What is the unit used to express hydrostatic bottom pressure?

• A. Pa
• B. bar
• C. kPa
• D. psi

Answer: (D)

Hydrostatic bottom pressure is the pressure created by the weight of a column of water above a surface. In water distribution practice, especially in the US, this pressure is typically expressed in pounds per square inch (psi) because gauges, fittings, and system design commonly use this unit. The pressure increases with depth according to P = rho * g * h, and for freshwater, each meter of water head is about 9.81 kPa, which is roughly 1.42 psi per meter. So, expressing hydrostatic bottom pressure in psi aligns with everyday field measurements and calculations, making it the standard choice in this context. The other units (Pa, kPa, bar) are SI or metric units, which can be used, but psi is the conventional unit for this topic in the relevant practice environment.