KANOMAX 6036 Multi-function thermal anemometer User Manual - Manual (Page 29)

28 Measurement Principles
U b a RI
  2
§ 9 Measurement Principles
Hot-wire Anemometer Principle
When the heated air velocity sensor is exposed to airflow, the sensor
temperature will change by the
heat drawn by the airflow.
Accordingly, the sensor resistance
value will change. This change in
the resistance value will vary
largely as the air velocity increases.
Therefore, if the relationship between
the air velocity and the resistance value
is known, the air velocity can be obtained by measuring the
resistance value (or current).
The Anemomaster anemometer is based
on the above principle. Generally, a
hot-wire anemometer employs a feedback
circuit to control the sensor to maintain
constant temperature. (Constant
Temperature Type)
When there is a change in the air velocity,
the heat drawn from the sensor (heat
dissipation) will change accordingly. In
order to maintain constant temperature,
current is applied to the sensor to
compensate this change. Thus, the air
velocity value can be obtained from the
amount of the applied current (i).
The amount of heat [H] drawn from the air
velocity sensor can be expressed by the
following formula.
Where; H: Heat Dissipation
T: Sensor Temperature
Ta: Air Temperature
U: Air velocity
a, b: Constant
The Heat Dissipation [H] can also be
expressed by the following form sensor
resistance (R) and current (i).
2
RI H

(R is kept constant regardless of the air
velocity change)
Thus:
As shown by this formula, the change in the air velocity “U” can be
interpreted as the change in the current “i”.
Current (i)
Air Velocity
Sensor Element
Velocity m/s
C
u
rr
en
t
Current
Wind
Cooling
-off
Sensor
(Platinum Coil）
H
eat
Di
ss
ip
at
io
n
[H]
H
eat
Di
ss
ip
at
io
n
[H]
Air Velocity [U]
Air Velocity [U]
Temperature
Compensation
Ta
1
< Ta
2
Ｋｉｎｇの式 ・・・・・・・・ ) )( ( Ta T U b a H
  
風
速
[
U
]
) )( ( Ta T U b a
 
Ta
1
Ta
2
T
a
1
<
T
a
2
……………King’s Formula