# Pump Question CERM Ch 18 Q7

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### #1 Dano_PE

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Posted 10 April 2012 - 10:58 PM

In Chapter 18 Question 7 of the CERM sample questions they solve for the total dynamic head without taking into account the atmospheric pressure.

For instance in question 1 it is solved by converting the pressure at inlet and outlet(gage pressures) to absolute pressures (Example ; Poutlet= 14.7psi + 20psi = 34.7psi(roughly))

In question 7 they do not do this. They just use to pressure gage and do not convert it to absolute pressure.

I understand everything else. I am just hung up on when and when not to convert the gage pressure to absolute pressure.

### #2 ksprayberry

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Posted 10 April 2012 - 11:52 PM

I'm Mechanical, but we have similar problems. Is it because both the inlet and outlets are both at the same reference pressure. ...Both at atmosphere? So when you carry it across the = sign in the equation, one 14.7 cancels the other out?..I can see doing it consistently whether you need to or not. Adding 14.7 to both gauges is maybe "good form" and develops habit...I don't know, maybe that's a stretch. I caught one or two in our book the same way and wondered why they didn't do it.

Just a possible thought.

### #3 Dano_PE

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Posted 11 April 2012 - 12:23 PM

That was my suspicion.

I attempted to do the problem with the atmospheric pressures to see if I got the same solution and I did not.

The inlet was P=0psi (I forgot that 0psi gage is the same as 14.7psi absolute).

Now it works out correctly. Thank you.

### #4 andyrich

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Posted 07 May 2012 - 04:35 PM

If you had a negative gauge pressure in the problem, you might want to convert both sides to atmospheric for convenience.

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