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Calibration and requirements for accuracy of thermal anemometers for indoor velocity measurements

Ziel

Standards for the determination of the thermal comfort of people indoors require the measurement of air velocity in rooms. Thermal anemometers with a hot-sphere type velocity sensor are recommended in the standards and are most commonly used for making the required velocity measurements. There are, however, large discrepancies in the results obtained with different instruments currently available on the market. Several factors, such as the calibration and the dynamic behaviour of the anemometer, the design of the transducer and its directional sensitivity, the measuring and sampling period, the natural convection flow generated by the heated velocity sensor, etc. all have an impact on the accuracy of the velocity measurements.

The aim of the project was to:
identify the requirements for thermal anemometers with a hot-sphere-type sensor that will improve the accuracy of low velocity indoor measurements of airflow;
develop an accurate test method for describing the dynamic characteristics of thermal anemometers with a hot-sphere-type sensor.
Mean velocity up to 0.6 m/s, turbulence intensity up to 70% and temperature fluctuations up to 3°C were identified in rooms. The frequency of the velocity and the temperature fluctuations that contributed up to 90% of the RMS values were 2 Hz and 1.2 Hz respectively. Test methods for describing the dynamic characteristics of thermal anemometers for low velocity measurements and the directional sensitivity of hot-sphere-type velocity sensors were developed. New requirements for low velocity thermal anemometers were developed. The results have been transmitted to CEN TC156/WG4, CEN TC122/WG11 and ISO TC159/SC5/WG1 and have been reported at conferences and published in journals.
During the first phase of the project a large data base comprising 2500 velocity and temperature measurements in test rooms and rooms in the field with different combinations of ventilation system, air supply device, airflow rate, occupancy, etc. was collected and analysed. A three-dimensional laser Doppler anemometer, low velocity thermal anemometers and specially developed thermometers allowing measurement of fast temperature fluctuations were used. During the second phase of the project airflows with characteristics similar to those identified in rooms were generated under laboratory conditions. The impact of the following factors on the accuracy of low velocity measurements by means of thermal anemometer with a hot-sphere-type velocity sensor was investigated: the amplitude and the frequency of the velocity and the temperature fluctuations; the directional sensitivity and the overheating temperature of the velocity sensors; the free convection from the velocity sensor; the measuring and the sampling time. Tests on the dynamic response of the thermal anemometers were performed.

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