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in question A2 part (ii) why P2 is considered to be zero

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  • $\begingroup$ Please edit your question to include what you have tried to do to solve your problem or what thoughts you have. This site will help you to find the flaws in your process, but we typically do not provide full solutions to homework. $\endgroup$ – hazzey Jun 20 at 12:48
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P2 is given as atmospheric and P1 is given as a gauge pressure of 80kN/m^2.

The gauge is assumed to measure the pressure against atmospheric ie it could be a Bourdon tube type gauge.

Which means if atmospheric is taken as 1 then the inlet pressure would be 80 + 1 kN/M^2 absolute. Then, one would subtract 1 from the 81 to get the pressure difference...

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  • $\begingroup$ atmospheric pressure is 1. however, in this particular question are supposed to consider the atmospheric pressure "zero" $\endgroup$ – azher Jun 19 at 16:11
  • $\begingroup$ @azher for the difference between gauge and absolute pressure, see en.wikipedia.org/wiki/Pressure_measurement. It is 0 because measured relative to atmospheric pressure. $\endgroup$ – am304 Jun 19 at 16:14
  • $\begingroup$ @azher Consider that what is important here is the pressure difference; which is a reason why gauge pressure can commonly be given in some applications instead of absolute pressure. $\endgroup$ – JMac Jun 19 at 18:57
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    $\begingroup$ @azher Aargh - don't mix up units! Atmospheric pressure is about 101.3 kN/m^2 at sea level, not "1". But of course the value doesn't matter since P1 is a gauge pressure not an absolute pressure. $\endgroup$ – alephzero Jun 19 at 19:29

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