Water Pressure 2

Does shape matter when calculation pressure? NO – Pressure at the bottom of a tank or pond or reservoir, no matter what the shape, is equal to the depth of the water in feet multiplied by 0.433 psi or divide by 2.31 feet. The weight of the water is directly related to the height of the water and can be expressed in feet of head or pressure head.

Atmospheric pressure on a water surface at sea level. An open tube is inserted vertically into the water; atmospheric pressure, which is 14.7 psia, acts equally on the surface of the water within the tube and on the outside of the tube.

Atmospheric pressure is acting on the surface of the liquid in the container (glass, bottle, jar, or box). A person drinking from a straw creates a partial vacuum in the straw by reducing the atmospheric pressure in the straw. The fluid rises because the atmospheric pressure in the straw is reduced to sub- atmospheric pressure creating a negative pressure in the straw. The liquid will stop flowing when atmospheric pressure is reintroduced by removing the straw from the mouth or by creating a hole in the straw.

As a practical example, assume the water pressure at a closed faucet on the top of a 100-foot high building to be 20 psig; the pressure on the ground floor would then be 63.3 psig.

If the pressure at the ground were to drop suddenly due to a heavy fire demand in the area to 33.3 psig, the pressure at the top would be reduced to -10 psig. If the building water system were airtight, the water would remain at the level of the faucet because of the partial vacuum created by the drop in pressure.

If the faucet were opened, however, the vacuum would be broken and the water level would drop to a height of 77 feet above the ground. Thus, the atmosphere was supporting a column of water 23 feet high.