Homemade inclined manometer
 

I have recently found that a water tube manometer is actually a very simple instrument. Basically, a measured amount of water is suspended in a u-shaped tube. One of ends of the tube is used for pressure measurements, the other can be left open to the atmosphere to measure static pressure, or can be attached to the low end of a pressure system to measure differential pressure. There is a scale behind the tubes for making measurements.

There are two basic types of water tube manometers, vertical and inclined. Since I plan to measure the external static pressure of my duct system and air handler, I chose to start with an inclined manometer because they can read much more precisely in a small range (0 - 1" H2O in this case).

I built this one from a length of clear 5/16" vinyl tubing, which cost me $2.90 for 10 feet at the hardware store. The base plate is just a piece of 1/4" plywood that I had laying around, cut to 8" x 12". The tubing is held to the base plate with zip ties. I measured at 1" intervals up from the bottom and made lines across for the scale.

Here is the semi-finished manometer held against my cold air return.

http://i735.photobucket.com/albums/w...6/IMAG0123.jpg

It's not nearly professional-looking, but it should serve its purpose. I have yet to get some food coloring for the water and fill it to the proper level. Once I do that I'll tape it to the return and run the scaled end of the tubing into the return, and the other end to the supply just above furnace.

Yep, they're quite simple. Most people go buy expensive meters when something like this works great!

Cool:cool:
Is there any problem with the water evaporating over time?

I'll let you know in a few months. ;)

Got it full of blue water, zeroed up, taped to the return duct, and tubes run to the return and supply.

http://i735.photobucket.com/albums/w...6/IMAG0125.jpg

Here's the whole setup. Kinda looks like an educated redneck put this together, hehe...

http://i735.photobucket.com/albums/w...6/IMAG0126.jpg

Reading with the t-stat set to FAN. Low speed fan operation.

http://i735.photobucket.com/albums/w...6/IMAG0127.jpg

Reading during a heating cycle. High speed fan operation. I noticed that it didn't quite return to zero after shutting the fan off the first time, so I removed the zip tie at the bottom of the 'U' and pulled the whole loop out a bit to zero it back up.

http://i735.photobucket.com/albums/w...6/IMAG0129.jpg

I wasn't quite sure how to properly read it. According to this Wikipedia article, "The difference in fluid height in a liquid column manometer is proportional to the pressure difference." Since I labelled the scale at 1" increments, the value read on the bottom tube needs to be doubled to account for the lowered height of the upper tube. Since this read about .2" WC on high, my total external static pressure is .4" WC.

I now realize that I could have increased the accuracy of this thing by decreasing the incline of the tubes and only having the scale read 1" either side of zero. That would give it the ability to read up to 2" WC with more accuracy in the range that I'm using it for. I just might make another base plate for it with a shallower incline when I get some free time. I have some tubing left over and I want to make a vertical u-tube with a larger scale for reading things like gas manifold pressure.

At least now I know what's going on inside the ducts, and I have a working gauge to keep an eye on it. :thumbup:

EDIT: It just dawned on me that I should have put the low side in the blower compartment downstream of the filter. That way I should be able to tell by looking at the gauge when the filter is restricting airflow.

I have one of these on my radon stack. Now that it's mentioned I am wondering about evaporation. There hasn't seemed to be any yet. I wonder what it's filled with.

It was actually the pictures of those radon stacks that gave me the idea. I noticed that they all have one of these manometers on them. I'm guessing they are just filled with colored water. Any other liquid would have a different density which would give inaccurate measurements.

So, are you using this as a gauge for when to change your furnace filter or what?

Pretty much. I was thinking that the ducts were more restricted due to many of the dampers being partially or fully closed, but it turns out I'm still within the .5" WC limit. I figure it will be a good tool to use for adjusting the duct dampers as well as monitoring the filter condition and any other restrictions (closed registers, furniture over return grates, toys in ducts, etc.)

Really, I just like having information displayed at all times. Maybe I'm just weird. :rolleyes: I bought the SG for the car, why not have a similar information display for the house?

Sounds like a good idea. I too have many dampers closed. Perhaps I should look at my own setup.

What is the recommended pressure drop? Is it to keep it under 1/2" water column?

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.