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Chris --
I think the best thing is an actual measurement. I do this using a modified oil cap and a special low-pressure differential gauge called a "Magnehelic." These meters come in different ranges (0-5 inches of water works well) and are often on eBay for a few dollars.
A pressure (or vacuum) equal to 1" of water is the amount of vacuum needed to suck water up a straw 1". Roughly 27" of water equals 1.0 psi.
However, a simple manometer can do a fine job instead of the meter. You can use water in the manometer (which gives you a direct reading in "inches of water"), or substitute motor oil, which only requires that you compensate for the difference in density.




Don Foster (near Cape Cod, MA)
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