Principle Of Measurement; Principle Of Hot-Wire Anemometer - Kanomax Climomaster S-6531-0E User Manual

§ 13

Principle of Measurement

Principle of Hot-wire Anemometer

Current
Cooling
Wind
-off
Sensor （Platinum Coil）
Current (i)
ｉ
Air Velocity
Sensor
Ta < Ta
1 2
Velocity [U]
Compensation
 
( a b U )( T Ta
Velocity [U]
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Velocity m/s
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.
the colder impact flow from the sensor can be expressed
by:
H
Where H: Heat diffusion quantity
the sensor
a, b: Constant
Ta
Also, heat diffusion quantity can be expressed by the
1
Ta
2
formula:
H 
Where R is resistance and I is current
(R is kept constant regardless of air velocity since the
temperature in constant).
Therefore,
RI
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.
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.
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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.
The amount of heat that is extracted by
  
・・・・・・
( a b U )( T Ta )
Ta: Air temperature
2
RI
 
2
a b U
The amplifier responds by immediately
Delivered power is converted into an
Depending on
King ' s fomula
T: Temperature of
U: Air velocity
Air flowing past the
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