The freezing conditions at the summit of Mount Washington make it the perfect setting for wind sensor data tests.
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Where do you go if you want to put ‘the world’s toughest wind sensors’ to the test? How do you prove your claim that your wind sensors will withstand the harshest environments that the planet can offer? FT Technologies, manufacturer of the FT7 Series of ultrasonic wind sensors, chose Mount Washington Observatory on the summit of Mount Washington in New Hampshire, USA.
THE WORLD’S WORST WEATHER
At 1,917m (6,288ft), Mount Washington Observatory is one of only a handful of permanently staffed mountaintop weather stations in the world. The self-styled ‘Home Of The World’s Worst Weather’ boasts an environment on a par with the polar regions, but offers readily accessible and consistently extreme conditions, with an average winter temperature of -14°C (6.8°F), and a record low of -44°C (-47.2°F), combined with an annual average windspeed of 16m/s (or 20m/s in winter). The summit’s distinctive feature is this combination of Arctic temperatures, high winds and high precipitation – an average of nearly 25cm (9.84in) in October and November. Mount Washington provides a challenging test environment that is impossible to reproduce in a lab.
Its freezing temperatures, moisture and high winds made it the perfect place in which to test the data availability of several different types of wind sensor. These hostile conditions can prove catastrophic to electronic precision instruments. To survive and to function, there must be adequate measures to oppose the effects of the cold. Even Mount Washington Observatory’s permanently sited pitot tube, which has 1,500W of heating, occasionally stops working due to components icing up.
FT ran tests in an icing wind tunnel to identify how much icing the sensor could resist and what power would be required to keep it ice-free. This two-part test was in accordance with US MIL-STD 810G. The first part is a de-icing test, where 45mm of ice is allowed to build up on the sensor. The sensor is then powered and must be able to de-ice itself within 15 minutes, at an ambient temperature of -14°C (6.8°F), which it did.
The second part involves spraying the sensor with water while it is powered in a 15m/s airflow at -14°C (6.8°F), until 37mm (1.46in) of ice has built up on a nearby test bar. Testing showed that with the sensor drawing 210W of power, an FT7 Series sensor could stay ice-free in an effective wind chill of -27.6°C (-17.7°F). Further testing was performed in the same icing wind tunnel with the sensor drawing only 96W of power. The sensor managed to stay ice-free down to -5°C (23°F) in 15m/s airflow, while being sprayed with water, equivalent to
-15°C (5°F) wind chill.
FT Technologies ran the
Mount Washington test
from March to May 2017.
Observatory staff erected
two FT742-DM acoustic
resonance wind sensors at the
weather station – one was set
to draw 216W of power and
the other, 288W. There was
also an unheated mechanical
sensor designed for icing
environments and another
heated ultrasonic sensor set to
draw 240W. Data availability and general
performance was datalogged using a Campbell
CR6 datalogger and the data was regularly
downloaded by FT engineers in the UK.
Ultrasonic wind sensors have no moving
parts. Mechanical anemometers are especially
vulnerable to icing conditions, which increase
mass on the cups (leading to inaccurate
windspeed measurements) before freezing
up. Maintaining ice-free performance requires
effective heating to prevent ice build-up
around and on the sensor. Consequently, the
materials used to construct the sensor must
be carefully chosen and the sensor well
designed, to ensure that heat flows evenly
over the surface of the sensor and into the
mounting system. In some circumstances the
sensor itself can be ice-free but still blinded
by ice accumulating on the mounting.
HIGHEST DATA AVAILABILITY
The lowest temperature recorded during the
test was -25.9°C (-14.6°F) with a coincidental
windspeed of 40.6m/s. At one point, glaze ice
was forming at a rate of 12cm (4.7in) per hour
in -50.9°C (-59.6°F) wind chill. Many days
had sub-zero temperatures, high windspeed,
cloud, heavy precipitation and freezing fog.
The ultrasonic sensors (unlike mechanical
sensors) can assess the quality of the data
they produce and output messages if the data
is of poor quality. The FT engineers looked at
the incidence of this data on an ongoing basis
to determine the overall data availability over
the 77 days of the test. The FT sensor
running at 288W showed 97.97% data
availability – higher than the other sensors –
and kept itself ice-free for 73.65 days. Each
icing event lasted on average just under 15
seconds, so the sensor was able to quickly
de-ice itself and continue delivering data.
Fred Squire, FT’s director of sales and
marketing, said of the test results, “We are
delighted. The FT sensors showed the highest
data availability of those tested. If our sensors
can survive on Mount Washington, they can
Detailed analysis of the data is continuing
and while the lab results showed that the sensor could stay ice free in a water stream at
a wind chill of -27.9C (-18.2°F), at Mount
Washington, icing events were occurring at
wind chills in the low -20s centigrade,
proving the usefulness of real world testing.
WORLD’S MOST TESTED
It’s not only cold temperatures and ice
that pose a threat to wind sensors. Heat,
lightning, heavy rain, sand and solar
radiation can also interfere with their
smooth functioning. When used in marine
applications, wind sensors are vulnerable
to corrosion and even bird attack.
FT Technologies sensors undergo
independent laboratory tests for shock, hail,
vibration, corrosion, altitude and lightning
protection. They were recently submitted to a
series of ingress protection tests designed to
simulate what happens to a sensor in heavy
rain driven by a typhoon or hurricane.
First, the sensors were sprayed with dust
for two hours. They were then sprayed by
powerful water jets at a flow rate of 100 l/min
(26.42gpm), from a distance of 2.5m (8.2ft),
for three minutes each time. They were also
immersed in 1m (3.3ft) of water for 30
minutes. These tests showed that the sensors
are dust- and water-tight and that they met
IP66 and IP67 ingress protection standards.
“The FT7 Series are probably the most
tested wind sensors in the world,” said
Squire. “They have passed over 28
independent tests and our own highly
accelerated lifecycle test (HALT) in which
they are temperature cycled from 125°C
(257°F) to -90°C (-130°F) while being
vibrated at 50Grms.
“Our wind sensors are 100% wind tunnel
checked before despatch, to ensure that they
will give many years of reliable service, even
in the harshest conditions. The wind tunnel
is calibrated annually and sensors are
regularly selected from production and their
calibration cross-checked in a MEASNET
wind tunnel. This ensures their accuracy.”
WINTER TESTING IN CANADA
This wasn’t the first time that FT had subjected
its sensors to this type of winter testing. In
2013, it worked with the TechnoCentre Éolien
in Gaspé, Canada, which helps businesses test and
adapt technology to the hostile northern climate.
The test site, near Rivière-au-Renard, Quebec, is
coastal, with mountainous topography. Two heated
FT702LT wind sensors were installed on a 126m (413ft)
mast, at 84m (276ft) and 122m (400ft)above ground
level. The test ran from January until May, with
performance comparisons provided by unheated and
heated mechanical cup anemometers in addition to
a conventional time-of-flight ultrasonic wind sensor.
Over the test period, nine heavy icing events
were reported at the meteorology mast. The lowest
recorded temperature was -26.2°C (-15.2°F), with
a total of 10 days below -20°C (-4°F). The maximum
windspeed during the test was 30.3m/s. The FT
sensors remained ice-free during the tests. The
heated and unheated cup anemometers experienced
icing rates of 3.4% and 13.8% respectively and the
time-of-flight ultrasonic wind sensor was destroyed
by an ice impact after 30 days in operation.
The TechnoCentre Éolien reported that both FT
Technologies FT702LT sensors achieved more than
99.9% data availability.