3 LEAK TESTING OVERVIEW
13
b) Equivalent internal volume calculation
Differential pressure can be converted into a leak rate by calculating the equivalent internal volume with
the following equation:
Ve = Vw + Vt + [ Ks × { 1 + ( Vw + Vt ) / ( Vm + Vt )} + Kw ] × (101.3 + P ) …………………
1
Where:
Ve:
Equivalent internal volume [mL]
Vw:
Internal volume of the Work side: tested part and the tubing [mL]
Vm:
Internal volume of the Master side: master and the tubing [mL]
Vt:
Tester internal volume (mL) Vt = 11 mL (Standard pneumatic circuit)
Ks:
Change in internal volume of the sensor per unit pressure change [mL/kPa]
Kw:
Change in internal volume of the tested part per unit pressure change [mL/kPa]
P:
Test pressure [kPa]
If the Master side has the same internal volume as the Work side and
both are so rigid that the test pressure does not physically change their
dimensions, then
Ve = Vw + Vt + 0.01(101.3 + P) …………………
2
Where:
Vw = Vm
Ks = 0.005 [mL/kPa] (measured value)
Kw = 0 [mL/kPa]
Ks {(1 + ( Vw + Vt ) / ( Vm + Vt)} + Kw = 2Ks = 0.01 [mL/kPa]
c) Calculation of a Leak Rate
If the volume of air that leaked out of a part is known, this can be converted to a leak rate by knowing for
how long (t) the pressure change (
Δ
P) was measured.
Mathematically this is expressed as follows:
Where:
Q:
Leak rate in volume per time [mL/min]
∆
P:
Pressure drop due to a leak [Pa]
Ve:
Equivalent internal volume [mL]
T:
Detection time
The LS-1866 uses the above equation for the calculation of the leak rate. Note that the tester uses
standard atmospheric pressure for this calculation. If the tested part is tested under standard pressure,
1.013 × 10
5
Pa, and standard temperature, 20°C, then the tester displays a standard volume per time.
Example: Converting the differential pressure,
Δ
P = 50 [Pa] under following test condition into leak
rate using formula
2
and
3
.
Test pressure (P) = 400 [kPa]
Part internal volume (Vw) = 80 [mL] (including tubing) = Vm
DET timer (T) = 3 [s]
Tester internal volume (Vt) = 11 [mL]
Differential pressure during DET stage (
ΔP
) = 50 [Pa]
Using formula 2,
Equivalent internal volume (Ve) = 80+11+0.01×(101.3+400) 96 [mL]
Using formula
3
,
(mL/min)
0.95
3
60
10
1.013
50
96
5
≒
×
×
×
Leak rate (Q) = 0.95 [mL/min]
T
60
10
013
.
1
P
Ve
Q
5
×
×
∆
×
=
3
Tester internal volume,
Vt, including CAL port
1) Standard: 11 mL
2) AS01 : 6 mL
