A standpipe with a height of 100 feet, containing water, would have a base pressure of ?

• A. 43.4 psi
• B. 50 psi
• C. 30 psi
• D. 60 psi

Answer: (A)

The base pressure in a standing water column comes from hydrostatic pressure: the pressure at the bottom equals the weight of the water above it. For water, this translates to the unit weight (about 62.4 lb/ft^3) multiplied by the height of the column.

Calculate the pressure: 62.4 lb/ft^3 times 100 ft gives 6,240 lb per square foot. Convert to psi by dividing by 144 in^2 per ft^2: 6,240 / 144 ≈ 43.3 psi. Rounding to one decimal place gives 43.4 psi.

So, with the top open to atmosphere, the gauge pressure at the base is about 43.4 psi. (Different heights would give other pressures: for example, a taller column would yield higher pressure, and a shorter one would yield lower.)