Leak Rate Testing
Base Testing of Apparatus
In order for accurate leak detection
the testing apparatus must have a leak rate less than that of the maximum leak
rates of the lids. To make sure the
apparatus met this requirement, the team preformed a pressurized bubble test on
the welds and fittings of the apparatus.
The team’s pressurized leak rate testing procedure is
covered in its own section below.
Figure 1 shows the team’s pressurized bubble
test of the testing apparatus’ welds to determine if the testing apparatus was
sound enough to hold a vacuum. The team
found two pinhole size cracks in the top and bottom welds where air was able to
flow freely. The problem was solved by placing RTV® silicone along the interior weld
surfaces of the test apparatus, and pressurizing the apparatus to force the
RTV® into the voids, solving the problem of leaking welds.
Figure 1: Bubble test of testing apparatus welds
The valve and fitting assemblies were also tested to ensure
leak rates less than 10^-3 Std cc/s but were tested
using a pressure gradient leak test.
Figure 2 shows the testing procedure, where the
pressure transducer was connected directly to the venturi/valve assembly using
a 1/8 inch NPT coupler. The
testing pressure gradient testing procedure is discussed
in the Pressure Gradient Leak Testing section.
Using this method, the team found the combined leak rate of the assembly
to be 10^-7 Std cc/s, well below the 10^-3 Std cc/s
required for the lids.
Figure 2: Pressure gradient testing of valve
Plan for Testing
Once the leak testing apparatus was determined to leak less
than 10^-3 Std cc/s, the team was ready to begin
testing of their lid/sealing system designs.
Lid/sealing systems designed by the team are run
through three tests (1) pressurized bubble
test, (2) Magnaflux Spotcheck®
dye penetration test, (3) pressure
gradient leak test. Each giving the
team increased leak rate detection sensitivity.
Pressurized Bubble Test:
Team Leaks for Weeks tested each lid/sealing system first
using the pressurized bubble test. The
pressurized bubble tests can produce evidence of leak rates down to 10^-4 Std cc/s, and is
relatively easy to perform by simply creating a pressure gradient between two
volumes, and spraying soapy water on the surface with lower pressure. Figure 2 shows bubble testing on the first
attempt at using an O-ring.
Figure 3: Bubble testing of aluminum contoured lid
Click here to see pressurized
bubble test video.
Magnaflux® Dye Penetration
Test:
If the lid/sealing system passed the pressurized bubble
test, the next step was to perform a dye penetrant test. The team used Magnaflux
Spotcheck® as our dye penetrant. Dye penetrates
are capable of showing evidence of leak rates to determine if the leak rate was
greater than 10^-5 Std cc/s. The team preformed the dye penetrant test by
using the venturi to pull a vacuum within the apparatus, then spraying the red
dye penetrant on the outside surface of the seal area. After waiting the recommended 30 minute
penetration time, the team opened the valve, pressurizing the apparatus, and
removed the lid for inspection. The
team then sprayed a developer on the inner seal surface to enhance ability for
visual inspection.
Figure 4: Magnaflux test of aluminum contoured lid
Pressure Gradient Leak
Testing:
Although the pressurized bubble and the dye penetrant tests
allowed the team quick visual inspection of sealing capability, a quantitative
test was required. The team chose to
use a pressure gradient leak test to quantify and standardize each lids leak
rate. Two pressure transducers were used to measure the apparatus’ internal pressure and
the atmospheric pressure. The voltage
across the two transducers was measured using two
5-digit multimeters that were connected to our test
computer using a GPIB connection. The
team created a LabVeiw® program to collect the
voltage measurements taken by the two digital multimeters. The team then analyzed the data using
Microsoft Excel®. The leak rate was calculated using the following equation derived by the
team:
Where
L = Leak Rate (Std cc/s)
P1= Initial Internal Pressure (psia)
P2= Final Internal Pressure (psia)
V= Apparatus Internal Volume (cm^3)
t= Time Between Readings (s)
Figure 5: Pressure Gradient Leak Test Apparatus
Figure 5: Leak Testing Computer, DMM, Power Supply
