How a Clamp-On Ultrasonic Flow Meter Works: The Transit-Time Principle

How a Clamp-On Ultrasonic Flow Meter Works: The Transit-Time Principle

A clamp-on ultrasonic flow meter measures liquid flow from outside the pipe. Two transducers are strapped onto the pipe wall, so there is no pipe cutting, no system shutdown and no contact with the fluid. Honno ultrasonic flow meters use the transit-time method.

Transit-Time Measurement

The upstream and downstream transducers exchange ultrasound alternately. One signal travels with the flow, and the other travels against it. The signal moving downstream arrives faster, while the signal moving upstream is delayed. The difference in transit time between the two directions is proportional to the flow velocity inside the pipe.

How the Converter Calculates Flow

The pipe outside diameter, wall thickness, material, lining and fluid properties are entered in the converter menu. The converter then calculates the transducer spacing and selects the V or Z mounting method. Single, dual or four channels average the velocity profile across the pipe, which is especially useful on large bore or disturbed-flow mains.

Signal Quality and Installation

Reliable readings depend on a good acoustic path. Provide about 10D of straight pipe upstream and 5D downstream; near a pump outlet or a half-open valve, allow about 30D. After mounting, check the signal strength and quality before trusting the reading: a signal strength of at least 60 and a Q value above 60 indicate a stable measurement.

Outputs and Heat Metering

The converter calculates volumetric flow and totalized volume. With PT100 supply and return sensors connected, it can also calculate heating or cooling energy. Common outputs include 4–20 mA analog, OCT pulse, relay or frequency, and isolated RS485 with Modbus.

Typical Applications

Clamp-on ultrasonic meters are used in water supply and drainage, water treatment and sewage, HVAC and district heating or cooling, industrial process water, chemical and petrochemical plants, pump and pipeline monitoring, and retrofit of existing pipe systems.

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