Problem:

Inlet P: 19.7 psia
Outlet P: 17.7 psia
DP: in gauge?

Solution 1:
DP = 19.7 psia - 17.7 psia
= 2 psia
= 2 psia - 14.7 psi atm
= -12.7 psig

Solution 2:
DP = 19.7 psia - 17.7 psia
= (19.7 - 14.7) psig - (17.7 - 14.7) psig
= 5 psig - 3 psig
= 2 psig

Which of the above solutions is correct?

 

The correct answer is the differential pressure is 2 psi, neither gauge nor absolute. Some would call it 2 psid, where the "d" is for differential.

 

I would go so far as to say that the differential pressure is only properly stated as 2 psid and not as 2 psi.

The absence of the letter g (for gauge pressure) infers that the units are gauge and not differential or absolute.

psi = psig
psi does not equal psia
psi does not equal psid

 

I would think neither.

DP = differential pressure no reference to atmosphere
19.7 - 17.7 = 2 psi dp.

Regards,
cww

 

Hi there,

This can be a bit confusing so always make sure you use the SAME pressure platform when you do these calculations. Do not change platforms halfway through the calculation.

0 Psia = -14,7 Psi OR 0Psi = 14,7Psia
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Using only the absolute pressure platform:

-14,7 Psi + 19,7 Psia = 5 Psi
And,
-14,7 Psi + 17,7 Psia = 3Psi
So,
5-3 = 2Psi

Also
19.7 psia - 17.7 psia = 2Psi
Because,
19,7Psia = (-14,7 Psi + 19,7 Psia = 5 Psi)
And,
17,7Psia = (-14,7 Psi + 17,7 Psia = 3Psi)
So again,
5-3 = 2Psi