What is the primary purpose of using a plumb bob?

The primary purpose of using a plumb bob is to check if a pipe is completely vertical. A plumb bob is a simple tool that uses gravity to provide a true vertical reference. When suspended from a string or line, the heavy, pointed end of the bob will settle in a direct line to the ground, allowing the user to determine if a surface or installation, such as a pipe, is aligned vertically. This is crucial in drainage work as proper vertical alignment ensures effective water flow and prevents issues like blockages or backflow.

Other choices involve tasks that do not utilize a plumb bob effectively. Measuring water flow requires specific flow rate instruments, determining the length of a pipe typically depends on using measuring tapes or lasers, and assessing drainage efficiency involves analyzing the system's performance rather than checking vertical alignment.