When testing a hydrant at full flow, it is best to use a _______.

• A. Diffuser
• B. Gauging nozzle
• C. Nozzle cap
• D. Venturi meter

Answer: (A)

When testing a hydrant at full flow, you want to manage the energy of the water leaving the hydrant so readings are safe and stable. A diffuser attached to the hydrant outlet spreads the jet and slows it down, reducing turbulence and velocity. That makes it easier to read discharge accurately with measuring devices and minimizes splash, erosion, and potential water hammer effects as the hydrant is opened or tested. The other options either restrict flow (gauging nozzle), block flow (nozzle cap), or serve as a separate flow-measuring device not typically used to enable true full-flow testing at the hydrant outlet (venturi meter).