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Assume you have a horizontally placed cylindrical pipe with constant diameter. There is a flow of water, through this pipe. You want to measure the static pressure inside the tube, which should be the same everywhere along the tube if we assume there are no pressure losses due to friction. On the top of the pipe, two vertical measuring tubes are placed to measure the static pressure in these two places. But like we said, the static pressure should be the same in both places. The two measuring tubes have different diameter, where the second one has twice the diameter of the first. Will the height of the water column in the measuring tube with the larger diameter be half that of the other? If so, how could you conclude that the static pressure in both places is still the same?

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  • $\begingroup$ What makes you think that the height of the water volume in the measuring tube with the larger diameter will be half that of the other? $\endgroup$ – Chet Miller Mar 13 at 22:39
  • $\begingroup$ @ChesterMiller I said the height of the water column, not volume :) I thought this was the case due to Jurin’s Law, which says that if you double the diameter of a capillary tube, the height will half. What I am unsure of is if this will also happen in this case, with a fluid in motion? $\endgroup$ – curiousmaths Mar 14 at 5:49
  • $\begingroup$ I know what you said because I copied it word-for-word. Compare what I wrote with your own words. In any case, for fluid flow, aside from the effects of surface tension (which can be minimized by using large enough diameter measuring tubes), the heights of the two columns will be equal. $\endgroup$ – Chet Miller Mar 14 at 11:13

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