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Old 02-07-12, 06:10 PM   #1
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Default 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.



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.


Last edited by abogart; 02-08-12 at 11:02 AM.. Reason: Changed title to "inclined manometer," as this is no longer a u-tube design.
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Old 02-07-12, 08:09 PM   #2
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Yep, they're quite simple. Most people go buy expensive meters when something like this works great!
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Old 02-08-12, 12:22 AM   #3
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Cool
Is there any problem with the water evaporating over time?
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Old 02-08-12, 06:25 AM   #4
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Quote:
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Cool
Is there any problem with the water evaporating over time?
I'll let you know in a few months.
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Old 02-08-12, 07:18 AM   #5
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Got it full of blue water, zeroed up, taped to the return duct, and tubes run to the return and supply.



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



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



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.



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.

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.

Last edited by abogart; 02-08-12 at 07:24 AM..
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Old 02-08-12, 07:56 AM   #6
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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.
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Old 02-08-12, 08:05 AM   #7
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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.
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Old 02-08-12, 08:56 AM   #8
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So, are you using this as a gauge for when to change your furnace filter or what?
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Old 02-08-12, 09:17 AM   #9
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Quote:
Originally Posted by Daox View Post
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. I bought the SG for the car, why not have a similar information display for the house?
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Old 02-08-12, 09:24 AM   #10
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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?

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