10. Measurement Principles
10.1 Hot-wire Anemomaster 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 values
will change. This change in the resistance values will
vary largely as the air velocity increases. Therefore, if
the relation 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 uses this above mechanism. 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 formula from
the sensor resistance (R) and current (i).
(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”.
Temperature Compensation
When the air temperature changes, the measurement values will also change
since the amount of heat dissipation will change accordingly even when the air velocity is constant. Thus,
Anemomaster
employs a temperature compensation circuit to enable accurate air velocity measurement by
eliminating the influence of the temperature change. For this purpose, a temperature measurement sensor Rc
having the same temperature coefficient as the air velocity is provided at the opposite side of the bridge, and the
bridge is adjusted to keep the difference with the air temperature (T-Ta) constant.
10. Measurement Principles
33
Cooling
Current
Airflow
Air velocity sensor
(platinum coil)
Air Velocity m/s
C
u
rr
en
t
Velocity Sensor
Current (i)
fomula
s
King
Ta
T
U
b
a
H
・・・・・・
'
)
)(
(
−
+
=
Temperature
Compensation
Air Velocity [U]
Ta
1
< Ta
2
H
ea
t
D
is
si
p
at
io
n
[
H
]
Air Velocity [U]
)
)(
(
Ta
T
U
b
a
−
+
H
ea
t
D
is
si
p
at
io
n
[
H
]
Ta
1
Ta
2
2
RI
H
=
U
b
a
RI
+
∝
2
