The UNI EN ISO 7726 standard prescribes strict limits regarding the accuracy and response times of the sensors for air speed measurements, both in comfort and thermal stress conditions. Air velocity must be described both by the average velocity over a period and by its standard deviation. Speed fluctuations, indicated by the turbulence index TU, must also be monitored.
LSI Lastem is among the few companies in the world to design and manufacture anemometers based on true “hot wire” technology, in which the sensitive element is not a thermistor but a tungsten wire, with all the advantages deriving from an accurate sensor, quick in response and adhering to the specifications of ISO 7726 in the accuracy above 0.5 m/s.
To be able to meet the requirements of the standard in the comfort application, including a measuring range from 0.05 to 1 m/s, an accuracy of ± (0.05 + 0.05 * va) m/s, where va is the average speed over the measurement period, and a response time of 0.5 s, there is a need to have a stable, fast and accurate sensor. Just as it is essential to have short response times to capture the smallest fluctuations in air velocity necessary to obtain the turbulence index.
The LSI Lastem hot wire anemometer, made with a 9 µm tungsten wire, thanks to its very low thermal inertia, has a response time of only 0.01 s, optimal for calculating turbulence and adequately monitoring changes of air speed.
The measuring range of the sensor is incredibly wide, from 0.01 to 20 m/s, a feature that makes the device suitable for measurements in the field of comfort and thermal stress, as prescribed by the UNI EN ISO 7726 standard. The LSI Lastem hot wire is sensitive to air currents from a direction of 0-300 ° in the plane perpendicular to the wire, without prejudice to the presence of the support.
Although hot wire technology has been used in indoor measurements for several decades, it remains among the best for reliable and fast measurements; many manufacturers on the market improperly call their anemometers “hot wire”, when instead they present hot spheres or thermistors as sensitive elements.
From laboratory comparisons, it emerges that thermistors or hot spheres have limits both in terms of directional response, when their orientation in space is changed, and high thermal inertia, which cause these sensors to have a too slow response to guarantee a valid measurement.
