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 Inclined manometer help 
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Joined: Thu Mar 22, 2018 10:46 pm
Posts: 5
Hi guys, I'm a new member and have some pretty basic questions about inclined manometers and calibrating our bench, I'm not sure if I'm fully grasping the theory behind them. Sorry for the lengthy post, but I'd like to introduce myself and explain my project and I'll post my questions at the end.

I'm a junior in college studying Automotive Engineering Technology. I'm currently taking an Airflow Analysis class, and for our final project I am building a flow bench. I started to build it before I found this website otherwise I would have changed the design a little bit. It currently only flows intake, but if we can get it accurate we have plans to make it flow intake and exhaust. We have access to flow benches at school and made our own orifice plates, which we have tested at 10-28" so we know how much they flow. I can provide a spreadsheet with these values if needed. We are having trouble getting accurate measurements from our own flow bench, and I think our problem is our inclined manometer.

I'm a little confused on how to design the inclined manometer. I found the scale generator, but I am not sure how long to make it or what angle it should be at, and I cant find any information on it. Does it matter what angle the manometer is at or how long it is? After doing some more reading today I realized we might just need to calibrate the manometer with our known-flow orifice plates. So does the following procedure sound correct?

If our inclined manometer is 19.5" long with an angle of 30 degrees, we need to calibrate it by making the angle adjustable. So lets say we have a 1.5" orifice plate that flows 195 CFM at 25" of water. We can put our 1.25" plate that flows 113 CFM at 25" of water and know that our inclined manometer should read about 58%. But if our inclined manometer actually reads 50% of flow (I dont remember if that was our actual measurement, but our numbers were coming up low. I can provide actual test data tomorrow night) we need to adjust the angle of it until it reads 58%. Does this sound correct?

Thanks a lot for any help and input, I think I am starting to understand it better and this site has been a lot of help.
I tried uploading pictures but it said the file is too big, If i can figure it out i will add them later


Sun Apr 15, 2018 1:50 pm
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Joined: Wed Mar 03, 2010 3:40 pm
Posts: 1298
Location: Melbourne, Australia
Hello Colin, welcome to the Forum.

The first thing to realize is that we are not really interested in the exact angle of slope. What really matters to us, is the rise from one end to the other.
Differential air pressure forces the water level to rise, and if there is ten inches of pressure differential, the water will be forced up ten inches, regardless of the length or angle of the manometer tube.

You can make the length of the sloping manometer any length you wish, but bear in mind that to mark off an accurate scale, its a lot easier to find (say) the 0.647 point along the scale if the scale length is made 1,000 mm long, rather than say 33 1/4 inches !

The other thing to think about is that a vertical manometer can have a slight bend in it, or be slightly off vertical and still read very accurately. A very long sloping manometer needs to be dead straight, and levelling becomes an important issue if you have an uneven floor and the bench can be moved around. In that case, a very long sloping manometer at a shallow angle might be best permanently bolted to a solid wall.

Its all a compromise. Once you have your orifice and manometer made, you will need to adjust the slope of the manometer for final flow calibration. At no time will you need to know or measure what the actual angle is. Its the difference in height between the two ends that matters.

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Sun Apr 15, 2018 8:44 pm
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Joined: Thu Mar 22, 2018 10:46 pm
Posts: 5
Awesome, thanks for the explanation! I will try and get it working tonight and see if I can upload some pictures of the finished bench some time


Mon Apr 16, 2018 5:42 pm
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Joined: Thu Mar 22, 2018 10:46 pm
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Alright, I have tried varying the angle and it didn't change anything. We went from about 15 degrees, all the way to over 45 degrees and were still getting the same results. We are seeing about 30% of flow when our calibrated plate should be reading 58% of flow. Changing the angle had no effect on the reading. Not really sure where we are going wrong here. We have checked for leaks and havent found any


Mon Apr 16, 2018 9:17 pm
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Joined: Wed Mar 03, 2010 3:40 pm
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Location: Melbourne, Australia
How can changing the angle have no effect ?

Surely if you lower the angle the water will travel further along the scale for any given pressure differential.

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Mon Apr 16, 2018 9:38 pm
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Joined: Thu Mar 22, 2018 10:46 pm
Posts: 5
I'm sorry youre right, we figured it out. our manometer fluctuates a little bit so it was hard to see the change on the smaller end of the scale with the small adjustments we were making. I believe we have it set right now, just need to do a little more testing. The angle is much smaller than I thought we were going to need


Mon Apr 16, 2018 10:02 pm
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Joined: Wed Mar 03, 2010 3:40 pm
Posts: 1298
Location: Melbourne, Australia
Bouncing manometer syndrome is a far from unknown problem, and there are several possible remedies you could try.

The measurement point must be seeing some very turbulent air. So the first thing is to try to locate your pressure pick up point in a quiet corner somewhere, if you can find one....

Next ting to try is a piece of foam rubber to cover and protect the pressure sensing point and shield the pressure pickup tube end from direct blasts of very violent angry air.
If all else fails (and it sometimes does) a small bore restrictor (with a very fine pin hole) at the pressure pickup point can sometimes help a lot.

If at that stage you are about to slash your wrists, take poison and jump off the roof, and extreme remedy might be installing multiple pressure pickup points and feeding all of those into an averaging manifold.
Aquarium shops have nice very low cost plastic manifolds (with barbed fitting) for blowing bubbles into fish tanks.
Something like that might be worth a go.

Another version of the "averaging manifold" idea, is to run a blocked end pipe around the edges of the settling chamber, perforated with a few very widely distributed pin holes.

You will definitely solve the bouncing manometer problem, but it may take more than one attempt.

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Mon Apr 16, 2018 11:45 pm
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Joined: Thu Mar 22, 2018 10:46 pm
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Awesome thanks for the info! I'll definitely try some of those out, I'm sure our measurement point is in a pretty turbulent area, sometimes it fluctuates quite a bit. we designed and started building the bench before we found this forum otherwise we would have a slightly better design. It's working better than we expected though, just need to work out some small kinks like this


Tue Apr 17, 2018 12:03 am
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Joined: Wed Mar 03, 2010 3:40 pm
Posts: 1298
Location: Melbourne, Australia
Just about everyone here on the Forum has once been at the stage you are at right now.

Stick with it, the problems you are seeing are all pretty much par for the course.
There definitely are solutions, its just a case of working through the problems as they arise.

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Tue Apr 17, 2018 12:11 am
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