What device is used to test percent deflection in a sewer pipe between manholes?

• A. mandrel assembly
• B. poly pig
• C. air plug
• D. laser

Answer: (A)

Testing percent deflection in a sewer pipe between manholes measures how much the pipe’s interior has shrunk from its intended shape under load. A mandrel assembly is the tool for this job. You pull a mandrel—a rigid device with a defined diameter—through the installed pipe from one manhole to the other. If the pipe can accommodate the mandrel, the deflection is within the allowed limit; if the mandrel binds or cannot pass, that indicates greater deflection. The percent deflection is essentially the difference between the nominal inside diameter and the size of the largest mandrel that can pass, expressed as a percentage of the nominal diameter.

Other tools don’t fit this purpose: a poly pig is used to clean the pipe, not to assess deformation; an air plug is used for isolating sections during air-pressure tests; a laser can measure dimensions or alignment but doesn’t directly quantify pipe deflection through traversal like a mandrel does.