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Understanding Display Leak Rates and Test Gas Concentrations
An INFICON
sniffer leak detector can display leak rates and test gas concentrations,
which represent different physical values. Here we look at the connection
between a sniffed leak rate in mbar·l/s and a display in ppm.
A 'leak rate' is
typically described as a gas flow emitted
through a pinhole or a crack in a housing,
pipe, etc. The physical value is expressed
as "pressure
* volume / time," normally shown as mbar·l/s or Pa·m³/s
(less conventional calibrations include:
Atm·cm³/s, Torr·l/s,
sccm, lusec, etc.).
For sniffer leak
detection, operators often use ppm (parts
per million = 1/1000000 = 10-6) to describe
the leak rate. However, 'ppm' is
actually a unit used for gas concentrations
and not for a gas flow; this is due to
the sniffer line, where the leak rate
mixes with the ambient air. The resulting
concentration is a combination of the leak gas flow amount and the total
airflow through the sniffer line.
In any case, the
displayed rate of concentration is dependent on the leak rate. But
a different sniffer line with a different gas flow would result in
a different concentration, or a probe tip blocked by dirt can also
change the total gas flow and the concentration. The operator will
interpret both as a different leak rate. Obviously, it is more accurate
to derive the total gas flow through the sniffer line from the calculation
of leak rate and concentration:
| Example 1: |
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| Total gas flow through the sniffer line |
= 300 sccm |
= 300 Atm·cm/min |
= 5 mbar·l/s |
| Displayed concentration |
= 20 ppm |
= 20·10-6 |
= 2·10-5 |
leak rate =
concentration * total gas flow = 2·10-5 * 5mbar·l/s =
1·10-4 mbar·l/s
| Example
2: |
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| Leak rate |
= 1·10-4
mbar·l/s |
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| Total gas flow
through the sniffer line |
= 3000 sccm |
= 3000 Atm·cm/min |
= 50 mbar·l/s |
In both examples
we have the same gas flow coming out
of a leak → 1·10-4 mbar·l/s. But the
different flow through the sniffer line effects the resulting indication
in ppm.
An intelligent
sniffer leak detector will always calibrate through the sniffer line
and connect the gas flow to the leak rate and concentration indications.
It will also control the gas flow under operating conditions and continuously
correct the indication.
Other applications
also need to display a given concentration
of test gas in a chamber or in the ambience.
We will define the concentration in accumulation leak detection in
an upcoming "Tech Talk."
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