What does “drainage fixture unit (DFU)” measure?

“Drainage fixture unit (DFU)” is a measurement that specifically quantifies the probable discharge into the drainage system from a fixture during peak usage times. This concept is essential in plumbing design and systems engineering because it helps determine the size of the drainage piping needed to handle the maximum expected flow.

The DFU value takes into account not just the volume of water a fixture can discharge, but also the frequency and timing of its use. By understanding how much waste water a fixture discharges during peak times, plumbing engineers can design systems that prevent overloading and ensure that drainage systems operate efficiently. This prevents issues such as backflow or clogs, ultimately facilitating smooth waste removal.

In contrast, the other options address aspects not measured by DFU. For example, the total volume of water used by fixtures pertains to overall water consumption but doesn't reflect peak drainage performance. Similarly, the length of time water is retained deals with water pooling or standing, which doesn't connect directly to discharge rates. Lastly, the pressure in a drainage system relates to its hydraulic performance and integrity, but does not involve the measurement of discharge rates associated with individual fixtures. Thus, measuring probable peak discharge is what defines DFU and is critical for effective plumbing system design.