Pressure

[pewant2be]

I am taking water resources afternoon, but still make the mistakes on psai and psig. If pressure is given in psig, do you subtract it from Patm (14.7 psia) to get the corrected pressure at ATM ? I have seen many problem s in which they do and don't bother to convert.

If that's the case, when do you use the psig given and forget psia in the calculation. I see these a lot in pumps problems.

BTW, what exactly is absolute pressure..

You help is well appreciated.

:smileyballs: :unitedstates:

 

[IlPadrino]

I am taking water resources afternoon, but still make the mistakes on psai and psig. If pressure is given in psig, do you subtract it from Patm (14.7 psia) to get the corrected pressure at ATM ? I have seen many problem s in which they do and don't bother to convert.
If that's the case, when do you use the psig given and forget psia in the calculation. I see these a lot in pumps problems.

BTW, what exactly is absolute pressure..

You help is well appreciated.

:smileyballs: :unitedstates:

Absolute and Gauge Pressure...

Absolute Pressure (psia) = Atmospheric Pressure + Gauge Pressure (psig)

Atmospheric Pressure changes with weather conditions and altitude though is usually assumed to be 14.7 psi at sea level.

Pressures are usually given in psig, except for compressible flow.

For the WR problems, you're usually talking about pressure changes, so it doesn't really matter which is used.

 

[MA_PE]

or in other words: absolute pressure is referenced to a pure vacuum (0 psi) and gauge pressure is referenced to typical atmosphereic pressure.

hope that helps.