# Is psi the same as psia?



## xuhq

Can anybody pls confirm?


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## civilized_naah

xuhq said:


> Can anybody pls confirm?


psia is still psi. The 'a' at the end represents Absolute (real) pressure. As opposed to psig, which is also psi, but a Gage pressure, which is the pressure (difference) recorded by a gage or device, which has to have a reference pressure. The most convenient reference pressure is atmospheric pressure. Thereforesig = psia - reference pressure in psi


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## Sschell

^close... but not quite.

PSI is a unit

in PSIA the unit is PSI, but zero is defined as absolute zero.

PSIG is gage pressure, PSI is still the unit, but zero is defined as ambient atmosphereic pressure (so the difference between PSIA and PSIG is typically 14.7 PSI i.e. 0 PSIG = 14.7 PSIA)

PSID is differential, again, PSI is the unit, but this refers to the difference between two measurement points. This is frequently used to measure pressure drop across an orfice (for example). PSIG can be seen as a subset of PSID, where your low side measurement point is ambient pressure.

edit: oh yeah, if something specifies only "PSI" with out the word atmosphere or difference/differential then it means PSIG. This is standard useage.


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## xuhq

Water flows at 5 gpm in a 1 in schedule-40 rigid steel pipe. If the initial pressure in the pipe is 20 psig, the maximum pressure that results from a rapidly closing valve is most nearly

a 80 psi

b 120 psi

c 140 psi

d 160 psi

The deltaP i calculated is 122 psi, which gives 122+20=142 psig. But the absolute pressure should be 122+20+14.7=156.7 psia.

If psi means psig, then © is right. But if psi means psia, then (d) is right.

Can anybody check the result? I don't have it.

Thanks,



sschell_PE said:


> ^close... but not quite.
> 
> PSI is a unit
> 
> in PSIA the unit is PSI, but zero is defined as absolute zero.
> 
> PSIG is gage pressure, PSI is still the unit, but zero is defined as ambient atmosphereic pressure (so the difference between PSIA and PSIG is typically 14.7 PSI i.e. 0 PSIG = 14.7 PSIA)
> 
> PSID is differential, again, PSI is the unit, but this refers to the difference between two measurement points. This is frequently used to measure pressure drop across an orfice (for example). PSIG can be seen as a subset of PSID, where your low side measurement point is ambient pressure.
> 
> edit: oh yeah, if something specifies only "PSI" with out the word atmosphere or difference/differential then it means PSIG. This is standard useage.


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## Sschell

my answer would be c (from the information you provided, I did not check your calculations)


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## Ronin

Can you please explain how you got dp = 122psi?

5 gpm in 1" SCH 40 steel pipe has 1.85 ft/sec velocity

Velocity head = v^2/(2g) = 0.05 ft

Converting to psi = 0.05*62.4/144 = 0.02 psi


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## Sschell

^water hammer.


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## IlPadrino

water hammer maximum surge pressure = aΔV/2.31g, where a=speed of sound (4856 fps in water), ΔV = 1.86 fps (instantaneous stop), 2.31 is the conversion factor from ft to psi, and g = 32.2 ft/sec-sec

answer=121psi+20psig=141psig and so I'd choose c.


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