Tip of the Week
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Is it Leaking?
Most people know that the allowable leak
rate in Method 5 is 0.02 cfm. However,
it’s actually a little more complicated than that.
Section
8.4.4. of Method 5, which deals with post-test leak-check requirements, states:
“If the leakage rate is found to be no greater
than 0.00057 m3/min (0.020 cfm) or 4 percent of the average
sampling rate (whichever is less),
the results are acceptable…”
Most Method
5 and Method 5-type tests are run at a rate of 0.5 cfm or greater (which gives
you 30 cubic feet in an hour). Therefore,
the 0.02 cfm limit applies most of the time (4% of 0.5 is 0.02). However, there are occasions where you may
sample at a rate less than 0.5 cfm. In
those circumstances, the 4% rule will apply.
The most
common occurrence for this is in running controlled condensation (Modified ASTM
Method D3226). Typically, this test is
run at a rate of around 0.2 cfm.
Although the ASTM method does not specify an allowable leak rate, we at
CleanAir feel that it is a “best practice” when a rate is not specified to
default to the Method 5 criteria.
Therefore, the 4% criteria would apply, which means that your allowable
leak rate is 0.008 cfm. In other words,
not very much. Back in the old days when
I used to test, we called this “dead nuts”.