18
■
Specifying accuracy
See the tables for specifying accuracy on pages 20 and following. The accuracy
tables show the ranges when the gas is air. To convert to other application condi-
tions, calculate as shown below.
The Reynolds number (Re) used below is calculated using the formula
Re = (V
×
D)/
ν
.
V: velocity (m/s)
D: typical length (internal diameter of the
MVF
body(m))
MVF050
: 52.5mm,
MVF080
: 78mm,
MVF100
: 96.8mm,
MVF150
: 142mm
ν
: kinetic viscosity of the fluid (m
2
/s), =
µ
/
ρ
For instance, in the case of air (dry air) at 0°C and 101.3kPa,
Viscosity
µ
=17.24
×
10
–6
Pa·s
Density
ρ
=1.293kg/m
3
From these conditions, the kinetic viscosity
ν
=13.35
×
10
–6
m
2
/s.
Or, in the case of air (dry air) at 23°C and 700kPa,
ν
=1.883
×
10
–6
m
2
/s
As a calculation example, we will use the following conditions:
Installed flowmeter:
MVF080
Fluid: air (dry air)
Operating pressure: 700kPa
Fluid temperature: 23°C
Atmospheric pressure: 101.3kPa
We will calculate the following items:
1. Minimum measurable flow rate
2. Maximum measurable flow rate
3. Accuracy after temperature and pressure compensation (examples: for 100 and
150m
3
/h (normal))
1. Minimum measurable flow rate (volumetric flow rate (m
3
/h) and mass flow rate (m
3
/h (normal))
First, the minimum measurable flow speed is determined as the larger of 0.3m/s or
the velocity at Re 3500. The velocity at Re 3500 is calculated from the formula for
calculating Re:
V = Re
×
ν
/
D
.
Here, if Re = 3500,
ν
= 1.883
×
10
–6
m
2
/s, and
D
= 78
×
10
–3
m,
V = 3500
×
1.883
×
10
–6
/ (78
×
10
–3
) = 0.08m/s.
Since a velocity of 0.08m/s at Re 3500 is less than 0.3m/s, the minimum measur-
able velocity is 0.3m/s.
Now, the minimum measurable volumetric flow rate can be calculated as
Qactual (m
3
/h) = S
×
V
×
3600 = 5.2.
S: flow path cross-section of
MVF080
(m
2
) = (78
×
10
–3
)
2
×
π
/4
V: velocity (m/s) = 0.3
Therefore, volumetric flow rate can be measured down to 5.2m
3
/h.
Next, we can calculate the minimum mass flow rate Qnormal (m
3
/h (normal)) at
0°C and one atmospheric pressure, with temperature and pressure compensation.
Chapter 5. SPECIFICATIONS
