Principle of measurement, Principle of hot-wire anemometer, 13 principle of measurement – KANOMAX 6501 series Climomaster User Manual
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§ 13
Principle of Measurement
Principle of Hot-wire Anemometer
The principle of the thermal Probe is
based on a heated element from which
heat is extracted by the colder impact
flow. The temperature is kept
constant via a regulating switch. The
controlling current is directly
proportional to the velocity. When
thermal velocity Probes are used in
turbulent flows, the measured result is
influenced by the flows impacting the
heated body from all directions.
In turbulent flows, a thermal velocity sensor indicates
higher measured values than a vane Probe. This can be
observed during measurements in ducts. Depending on
the design of the duct, turbulent flows can occur even at
low velocities. The amount of heat that is extracted by
the colder impact flow from the sensor can be expressed
by:
Where H: Heat diffusion quantity T: Temperature of
the sensor Ta: Air temperature U: Air velocity
a, b: Constant
Also, heat diffusion quantity can be expressed by the
formula:
Where R is resistance and I is current
(R is kept constant regardless of air velocity since the
temperature in constant).
Therefore,
The Temperature Compensation
The air velocity sonsor is heated to an elevated temperature
relative to the surrounding air by means of control
electronics. The temperature compensation sensor senses
the ambient, or surrounding air temperature and forces the
velocity sensor to stay at a constant overheat above the
ambient. The circuit forces the voltage to be equal by
means of an operational amplifier. Air flowing past the
sensor tends to cool the sensor, thus driving down its
resistance. The amplifier responds by immediately
delivering more power to the circuit to maintain voltage
equilibrium. Delivered power is converted into an
electrical signal to display.
Cooling
-off
Current
Sensor （Platinum Coil）
Velocity m/s
C
u
rr
e
n
t
2
RI
H
U
b
a
RI
2
fomula
s
King
Ta
T
U
b
a
H
・・・・・・
'
)
)(
(
Current (i)
ｉ
Velocity [U]
Ta
1
< Ta
2
H
ea
t
D
iff
us
io
n
Q
u
an
ti
ty
[
H
]
Ta
1
Ta
2
Compensation
H
ea
t
D
iff
us
io
n
Q
u
an
ti
ty
[
H
]
Velocity [U]
)
)(
(
Ta
T
U
b
a
Wind
Air Velocity
Sensor