KANOMAX 6162 Anemomaster - Manual (Page 44)

7. Measurement Principle 38
7. Measurement Principle
7.1 Principle of Hot-Wire Anemometer
When the heated air velocity sensor is exposed to airflow, the sensor will be
cooled. As the sensor temperature changes, the residence value changes
accordingly. The faster the velocity is, the greater the residence value changes.
Therefore, if how air velocity is proportional to the residence value is understood,
air velocity can be calculated using the measured residence value (or electric
current).
Our Anemomaster uses this principle. In general, a hot-wire anemometer employs
a feedback circuit to keep constant temperature in the sensor area (Constant
Temperature Anemometer).
The sensor temperature is kept at constant temperature, and this will not be
affected by air velocity. However, the amount of heat drawn from the sensor
changes depending on air velocity. In order to compensate the drawn heat,
electrical current to be applied to the sensor. Based on the amount of the electrical
current (i), the air velocity can be calculated.
The amount of heat [H] that is drawn from the sensor is expressed by:
( )
( )
Ta T U b a H
− + =
----- King’s formula
H: Heat Dissipation T: Sensor Temperature
Ta: Air Temperature U: Air Velocity a, b: Constant
Also, the amount of heat diffusion can be expressed by the following formula;
2
RI H
=
(R is kept constant regardless of air velocity as the temperature is constant.)
Therefore, U b a RI
+ ∞ 2
As you can see from this above formula, the change of air velocity [U] can be
seen as the change of the current passed to the sensor [i].
Wind
Sensor (Platinum Coil)
風速素子
電流ｉ
Rc