4.3811.00.000_US-Anemometer-DSP_eng.doc
6 - 26
021511/10/06
2.2
Measuring principle: Acoustic virtual temperature
The speed of propagation of the sound depends on the absolute temperature of the air via a root
function although it is more or less independent of the air pressure and only depends on the air
humidity to a minor extent. We can thus make use of these physical properties for temperature
measurement of air with a known and constant chemical composition.
Here we measure gas temperature without the indirect step of thermal coupling involving this gas to
a sensor.
The advantages of this measuring method are firstly an inertia-free reaction to the actual gas
temperature, and secondly, the avoidance of measuring errors such as those that occur for
example when a solid-state temperature sensor is heated by radiation.
Due to the low dependence of the speed of propagation of the sound on the air humidity level the
"acoustic virtual temperature" relates to dry air (0% humidity) under the same pressure conditions
as those actually measured.
The deviation of the measured "acoustic virtual temperature" from the real air temperature shows
linear dependence on the absolute humidity level of the air.
The amount of water vapour in the air proportionately increases the speed of the sound as H
2
O
molecules only have around half the mass of the other air molecules (O
2
and N
2
).
The increase in the speed of the sound results in an apparent (virtual) increase in the measured
temperature of humid air in comparison to dry air of the same temperature.
The deviation of the measured virtual temperature of humid air from the real air temperature can be
corrected for instance according to the following relationship if the absolute humidity is known:
T
r
= T
v
– 0.135 K * m
3
/ g * a
where
T
r
is the real air temperature,
T
v
the measured acoustic virtual temperature and
a
the
absolute humidity in grammes H
2
O per m
3
air.
With an air temperature of 20°C the virtual temperature is around 2 K too high with 100% rel.
humidity.
3
Preparation for Operation / Installation
Attention:
The working position of the anemometer is vertical (sensor
arms "above", connector "below").
During installation, de-installation, transport or maintenance of
the anemometer it must be ensured that no water gets into the
shaft or connector of the anemometer.
When using a lightning rod it must be borne in mind that it
should always be installed less than 45° to a measurement
path; otherwise there will be deviations in the measured values.
