmi Nozzle designs.


If the EPA or other authorities question your wastewater discharge measurement, Primary Flow Signal’s reliable open channel flow elements and design service are what you need.