You are given an open-ended mercury manometer and asked to determine the
pressure in a pipeline that supplies feed gas to a reactor. However, when you connect the
manometer to the pipeline, you notice that the calibration markings in the leg that
connects to the pipeline are faded and not readable. You need to improvise.
(a) With the manometer still connected, you turn off the gas flow and use a bourdon
gauge to measure the pressure in the pipeline at several positions. Would you
expect any difference between the gauge readings? Briefly justify your answer.
(b) With no gas flowing, the bourdon gauge reading is 6.2 psi and the mercury level
in the open leg is 800 mm above the base of the manometer. What should be the
corresponding height of mercury in the manometer leg connected to the pipe line?
(c) Now you turn back the gas flow and observe that the manometer level in the open
leg increases by 30 mm. What is the gauge pressure in the gas pipe line?
Pabs = Pgauge + Patm
The Attempt at a Solution
a.) My answer would be no. There's no fluid in the pipe and so there won't be a change in pressure along the path of the pipe.
b.) My first question is: why is there a pressure in the pipe? There's nothing colliding against the walls. That aside, 6.2 psi = 321 mmHg, that means it will displace 321 mmHg. This will result in a height of 1121 mmHg.
c. 30 mmHg = .5801 psi, the total gauge pressure would just be 6.7801, correct?
I feel as if I'm missing some important concepts here. I just don't have an intuitive grasp of the concept of pressure despite it simply being the ratio of a force to an area.