There is only one simple way to determine this and it is with the use of a standpipe in your breathers. Breathers without a standpipe will always expire from the bottom first regardless of where the moisture is coming from. Breathers with a standpipe will expire from the top first if there is moisture in the headspace and from the bottom first if there is moisture in the atmosphere.
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Tip Provided by Mark Barnes, VP, Reliability Services