10. Measurement Principles
34
Air Temperature Measurement
An air temperature element (platinum thin film element) whose resistance
value changes by the air temperature is incorporated in one side of the
bridge. The air temperature can be obtained by measuring the variance in
the resistance value.
Pressure Measurement (Optional)
A diffusion-type semiconductor pressure sensor is employed to measure pressure. This diffusion-type semiconductor
pressure sensor is based on the Piezoresistance Effect, in which the resistance value changes when pressure is applied.
It is configured with four (4) diffusion resistances (sensor chips) located on a thin silicon diaphragm (Fig.1).
When pressure is applied from to the pressure sensors and the diaphragm is deflected as shown in Fig. 2, compressive
stress is applied to R3 and R4 located at the center of the diaphragm while tensile stress is applied to R1 and R2. The
resistance value of the diffusion resistance changes in accordance with the strength of the stress applied.
By configuring the bridge of the detection circuit (Fig.3) with these diffusion resistances, voltage that is proportional
to the pressure can be obtained. In addition, since the diffusion resistance is dependent on temperature, temperature
compensation resistance is employed for the resistance.
10.2 Air Volume Calculation
Average air velocity inside a duct multiplied by the cross section of the duct equals
air volume inside the duct. To
calculate the average air velocity inside the duct, divide the cross section of the duct into several divisions.
Measure the air velocity of each division and average out the air velocity in the each division.
Air Volume: Air volume per unit time [m
3
/min, m
3
/h, ft
3
/min, ft
3
/h]
Air Volume (Q) = Average Air Velocity (U) × Cross section (A)
The right illustration shows an example of
measuring points in a duct as described in the
JI-B8330. According to JIS specifications, the
cross section of a square duct shall be divided
into 16 or more divisions and the center point
of each division shall be the measuring points.
For a round duct, there are 20 measurement
points that have a right angle to each other on
the cross section. For more details refer to JIS-B8330.
Air Temperature Element
Output
Constant Voltage
Pressure
Fig.2
Fig.1 Pressure Sensor
R1
R2
R4
R1~R4: Diffusion Resistance
R3
Diaphragm
Fig.3 Detection Circuit
R1
R2
R4
R3
C
o
n
st
an
t C
u
rr
en
t (
i)
Output
r1=0.316R
r2=0.548R
r3=0.707R
r4=0.837R
r5=0.949R
Measuring
Poin
t
Round Duct
R
r1 r2 r3
r4
r5
Square Duct
