7. Measurement Principle
40
7.3 Influence by Gas Composition to be Measured
Hot-wire Anemometers indicate air velocity based on the amount of radiation heat
which is the heat quantity deprived from the sensor to fluid. Depending on the fluid
to be measured, the amount of radiation heat varies, and the air velocity reading, too,
will be affected. Since all of the Anemomaster are calibrated in air at the normal
temperature and pressure, the indicated value requires to be compensated when you
measure mixed gas. Therefore, you need to know the physical property value of the
mixed gas beforehand in order to compensate the air velocity of mixed gas.
Below Figure 2 shows how to obtain the heat quantity diffused by the forced
convention from a cylinder (sensor).
Heat Dif
fusion
Quantity
Q
U
O
U
M
Figure 1
Changes in air velocity values
Figure 2 Heat diffusion quantity due to forced convention from a cylinder
flow rate
U
diameter
d
nusselt
number
Nu=A×B
amount of radiation heat
(
)
a
u
T
T
N
Q
−
=
l
πλ
length
ℓ
coefficient
viscosity
η
density
r
specific heat
C
p
thermal
conductivity
λ
kinematic viscosity
coefficient
r
g
V
η
=
thermometric
conductivity
cpr
a
λ
=
prandtl number
a
r
3600
Pr
=
reynolds number
r
Ud
=
Re
Summary of Contents for Anemomaster 6162
Page 4: ......
