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.

Here is version 1.1. It looks nice, but unfortunately I don't think it measures accurately.

http://i735.photobucket.com/albums/w...6/IMAG0131.jpg

I was trying to make a well-type inclined manometer. But since it seemed like it might be a bit difficult to find something to act as a well and connect the tubing to it, I just used a loop below the incline, thinking that would work fine to keep the fluid level in check. I believe the problem is that the fluid in the inclined column has to move more than the fluid in the upright column. The fluid levels balance out at a point where neither is where it should be.

When applying 1" WC to the gauge, the fluid in the inclined column should move 5" across the scale, but only raises 1/2" in height. The upright column, however, should only move down 1/2" in height. Obviously the water can't expand, so they balance at a mid-point.

I think there is some math here which is a bit beyond my understanding. It seems that the well would have to be made a certain size so that the height drops at the same rate that the height in the inclined column rises. I guess I'll do some more research and try again.

Does it have anything to do with surface tension? The fluid appears to be sitting "square" in the inclined section of tube, the surface in that section does not seem to lie level. Does the tube have to be a larger diameter, or should there be an additive in the water? Some fire departments use an additive known as "wet water" that breaks down the surface tension and allows water to soak into surfaces easier.

I thought that was neat too. Every commercial inclined manometer that I've seen works like this. Some of them do use oil and change the scale to compensate. But from what I can see, the liquid pretty much stays square in the tube, even in the oil-based gauges. I'm guessing it has something to do with surface tension and pressure on both sides overcoming gravity. That's why when I made the scale for this one I made the lines perpendicular to the tube, so that the water level could be easily read straight across the lines of the scale.