Basics for measuring the airflow

The Right Flow Sensor For Any Measuring Task

For measuring the flow velocity, typically, three methods are used, which are particularly different from each other with regard to their measuring range and the operating temperature:

  • Pitot tubes
  • Rotating vanes
  • Thermoanemometer probes

Pitot Tubes

The air velocity is determined by the dynamic pressure and the static pressure. Pitot tubes are robust and are available in special steel or nickel-plated brass. They connect to ALMEMO® devices by silicone hoses and a differential pressure module.

Advantage:
suitable for high flow velocities and harsh operating conditions, high ambient temperatures possible, easy to clean

Disadvantage:
strongly directional, low flow velocities are not measurable, temperature-dependent, limited accuracy, sensitive to turbulent flows

Rotating Vanes

The flow velocity is determined through a frequency measurement. Our rotating vanes are sensitive transducers with diamond bearings that are very precisely adjusted. This ensures high accuracy.

Advantage:
high accuracy at medium flow velocities and medium ambient temperatures, insensitive to turbulent flows

Disadvantage:
sensitive sensor technology, sensitive to mechanical stress, directional

Thermoanemometers

Thermistors and hot wire anemometers are highly sensitive sensors. The measuring element is continuously heated up. A control circuit keeps the temperature of the element, which has cooled down by the air flow, on a constant value. The control current is proportional to the flow velocity.

Advantage:
even very small air speeds can be measured (e.g. draught measurements), direction-independent measurements are also possible

Disadvantage:
sensitive sensor technology, sensitive to mechanical stress and contamination, sensitive to turbulent flows, high current consumption, limited ambient temperature.

Correction Factors for Exact Measurements of the Air Speed

Air Temperature 940 mbar 960 mbar 980 mbar 1000 mbar 1020 mbar 1040 mbar
–30°C 0.942 0.932 0.922 0.913 0.904 0.895
–20°C 0.961 0.951 0.941 0.932 0.923 0.914
–10°C 0.980 0.970 0.960 0.950 0.941 0.931
0°C 0.998 0.988 0.978 0.968 0.958 0.949
10°C 1.016 1.005 0.995 0.985 0.975 0.966
20°C 1.035 1.024 1.013 1.003 0.993 0.983
30°C 1.051 1.040 1.029 1.019 1.009 0.999
40°C 1.069 1.057 1.047 1.036 1.026 1.016
50°C 1.085 1.074 1.063 1.052 1.042 1.031
60°C 1.102 1.09 1.079 1.068 1.057 1.047
70°C 1.118 1.106 1.095 1.084 1.073 1.063
80°C 1.135 1.123 1.111 1.100 1.089 1.078
90°C 1.151 1.139 1.127 1.116 1.105 1.094
100°C 1.167 1.154 1.142 1.131 1.120 1.109
150°C 1.242 1.229 1.216 1.204 1.192 1.180
200°C 1.314 1.300 1.287 1.274 1.261 1.249
250°C 1.381 1.367 1.353 1.339 1.326 1.313
300°C 1.446 1.431 1.416 1.402 1.388 1.375
400°C 1.567 1.55 1.534 1.519 1.504 1.489
500°C 1.68 1.663 1.646 1.629 1.613 1.597
600°C 1.784 1.766 1.748 1.73 1.713 1.696
700°C 1.884 1.865 1.846 1.827 1.809 1.791

The true air velocity depends on the air temperature and the barometric air pressure. Therefore, the measured value must be corrected according to the above table to obtain exact measurements of the air speed.

Example:
Measured air velocity 50m/s, air temperature 80°C, atmospheric pressure 960mbar. The measured value must be multiplied with the correction value 1.123. The air velocity is, therefore, 56.1m/s.

Air Speed For Selected Dynamic Pressures (Prandtl Pitot Tube, T = 22°C)

Dynamic Pressure [Pa] Dyn. Press. [mm h.o.water] Air Speed [m/s]
1 0.1 1.29
2 0.2 1.83
3 0.3 2.24
4 0.41 2.59
5 0.51 2.89
10 1.02 4.09
20 2.04 5.78
30 3.06 7.08
40 4.08 8.18
50 5.1 9.14
100 10.2 12.93