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Focus on Ultrasonic Liquid Flow Metering

Polysonics Hydra SX30 Doppler Liquid Flowmeter
  • Panametrics PT878 Ultrasonic Flowmeter
  • Panametrics PT878 Ultrasonic Flowmeter

Ultrasonic liquid flowmetering is a convenient method of determining liquid flow within rigid pipes. Instruments are portable, easily configured, self-powered, log data, and are non-intrusive, so no pipefitting or modification required, no need to halt production, no risk of contamination. They cater for a wide range of fluids and conditions by utilising one of two methodologies.

Method 1: Transit Time – Time of Flight


  • Potable water, Wastewater, Cooling and heating water, Ultrapure water and liquids, Water/glycol solutions, Crude oil, Refined hydrocarbons, Diesel and fuel oils, Chemicals, Lubricating oils, Beverages.
  • Establishing flows of water, waste, coolant, lubricant, fuel, beverages.
  • Determining consumption of water, waste, coolant, lubricant …
  • Energy auditing of heat exchangers


Two transducers are coupled to the surface of rigid pipe. Each acts as a transmitter and receiver. Ultrasonic signals are sent [and received] by both transducers alternately. Sound energy in a moving liquid is carried faster when it travels in the direction of fluid flow (downstream) than it does when it travels against fluid flow (upstream), therefore a differential in time of flight will result, and the instrument calculates from this the rate of liquid flow.

It is important that the fluid is homogenous and carries minimal solids and bubbles as these tend to scatter the sound pulses and reduce the signal strength.

Common transducer positions allow for one (Z) traverse where transducers are diametrically opposed but longitudinally spaced, two (V) traverses where the transducers are mounted on the same side of the pipe, or multiple even (W) traverses. Pipe diameter largely dictates which method is used.

Applicable Model: Panametrics PT878 Transit Time Liquid Flowmeter

Method 2: Doppler effect


  • Raw sewage, Primary sludge, Slurries, Carbonated Beverages, Paper pulp, Waste water, Grey water, Aerated fluids.


In contrast to Transit Time flowmeters ‘Dopplers’ are reliant on ultrasonic pulses bouncing off targets so do require a degree of aeration or suspended (not dissolved) solids. Each transducer sends pulses which are then reflected back at a rate dependant on the speed of the moving target (bubble / solid etc). The ‘Doppler Effect’ is widely known for the change in pitch when an object such as a vehicle (plane etc) approaches and passes an observer. The difference between the reflected frequencies and transmitted frequencies is directly proportional to the speed of the sonic reflectors, and thus the instrument calculates the rate of liquid flow.

Applicable Model: Polysonics Hydra SX30 Doppler Liquid Flowmeter

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