Carrier
Gas Leak Detection
Using sniffer leak
detectors on large objects or difficult to access test areas usually
results in long test times and overall long test sequences. Using carrier
gas leak detection, where the leak gas flow is transported to the leak
detector by an additional carrier gas flow under atmospheric pressure,
can help get the leak signal to the sniffer much faster.
To illustrate how
carrier gas leak detection works, let’s look at the design of
a test object; an electronic box with a weld seam, an O-ring sealed
lid, a lamp and three plugs. The test object is filled with tracer
gas under a light overpressure.
Normally, to test
this object with a sniffer probe you would need to point the probe
tip at all suspicious points and along the seams (see arrows).
Example
1: Integral test chamber
Another way is to carry out a leak test in
a chamber with a carrier gas flow. If the test object and the net volume
of the chamber are small, the sniffer probe can be positioned at the
outlet flow point to analyze the gas flow (see below).
Specifications
for Example 1:
Net volume of the chamber: 100 cm3
Pumping speed of the sniffer: 3000 sccm
= 50 cm3/s
The tracer gas
from a leak at any point of the test object will reach the sniffer
probe in approximately 2 seconds. Sensitivity of this test format is
100% because all the test gas is sucked directly into the sniffer probe.
Example
2: Larger test object and chamber
Detecting leaks from a more voluminous test
object requires a bigger test chamber and an increased carrier gas flow
for a quick response time. This flow is typically generated by a fan
(see below).
Detection of the
correct leak rate needs a bit more calculation when using this configuration.
Specifications
for Example 2:
Length of the test object: 600 mm
Net volume of the chamber: 50 liter
Air flow by the fan: 36m3/h = 10 liter/s = 104cm3/s
Pumping speed of the sniffer: 3000 sccm
= 50 cm3/s
The response time
is still relatively short, but sensitivity is reduced by a factor of
200 because of the high dilution of the tracer gas in the total carrier
gas flow.
According to the
requirements of the application, the fan speed should be carefully
calibrated to optimize response time and sensitivity.
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