After performing a hydraulic calculation
in WaterCAD when checking the results
I have some doubts, by subtracting the
elevation of the hydraulic gradient of
a node the value obtained differs in
decimals to the value of the pressure. Why does this happen?
I would also like to know what is the
difference between "pressure" and "pressure head"
"Pressure head" refers to pressure in head units such as feet or meters. "Pressure" is the pressure reported in pressure units such as psi, bars, kPa, etc. However, the user has the ability to report pressure in head units such as feet and meters.
I just checked some calculations and Pressure head = Hydraulic grade - Node elevation down to 6 decimal places and that is still correct. I do see that the Pressure in feet and pressure head can differ slightly . This appears to be due to a slight difference in the value of density used, but it is well within the accuracy you need for practical calculations. The differences are on the order of 0.1%,
We use a default specific gravity of water of 0.998 which corresponds to 20 Deg C. If you change that in the calculations options to 1.000, then these differences you noticed go away.
Pressure Head = P*ρg
where, ρ= mass density of water
g= acceleration due to gravity
ρg is almost equals 10. For eg. if the pressure at a junction is 1 Kg/cm2 then its pressure head turns out to be about 10 m H2O. Hope this helps. Happy modelling,
Mathematically when expressing pressure in m H2O and pressure head in meters both values should be equal but vary less than 0.25% in the model I have made. Effectively when the value of 1 is used for the specific gravity for the water it happens that, pressure = pressure head.
I verify that the correct equation must be Pressure Head = P / ρ or Pressure Head = P / (ρ * g) when working for the pressure with Kg / m2 for the first case and with N / m2 for the second case. But when working empirically, the Pressure Head = 10 * Pressure is correct for pressure in Kg / cm2 and pressure head in meters