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QUESTION: This type of tool is based on the "water seeks its own level" principle.
If there is air in the line, the water will be at different levels at the ends of the tube.  Why?
It would seem that the pressure would equalize throughout the tube and cause the water to reach the same level at both ends.

ANSWER: Hello Charles,

If there is air in the line, your tool will have 2 (or more) sections of water. Each section will work independently. There will be 2 upper surfaces for each section of water. One surface of both sections will be in the hose under the air bubble. The 2 surfaces of the left section will be at the same level. The 2 surfaces of the right section will be at the same level -- but probably not at the same level as the left section. The bubble will be in a high spot in the hose between the 2 ends.

Imagine that you started with the tool empty. Fasten the 2 ends at about the same level letting the hose sag down between them. Then grasp the hose at some point and lift it up to about the same level as the ends. Then add water to one end, stopping when the water level appears in the glass tube. The other upper surface of that body of water would be in the hose, level with the water level in the end you added the water. (Can you see the water in the hose? It would easier to understand what I'm saying if you can.) Now add a small amount of water to the other end of the tool. Too small amount for the water to appear in the glass tube. The 2 surfaces of that body of water will be at the same level, but not at the 2 levels of the other section of water.

So the key to getting accurate readings is to let the hose sag between the 2 ends so any air bubbles can float up to the ends and escape the system. Once you have let all the air out, slight down/up/down twists can develop in the hose without causing a problem.

I hope this helps,

---------- FOLLOW-UP ----------

You are correct in describing the behaviour where the middle section of the tube is raised effectively creating 2 sections wherein the water can level itself.

However, it is also possible to have all of the tube laying more or less flat on the ground, with some air trapped in a small rise, and have the water at the two ends be several inches different in their level.

I have observed this behaviour when using such a tool.  Somehow that bubble seems to be supporting a greater column of water on one side than on the other.


ANSWER: Hello Chaz,

If the vertical size of the bubble is at least the diameter of the hose, so the hose is basically dry for even a tiny percentage of its length, I'm saying that it's the same phenomenon as if the high point is more significant. If you can see the bubble and water levels on either side of the bubble in the hose, you should see the same several inch discrepancy there that you see at the ends of the tool.

If that's not the situation, I can't come up with a better explanation without experimenting with it myself.

I hope this helps,

---------- FOLLOW-UP ----------

I think I may have figured this out.
Think of the tube as containing 2 opposing columns of fluid that meet in the middle.
If the tube is completely filled with fluid the vertical part of the fluid columns will have the same height.
Now imagine an air bubble trapped in a slight rise somewhere in the mostly horizontal part of the tube.
I think the bubble is decreasing the total mass of one of the fluid columns.
The opposing fluid column can therefore push it to a greater height.
In all cases the fluid always settles to a point where the 2 columns have the same mass "levelled".
It just so happens that when the 2 columns are of equal mass, the height of the vertical sections will also be equal.
In using this as a levelling instrument, we are taking advantage of this special case.
Please let me know if this solution "holds water".  :-)

Hi Chaz,

You may be saying the same thing I have been saying. If so, I agree.

I suggest that you prevent or eliminate the existence of a bubble in the tube. It should be fairly easy with 2 people: On level ground, hold the 2 ends chest high as far apart as practical. So the tube sags between the 2 of you but does not touch the ground. That should let the air out. Agitate slightly if necessary.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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