Ultrasonic anemometer vs cup anemometer

An ultrasonic anemometer measures wind by transmitting sound pulses between fixed transducers, while a cup-and-vane anemometer uses rotating cups to measure wind speed and a vane to determine direction.

The two technologies measure the same fundamental variables but behave differently because one relies on mechanical movement and the other does not. That difference affects starting threshold, response, maintenance, environmental performance and whole-life cost.

For applications where sensors are difficult or expensive to access, the absence of rotating measurement components can be an important consideration. It is not, however, the only factor that should determine the specification.

The two designs side by side

The difference becomes most relevant when the sensor has to provide reliable measurements over long periods, particularly where inspection or maintenance requires access to a mast, vessel, roof or other exposed installation.

How mechanical wear changes the measurement

Mechanical wear does not necessarily produce an obvious sensor failure.

As bearings become less free-moving, the force required to start the rotor increases. The effect can be a gradual change in low-wind measurement before the instrument stops functioning altogether. The resulting data may still look credible, making the degradation difficult to identify from the output alone.

This matters because low-wind conditions are often part of the measurement requirement, rather than simply the edge of the operating range. A sensor can therefore remain apparently operational while its measurement behaviour has changed.

Ultrasonic sensing removes this particular mechanical degradation path. There are no bearings or rotating components affecting the wind measurement, and Gill ultrasonic sensors provide instrumentation status information with the output to identify sensor faults.

That does not make ultrasonic sensing immune to failure. Environmental exposure, contamination, installation and electronic faults still need to be considered. The difference is that there is no rotating bearing whose condition progressively changes the wind measurement.

Where a cup anemometer still makes sense

Cup-and-vane anemometers remain appropriate in some applications.

Where a long-term measurement series has historically been collected using cup-and-vane instruments, maintaining the same measurement principle can be important for continuity. Introducing a different sensing technology can create a change in the data series that needs to be understood when historical and new measurements are compared.

They can also be appropriate where the installation is temporary, readily accessible or subject to a specification that calls for mechanical measurement. In these circumstances, acquisition cost may carry more weight than the maintenance implications of rotating components.

The decision therefore depends on the application rather than a simple technology hierarchy. Exposure, accessibility, measurement requirements, existing data, maintenance arrangements and whole-life cost should all form part of the specification.

Reading a specification honestly

Comparing anemometers requires more than placing headline accuracy figures beside one another.

• Compare accuracy at a stated wind speed. Gill 2-axis sensors are specified at 2% RMSE for wind speed and 2° RMSE for direction at 12 m/s. Accuracy figures without a reference wind speed are difficult to compare meaningfully.
• Look at the starting threshold. This indicates how the instrument behaves at low wind speeds and can be particularly important where low-speed measurements form part of the application.
• Consider response characteristics. A rotating instrument has mechanical inertia. An ultrasonic instrument does not, so the two technologies can behave differently as wind conditions change.
• Understand the maintenance requirement. A mechanical sensor’s expected service life depends on the bearings and other moving components operating in the conditions where the instrument is installed.
• Consider the cost of access. Purchase price is only one part of whole-life cost. Inspection, calibration, replacement and access to the installation can all contribute.
• Check the environmental specification. Wind range, temperature, ingress protection, heating and installation conditions should be considered alongside measurement accuracy.

The best specification is therefore the one that considers how the instrument will perform at the site, over the intended measurement period and within the wider system, rather than comparing a single headline figure.