7. Measurement Principle
39
7.2 Temperature Compensation
When the air temperature changes, the amount of heat dissipation changes
accordingly even when the air velocity is constant. By providing a
temperature measurement sensor Rc having the same temperature
coefficient as the air velocity at the opposite side of the bridge, the constant
difference between the air temperature and sensor temperature is kept.
By fixing the bridge constant as described above, the amount of heat
dissipation can bear a constant relation to the air velocity regardless of the
air temperature.
When implementing the temperature compensation sensor, the sensor with
significant resistance value shall be used in order to avoid self-heating due
to the current flow. Consequently, the sensor tends to be big. The more the
sensor is big, the worse the response against the air velocity sensor
becomes. Then, when the air temperature changes rapidly, it becomes
difficult to compensate the temperature. Given this factor, to improve the
response of Model 6261 Anemomaster, sub-bridge is being used. If
feedback is provided to amplifier in order to counterbalance this sub-bridge,
the combined resistance will almost be R(1+r2/r1) when this bridge is
viewed as one resistance. In other words, if r2 << r1 is selected, the
resistance for temperature detection (R) can be relatively small. Therefore,
we managed to keep the temperature detection sensor small, which enables
us to achieve the compensation with good response.
Temperature
Compensation
Air Velocity [U]
Air Velocity [U]
Ta
1
< Ta
2
)
)(
(
Ta
T
U
b
a
−
+
[H]
[H]
Ta
1
Ta
2
Constant
Output
R
R2
R1
RH
Air velocity sensor
r1
r2
Air temp. detection bridge
Air temp. sensor
Display
CPU
Summary of Contents for Anemomaster 6162
Page 4: ......
