My furnace manual says that external static pressure should not exceed 0.5" WC. External static pressure is the differential pressure between the supply and return sides. ESP can be determined by finding the pressure relative to atmospheric on both sides and adding the supply pressure to the negative return pressure. So .2 (rise) on the supply plus .2 (drop) on the return gives an ESP of .4" WC. Basically what the blower is working against on both sides. The manometer does this in one step by connecting the high side to the supply and the low side to the return.
I have been looking at some mass produced inclined manometers and plan on changing my design. Will post when I get version 1.1 built.
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