What is the meaning of absolute pressure in this fluid mechanics question?

Why does 6.8 m of water take absolute pressure height not gauge (pressure at point C)? what is the logic? If anyone assumes that local atmospheric pressure 755 mm of mercury is the pressure of the left side of tube,then what is the logic? because this is not mentioned in question.

Name of the book and the author: A Textbook of Fluid Mechanics & Hydraulic Machines By R K Rajput.

Name of the publisher: S. Chand

You say "if anyone assumes that local atmospheric pressure of 755 mm of mercury is the pressure of the left side of the tube...". Not sure what you mean by this; left side of tube is closed and at C the absolute pressure is the vapor pressure of Hg at its temperature, and the problem says neglect this, so absolute pressure at C is $$0$$. Also the right side is closed, hence the negative gauge pressure at L.

• A friend of mine said to me that "local atmospheric pressure 755 mm of mercury is the pressure of the left side of the tube",so this pressure is taken as absolute pressure.This is the reason why I am saying this is that . Commented Feb 11, 2021 at 18:18
• you said absolute pressure at c =0 but the book is saying 6.8 m of water .Is it(answer of the book ) wrong ? Commented Feb 11, 2021 at 18:25
• This is a closed system. The atmospheric pressure is 755 mm everywhere outside the closed system. The atmospheric pressure is only used to convert the true (absolute) pressure in the system to gauge pressure. Commented Feb 11, 2021 at 18:28
• Any liquid with an open space will evaporate and produce a pressure equal to its saturation pressure at its temperature. The problem says ignore this vapor pressure, so the absolute pressure at C is 0. The top of the left end tube is closed as the figure says, so is is not exposed to the atmosphere, and ignoring the small vapor pressure of Hg the pressure at C is 0. Commented Feb 11, 2021 at 18:43
• Absolute pressure is the actual pressure. At point D the absolute pressure is the absolute pressure at C, which is 0, plus the pressure due to the 0.5 m of Hg. Gauge pressure is by definition: pressure relative to the atmosphere = absolute pressure minus atmospheric pressure. The problem calculates absolute pressure at D (same pressure at A) then converts to gauge pressure. Here is an analogy. The third floor of a building (absolute height) is 15 m above ground. The third floor of a building relative to the top of the building (top at 100 m) is -85 m (relative height). Commented Feb 11, 2021 at 19:22

They are using meters of water as a unit of pressure. It is the height that a given pressure can support a column of water. Since Hg is 13.6 times as dense as water, if 0.5 meters of Hg is supported, this is equivalent to supporting a column of water 13.6 times as high. Similarly, 0.755 meters of Hg is equivalent to a column of water 13.6 times as high.

• Your answer is not clear to me.I have asked that why does 6.8 m of water take absolute pressure height not gauge ? Commented Feb 11, 2021 at 18:21
• Why do you think it would represent gauge pressure? Commented Feb 11, 2021 at 19:10
• Because ,Author neglects vapor pressure and also system is closed(there is no atmospheric pressure ) .He only counts manometer reading. Commented Feb 11, 2021 at 19:13
• That is the very reason it is absolute pressure. You have essentially vacuum at the top of the column at C. Commented Feb 11, 2021 at 19:22