12 3 LEAK TESTING OVERVIEW
3.1.2 Internal pressure changes of the tested part and master
In the BAL and the DET stage, the differential pressure
resulting from leaks rises at a constant rate with time. In the
DET stage, the differential pressure sensor (DPS) output is
zeroed through an auto-zero operation before a differential
pressure reading is produced.
3
3
.
.
2
2
L
L
e
e
a
a
k
k
R
R
a
a
t
t
e
e
T
T
h
h
e
e
o
o
r
r
y
y
a
a
n
n
d
d
E
E
q
q
u
u
a
a
t
t
i
i
o
o
n
n
s
s
When a differential pressure is detected, it can be converted to leak rate unit mL/min by using a conversion
equation derived from Boyle's Law. Using the unit's leak calibration facility removes the need for
calculations based on the conversion equation.
(See Section 4.2. → )
a) Pressure and Volume Relationship
The relationship between pressure and volume is stated in Boyle’s law. Boyle’s law: for an ideal gas,
pressure multiplied by volume is a constant if the temperature is constant. In mathematical form:
PV = constant (where P is in absolute pressure units.)
A leak is measured by a change in pressure. The amount of leakage to atmosphere is calculated and
expressed by the following equation derived from Boyle’s law.
Leakage (ΔV
L
) = Ve × ΔP / Patm
Where:
Ve: Equivalent internal volume
∆P: Pressure drop due to a leak
Patm: Atmospheric pressure
The definition of internal equivalent volume (Ve) is the volume of air of
the entire Work-side pneumatic circuit at a particular test pressure. It
also includes a diaphragm deformation factor of the DPS. Ve is used
as leak coefficient K(Ve) in the leak rate calculation.
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