I've googled around and can't find quite what I'm looking for.. this may be a VERY basic question, but I'd appreciate if it could be explained in layman's terms..
Traditional fuel injection engines have the ability to pressurize a fuel line to ~50psi.
Gasoline Direct Injection/Diesel engine fuel pumps can pressurize a fuel line to 30,000psi or more.
Power washers can pressurize water to 4,000psi +
but all these fluids are 'assumed to be incompressible'…
The explanation that the measured pressure is the force on external walls of a reservoir under pressure doesn't quite make sense to me, in that in order for a High-Pressure fuel reservoir to be under greater than atmospheric (30,000 psi) pressure, the fluid within must also be pressurized.
I don't understand!
I've read that the 'assumption' of incompressibility is not entirely true, but for a liquid to be compressible to 2000bar, the 'assumption' must be entirely false!?
Best Answer
The measured pressure is not a force but it is related to force.
Pressure is the force a fluid exerts on the walls of a container (for instance) expressed per unit of surface area:
$\Large{p=\frac{F}{A}}$, with $p$ the pressure, $F$ the force and $A$ the surface area.
In S.I. units, for example $10 \text{ Pa}$ ($10 \text{ Pascal}$) means the pressure exerts $10 \text{ N}$ ($10 \text{ Newton}$) of force $F$ per $m^2$ of surface area $A$.
That is correct.
I think you may not understand incompressibility completely. Liquids are quasi-incompressible because when you apply pressure on them their volume almost doesn't change. Mathematically we can write:
$\frac{\Delta V}{\Delta p} \approx 0$, where $\Delta V$ is the decrease in volume for a given increase in pressure $\Delta p$. So even for high pressures like $2000 \text{ bar}$ the decrease in volume is quite small (but not quite zero either).
As regards your title question, pressure is measured by means of a manometer.