Problem: If the static pressure in a pipe leading from an elevated water storage tank is 39 psi, what is the elevation of the water above the pressure gauge?

Solution: Recall that 1 psi = 2.31 ft and the pressure at the gauge is 39 psi, so:

The basic relationships between the weight of water and the weight of air can be described as follows:

• If a tube is placed in a body of water at sea level (a lake, a water storage reservoir, a container, or a glass), water will rise in the tube to the same height as the water outside the tube. The atmospheric pressure of 14.7 psi will push down equally on the water surface inside and outside of the tube.

• On the other hand, if the top of the tube is tightly capped and all of the air is removed from the sealed tube above the water surface, forming a perfect vacuum, then the pressure on the water surface inside the tube will be 0 psi. The atmospheric pressure of 14.7 psi on the outside of the tube will push the water up into the tube until the weight of the water exerts the same 14.7 psi pressure at a point in the tube even with the water surface outside the tube. The water will rise:

• Because it is impossible to create a perfect vacuum in practice, the water will rise somewhat less than 34 ft; the distance it rises depends on the amount of vacuum created.

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