How can hydrostatic pressure be zero?

1. Aug 13, 2012

minik

I was reading a paper and I saw a statement that says zero hydrostatic pressure. I was wondering what it means..

2. Aug 13, 2012

andrien

some people like to measure hydrostatic pressure by cutting off the atmospheric pressure part.they just assume atm. pressure zero.they measure it above or below atm pressure standard line ,also it depends on where you read it.

3. Aug 13, 2012

Studiot

You are referring to what is known as gauge pressure, but your conclusion is false.

Gauge pressure and hydrstatic pressure only differ by a constant and gauge pressure varies with a fluid just as does hydrostatic.

A simple answer would be what is the hydrostatic pressure of water at a tank or lake surface?

But your reference may be referring to moving fluids.

Last edited: Aug 13, 2012
4. Aug 13, 2012

minik

Here is the sentence
"If the water is under zero hydrostatic pressure it will come to rest on a porous surface in some position."
I was also guessing that it means the surface of the water but then it didn't make sense that, is the water layer very thin that we assume hydrostatic pressure to be zero everywhere or is there something else that I am missing..

5. Aug 13, 2012

Studiot

A flowing fluid has constant total energy.
It divides this between the kinetic energy of motion (flow) and the potential energy, known as the hydrstatic pressure energy.
The faster it flows the more KE it has and the less PE or hydrostatic pressure.

A fluid that is initially at rest and then allowed to flow (eg by opening a tap in the side of a tank) converts some of the hydrostatic energy to kinetic energy when it starts flowing.
The hydrostatic energy depends upon the depth of the fluid.

Conversely a fluid that is spreading out over a surface in a thin layer has little hydrostatic energy as it has little depth.
So it has little energy to drive and maintain the flow.

Further if the surface is rough ie has high friction as a porous surface will, it dissipates some of its energy working against friction so there is a minimum depth which will enable flow over such a surface.

A monomolecular layer of fluid will have zero hydrostatic pressure and no tendency to flow. ie it will come to rest.

6. Aug 14, 2012

andrien

I have not said any word about gauge pressure.I was really mean that the constant of integration in simple pressure formula if put zero will give some absolute measurement of force and there will not be net force.if you have really gotten some conclusion out of my first post then it was not meant that way i.e. gauge pressure.however are you saying that the thickness of monomolecular level is zero so that it does not give rise to any hydrostatic pressure.