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§ 13
Wind
Sensor
Platinum Coil
Velocity [U]
Compensation
(
a
b
Velocity [U]
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Principle of Measurement
Principle of Hot-wire Anemometer
Current
Cooling
-off
In turbulent flows, a thermal velocity sensor indicates
higher measured values than a vane Probe. This can be
observed during measurements in ducts.
the design of the duct, turbulent flows can occur even at
low velocities.
Current (i)
the colder impact flow from the sensor can be expressed
Air Velocity
by:
Sensor
Where H: Heat diffusion quantity
the sensor
a, b: Constant
Ta
Also, heat diffusion quantity can be expressed by the
1
Ta
2
formula:
Where R is resistance and I is current
Ta
< Ta
1
2
(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
)(
)
U
T
Ta
the ambient, or surrounding air temperature and forces the
velocity sensor to stay at a constant overheat above the
ambient.
means of an operational amplifier.
sensor tends to cool the sensor, thus driving down its
resistance.
delivering more power to the circuit to maintain voltage
equilibrium.
electrical signal to display.
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
Velocity m/s
turbulent flows, the measured result is
influenced by the flows impacting the
heated body from all directions.
The amount of heat that is extracted by
H
(
a
b
U
)(
T
Ta: Air temperature
2
H
RI
2
RI
a
b
U
The circuit forces the voltage to be equal by
The amplifier responds by immediately
Delivered power is converted into an
+51 960 810 089
Depending on
Ta
)
King
'
s
fomula
T: Temperature of
U: Air velocity
Air flowing past the
53
ventas@lazodecontrol.com
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