How did I screw up a tube manometer? - It's not reading right... Help

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gav1965

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OK, as the title says, I built a tube manometer to check the pressure in my LP system, and although not very pretty, it's basically like every one I see demonstrated online..... But when I hooked it up, it read 19 inches w.c. instead of the expected 11 or so..... I tried it about a billion times, and read a zillion posts online to see if I could spot something I was doing wrong, all to no avail.... Thinking maybe just maybe my regulator was bad, I swapped in a brand new one that I happen to have sitting around, and it still reads 19 inches....  :mad: :mad: :mad:

I'm pulling my hair out here (and it's a bummer 'cuz I like my hair)... I'd be so ridiculously grateful if someone could give me a clue as to what I have messed up....

Shouldn't matter, but this is on my homebuilt camping van, with a horizontal tank under the body..... I have the manometer connected to a stub line coming out of the regulator, but I can't see how pressure there could be different than pressure at the stove.....

Thanks for listening....
 
How did you calibrate the graduations on the scale?
 
Two things.

1 - you can skip the tube markings and just use a tape rule if you want to measure the difference.

2 - the 11" is probably meant to be measured in use. The pressure may go high at zero flow. Light the stove and then measure.
 
Thank you for the responses..... I was measuring the difference between the columns - my manometer isn't very fancy so I didn't make a graduated scale on it.....

I will try hooking it all up and testing with the stove on - I hadn't thought of that... It seemed to me that the pressure should be constant no matter what, but I'll see if the flow makes a difference

Here's hoping...

 
Measuring the vertical difference between the two columns is the right way to do it.  The side connected to the gas line should go down 5 1/2 inches while the side open to the atmosphere should go up 5 1/2 inches for 11" WC.

It may be obvious, but make sure the measuring stick (or tape) is vertical - if it's leaning at an angle you'll get a different reading.

Be careful about measuring the pressure at the stove - some have their own internal pressure regulator so measuring the pressure at the burners won't accurately reflect system pressure.
 
I use distilled water in mine. (store bought manometer) That's what the directions said to use. Apparently various tap water can give an "off" reading. Personally, I would think it would have to be algae laden swamp water to make a difference, but what do I know?
 
There is a difference in the density of water that contains mineral salts, algae or whatever. Ocean salt water, for example, is about 2.5% more dense than pure fresh water. That doesn't begin to explain the 19" vs 11" difference, though.

As Lou says, if you measure at some appliance, measure at the input to it, not somewhere internal to the stove or whatever. The pressure once it is within the appliance may regulated to something different.
 
Based on my experience with compressed air systems and paint regulators, I'd say Markbarendt may be on to something. 
Trying to adjust the pressure on my paint gun, in order to set it right, I'd always have to pull the trigger to let air flow while adjusting the pressure.  When I'd release the trigger the pressure would always climb a bit.  Of course it would drop right back down when the trigger was pulled again.
 
You need to check with some flow, almost all regulators will creep up with no flow and drop immediately with flow.  I don't think I have ever seen a regulator that did not creep, some worst than others.
 
Agree with the above comments about creep, but 19" vs 11" doesn't sound like "creep". That's more than 50% over!

Easy enough to test, though. Just open one stove burner a bit to allow some flow while the measurement is taken somewhere along the main line.
 
Ok, thanks so much for the input.... At the time, my plumbing was apart, as I was replacing a section of tubing, hence connecting the manometer close to the regulator..... It took longer than I expected to get the tubing replaced (surprising, right??) but at last it's together.....

And.....

Wait for it....

Now it reads about right.....

When I first hooked it up, it read 14 inches, but then I opened the valve on the stove, and it dropped right down to 12..... It always reads close to 12 now, with the stove running or not.....Giving me an idea... I think when it was hooked up so close to the regulator, there wasn't much room for expansion, so when I would turn on the propane flow to the system, it would shoot up before the regulator could lock.... I know I'm not probably saying that quite right, but hopefully you see my meaning.... If I'd have put  tee in my tubing to let me bleed off the excess, it probably would have stabilized and read correctly.....

Does that make sense?

Now, it reads 12 inches on what is, to repeat, a brand new regulator, should I assume the difference between 11 and 12 is due to things mentioned above, like minerals in the water or some such? I'm using filtered water, but not distilled.....
 
I think you're right - it takes a finite amount of time for the regulator diaphram to close and if you don't have much volume after it to disperse the excess flow the pressure will spike.  You need room in the pipe to allow for the time it takes for the regulator diaphram to move from open to closed.  You're supposed to have propane flowing through the system (light a burner) to avoid this kind of false reading when measuring the system pressure.

I wouldn't worry about about reading 12" instead of 11".  You're working with very low pressures, around 0.5 PSI, and that's less than a 10% difference.  Friction between moving parts in the regulator that haven't fully broken in yet could account for that.
 
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