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Adjusting for incline with manometer

  1. Dec 6, 2013 #1
    Hello.

    I am not skilled in math or physics. I'm interested in building an inclined manometer accurate only enough to measure water column (w.c.) inches scaled to .5 w.c to about 10 percent accuracy.

    A very basic manometer can be built to measure inches of water with a utube of small diameter clear plastic hose. Set the zero about 7 to 10 inches from bottom of loop and mark off a one inch increments above it. I don't know if this would measure even within my range of accur. Accurately determining within .5 w.c. is what I'm after. Don't want to buy a 60 dollar digital manometer from ebay since I'm only using this twice and then may not use it again for five years.

    I'm wondering if there is a simple calculation to adjust that scale according to the incline of the tubing? "Inclined" u-tube manometers I've seen on the web generally maintain the plane of the unit horizontally while inclining the tube itself approx 70 degrees to the left of the vertical axis of that plane (the board that the tubing is attached to).


    Can one assume that the same effect could be had by inclining the basic u-tube manometer backwards 45 degrees and adjusting the scale accordingly? I only ask because it would seem easier to build accurately.

    Inclination of the tube results in finer (though narrower overall) useful adjustment, no? Again, I'd like to make an inclined manometer that measures 0 to 15 w.c. scaled to .5 w.c with perhaps 10 percent accuracy.

    Plans on the web would be ideal but I can only find plans for the standard w.c. manometer.

    Lastly, It's often mentioned in these diy designs that the liquid used is not terribly important. One inch of mercury is equal to approx 13.6 inches of water weighed against atmospheric pressure. So while the weight of most other oils and fluids at hand to fill a manometer are generally equal, are the differences in weight between them appreciable enough to care about in my homemade manometer? Water, hydraulic fluid, light motor oil, etc? Viscosity would not be a concern I'm guessing. That might only affect a damping factor but the measurement would remain constant.

    I'm measuring a stable steady pressure so damping is not important. Tinted water may just be ideal.

    Thanks for any comment.
     
  2. jcsd
  3. Dec 6, 2013 #2
    bumpity bump. c'mon guys. i'm not even schooled in physics past the 11th grade but i know this has to be about as rudimentary as it gets. 109 views and no bites? really? who inhabits these boards?:smile:
     
  4. Dec 6, 2013 #3
    I did a quick knock up of this.

    SDC15155_zpsd262c1ab.jpg

    I'm measuring the gas pressure on my furnace.

    Is there anything immediately wrong with the manometer as you can see it in the image?
    There is a valve at the bottom that is no obstruction to the flow. It's there only to drain the manometer for easily zeroing the fluid.
    I assumed that inches of water was almost foolproof.
    The furnace specs measure gas pressure in w.c.
    Pressure on the output side of the regulator is spec'd at 3.5" w.c. with the furnace running.
    Gas pressure from the household supply side is spec'd at 7" w.c. with the furnace running.
    My manometer measures the output at 1.75 and the supply at 2.95.
    These numbers are highly suspicious since the furnace ought to be pretty close to 3.5 w.c. (I was only checking) and the household supply comes directly from the meter with no other valves or obstructions in the pipe and should definitely be close to the standard 7" w.c.

    So I suspect that my manometer is off by some magnitude. How can that be? Inches of water is inches of water. It's damn basic stuff. No?
     
  5. Dec 6, 2013 #4

    AlephZero

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    Homework Helper

    Assuming one side of the manometer is open to the air, you measure the difference in heights of the two columns, not the change from the zero position. So the picture shows a reading of about 3.5 in, not 1.75 in.

    The specific gravities of those fluids could vary by 10 or 20 percent from water. If you want accurate results, use the correct value (or use water!)

    The other accuracy issue is avoiding leaky joints. Waggle the flexible pipes around close to the joints. If the reading stays stable, you are probably OK.
     
  6. Dec 7, 2013 #5
    Thank you for that. I found half a dozen links to DIY manometers on the web and somehow missed that, pretty dang important, part.
     
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