The classic equation for pressure at depth is P=rgh but most on line calculators use P=rh, what am I missing?
I’m working on a hoop stress equation for a fabric water tank and kept getting crazy answers from HS=PD/2t so I went on line to check my numbers and noticed that every web calculator I could find used P=rh to calculate pressure, even if the P=rgh equation was listed right next to the calculator. When I checked for known pressures at depth to check the calculators, all the answers I could find match the P=rh formula, while every equation I could find matches P=rgh. This should be simple but now I’m just confused.
Austin:
GRAVITY must always be taken into account when you are working with densities and pressures. The g in those equations usually means gravity.
I am assuming that
r = rho for density and not radius
h = height
To confirm this make sure that your units work out properly.
Niel
Of course the units are very important. It depends also the system used, SI (international) or anglo-sax. (inches, pound, yards…)
If SI: P=rgh ; r=kg/m3 ; g=m/s2 ; h=m ; So P is in Pa or in N/m2 or in kg/(m*s2), they mean all the same, and where 2 or 3 are exponents.
If Anglo-sax: P=rh ; r=pounds/in3 ; h=inches ; P is in PSI (pound per square inche). Again 3 is the exponent.
I hope that will help you.
i believe that since g (gravity) is constant almost every where on this planet, it is ignored by web calculator to probably simplify the calculation of pressure.
michael adjin
