Displayed Pressure Calculation
The pressure displayed by the CPD8000 is calculated according to the following formula
(
)
(
)
(
)
(
)
Pvac
t
P
g
g
N
N
Kn
P
m
an
m
ac
PC
PC
n
l
k
+
−
−
×
−
×
−
×
×
−
×
×
×
=
ρ
ρ
ρ
ρ
α
λ
20
1
1
where:
−
Kn
is the specific coefficient of the piston-cylinder
assembly
−
N
is the indication of the measuring force cell in count
−
N
k
is the sensitivity of the measuring force cell
−
g
l
is the local gravity in m.s
-2
−
g
n
is the normal gravity in m.s
-2
−
λ
λ
λ
λ
PC
is the pressure distortion coefficient of the piston-
cylinder assembly
−
α
α
α
α
PC
is the thermal dilation coefficient of the piston-
cylinder assembly
−
t
is the temperature of the piston-cylinder assembly in
°C
-
ρ
ρ
ρ
ρ
ac
is the air density during the adjustment of the
measuring force cell in en kg.m
-3
. This parameter is
null when the CPD8000-A operates in absolute mode.
-
ρ
m
is the density of the adjustment mass in kg.m
-3
-
ρ
an
is the normal air density in kg.m
-3
-
P
Vac
is the residual vacuum in the reference chamber.
Constant Parameters
The constant parameters entering in the computing of
pressure are stored in the non-volatile memory of the
CPD8000:
•
Kn of the piston-cylinder assembly,
•
Sensitivity of the measuring force cell (N
k
),
•
Normal gravity (g
n
),
•
Local gravity (g
l
),
•
Pressure distortion coefficient of the piston-cylinder
assembly (
λ
PC
),
•
Thermal dilation coefficient of the piston-cylinder
assembly (
α
PC
),
•
Density of the calibration mass (
ρ
m
),
•
Normal air density (
ρ
an
).
Some of these parameters are specific to each piston-
cylinder assembly and are determined during the
calibration. You can easily modify them if necessary and
the internal memory allow to save the parameter of three
different piston-cylinder assemblies
Variable Parameters
The variable parameters affecting the calculation of the
pressure are automatically measured and integrated in the
expression of the pressure :
•
temperature of the piston-cylinder assembly (t),
•
ambient temperature ,
•
humidity,
•
atmospheric pressure.
•
residual vacuum (P
Vide
)
The pressure is automatically converted into any of the
common pressure units and the user has the possibility to
configure the system to special units.
This rigorous metrology enables an ease of use as well as
a speed of measurement operation.
Defining the air density
during the calibration
(
ρ
ac
)
Page 5 of 8
Desgranges & Huot WIKA Data Sheet CPD8000 04/2012
