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.
Tip Provided by Mark Barnes, VP, Reliability Services
