Why Ultrasonic Anemometers are more cost effective and reliable than the traditional Electro-Mechanic.

1. Gill Instruments: environmental & industrial monitoring solutions
Technical Key Note
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Key Note Series Number KN1804
Subject KN1804 Gill Ultrasonic vs mechanical wind sensors
Applicable models All Gill ultrasonic anemometers
Date: 29th
January 2016
Introduction
Mechanical wind sensors have been used worldwide for over a 100 years. However, although they
have given good service, there are inherent problems which are solved by using ultrasonic
technology.
Ultrasonic wind sensors have been widely in use for over 30 years. However, in the last 15 years they
have become more cost effective and accepted by meteorological institutions worldwide, so they are
more competitive than mechanical sensors.
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Benefits of ultrasonic wind sensors
 Wind speed and direction from a single sensor avoids multiple mounts, cables and
connections to a datalogger.
 No moving parts:
o No wear & tear to increase the ongoing costs of ownership such as maintenance and
servicing costs.
o No requirement for expensive yearly/bi-yearly recalibration and No wear & tear to
affect data which can reduce responsiveness and dramatically affect data quality with
time.
 Self-checking via digital status codes provides users with confidence in the data and
knowledge of whether a sensor has failed (and what exact issue the sensor may have, for
example a blocked path, electronics failure, etc.)
 Intelligent processing (as used by Gill) avoids the possibility of incorrect data and therefore
provides users with confidence and better quality data in more extreme conditions than would
otherwise be possible with a mechanical sensor.
 Robust sensors to survive installation and extreme conditions worldwide as there are no
moving parts, they do not damage should conditions ice up. As the ultrasonic anemometers
are made out of very robust polycarbonate or metal, they can withstand far more abuse
before any damage occurs compared to the very lightweight and flimsy mechanical sensor
material required to respond with enough sensitivity to changing conditions.
 Multiple output types from a single sensor offers flexibility when selecting dataloggers or when
making network connections, for example to SCADA systems or MODBUS networks.

2. Gill Instruments: environmental & industrial monitoring solutions
Technical Key Note
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 User selectable outputs allow a choice of output frequencies and formats, including options
such as SDI-12 and NMEA as standard. User selectable scaling for site and application
specific conditions, such as road tunnels where the speed ranges may only require 0-30 m/s
scaling with excellent resolution.
 Ultrasonic sensors measure without any resistance ensuring accurate measurements are
made, even at low speeds as they do not need to overcome a starting threshold like
mechanical sensors (to overcome the initial friction of the grease and bearings). For this
reason, ultrasonic anemometers can measure from 0.001 m/s wind speeds whereas
mechanical sensors typically start at 0.3 or even 0.5 m/s.
 Instantaneous measurements are possible allowing for accurate recording of fast and slow
speed/direction changes. This is particularly important in gusting conditions as mechanical
sensors can only slow back down to the mean wind at a set rate, this means speeds are
recorded higher than reality for longer (also known as over-speeding).As ultrasonic
anemometers have no moving parts, they provide vectorial outputs instantaneously,
producing a better ‘picture’ of the changing weather than mechanical sensors which can only
output values in scalar mode due to the equal force applied to both sides of a cup
anemometer as it spins around.

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Real world tests for mechanical vs ultrasonic sensors
In real world conditions all mechanical sensors such as cups & vanes and propellers will have
limitations that are not associated with ultrasonic wind sensors. Below are some simple wind tunnel
tests that can be carried out to simulate real-wide conditions:
1. Speed changes including gust conditions, i.e. fast changes from 5 to 15 m/s and back
again, will test how quickly the cup & vane / propeller can indicate the changes and stabilise.
However, gusting will cause issues with the cup & vane / propeller. The response time to the
gust will be poor and the direction and speed values will be slow when trying to see changes
quickly.
Ultrasonic wind sensors will provide instant and accurate responses to changes in
wind speeds.
2. Directional changes, in the real world wind is not constant and uniformly from one direction
only and shifts in direction of around 30-45 degrees are frequent. A cup & vane / propeller
will be constantly move back and forth, so will have an additional error in the values it ‘sees’.
This could be tested to a degree by putting the vane / propeller 90 degrees to the tunnel flow
and then do a slow wind speed of around 1 m/s…to measure the duration taken by the vane
/ propeller to align properly and measure accurately.
Ultrasonic wind sensors provide instant and accurate wind direction readings.
3. Starting threshold, start a speed test at 0.01 m/s and increase at this step resolution up to
1 m/s. See how a cup doesn’t react until around 0.15 or 0.2 m/s and a propeller
anemometer doesn’t react until around 0.3 or 0.5 m/s.
Ultrasonic wind sensors will provide wind speed data from 0.01m/s (or even 0.001 m/s
on some models) and wind direction once wind speed reaches 0.05m/s, 3-10 times
sooner than a cup and vane / propeller.
4. Ramp test, to show slow reaction to quick changing winds in the real environment. Start
at a high speed and quickly but uniformly reduce speed down to 10 m/s, log data from both
the tunnel and a cup / propeller and watch how a cup / propeller constantly reads high as
the bearings need time to react to the reduced speed (imagine a gust in real life, a cup /
propeller will not see the gust until after a small delay and then will not see the peak value
correctly if the gust is short and then when the gust stopsand the speed reduces to ‘normal’
values, a cup & vane / propeller will over read for several seconds before correcting again.
Ultrasonic wind sensors will not over or under read as the measurement is
instantaneous.

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5. Marine use of wind sensors has grown exponentially over the last decade and so salt
water test/long life testing is imperative. With any moving parts system, accelerate the life
cycle by running a constant use test for 6 weeks and then do the same tests again to see
how well the unit then performs, it will have a significantly different response due to the
seizure / reduced amount of grease in the bearings and could be too fast or too slow
compared with the reference. This behaviour changes throughout the life cycle (and not in a
linear fashion so is very hard to compensate for with mechanical sensors) and if run for
another 6 weeks, the test results may show further degradation. Additionally, by placing a
cup & vane / propeller in a salt water long life test (to simulate coastal environments), it will
be possible to observe how the performance changes as corrosion begins to effect the
performance of the bearings which will degrade overtime.
Ultrasonic wind sensors are used extensively in the marine environment and
performance does not degrade over time, the quality of data remains constant
throughout the life cycle of an ultrasonic and there is no need to recalibrate for very
long periods of time all of which mean better data quality and reliability with lower
running costs.