Pulsation Errors in Compressible Flow Measurement

As part of a study of the effects of varying aeration patterns and diffuser types on an extended aeration wastewater treatment system, researchers employed float-type rotameters with a range of 0-80 L/min (0-2.9 cfm) operating between 0 to 138 kPa (0-20 psi). During operation of the system, it was discovered that readings on the influent air meter and internal flow meters showed a consistent discrepancy of over 16 L/min (30% loss). This error did not occur when the air system was run without water in the tank. The simple structure and basic principle of operation should have made the flowmeters reliable and easy to work with. This discrepancy led the researchers to investigate several possible problems, including leaks, defects in the flowmeters and pressure, and temperature effects and pulsation. It was determined that pulsation from the compressor was the source of the discrepancy. Flow measurement errors in the system were corrected by installing a small, inexpensive accumulator.