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11. Air Velocity Compensation
When the heated air velocity sensor of the instrument is exposed to airflow, the heat is drawn from the sensor. The
instrument obtains air velocity readings by using this relationship between the amount of heat removed (heat
dissipation) and air velocity.
Since the instrument is calibrated with clean airflow with normal temperature and pressure, when the condition of air
to be measured is different from that of the air used for calibration, the heat dissipation amount will differ even when
the velocity is consistent (i.e. velocity reading is influenced by the condition of air).
11.1 Influence of Air Temperature
The instrument is a hot-wire anemometer, which measures the air velocity by using the heat dissipation amount. Thus,
if temperature compensation is not provided, air velocity readings will by affected by the ambient air temperature
change even when the air velocity is consistent. In order to prevent such influence, the instrument is equipped with a
temperature compensation circuit for measuring and compensating the air temperature in the range of 5°C to 60°C.
11.2 Influence of Atmospheric Pressure
The instrument is calibrated under atmospheric pressure of 1013hPa. Since change in the atmospheric pressure will
influence the heat dissipation amount, compensation of the atmospheric pressure is required. Compensation can be
provided by using the following formula.
=
Um
Where, Um: Actual Air Velocity [m/s]
11.3 Influence of Air Composition
Compensation is required if the measurement is to be performed in an environment including any gas other than air.
Compensation shall be performed by calculating the heat dissipation amount from the physical properties of the gas,
and comparing it with the heat dissipation amount of the air.
1013
×
Uc
Pm
Uc: Air Velocity Reading
Pm: Atmospheric Pressure at the Time of Sampling [hPa]
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