What does the term "fixture unit" measure?

The term "fixture unit" is a crucial concept in plumbing, as it measures the probable demand on a plumbing system caused by a fixture. Each type of plumbing fixture—such as a toilet, sink, or shower—has been assigned a specific value in fixture units based on its expected usage and peak demand. This measurement helps engineers and plumbers design systems that can adequately support the water flow and pressure requirements of various fixtures without risking insufficient supply or pressure drops.

The use of fixture units allows for standardized calculations when sizing pipes and determining the overall capacity of plumbing systems. By estimating the cumulative demand from multiple fixtures in a building, professionals can ensure that the system is capable of handling the expected load during peak usage times.

In contrast, other concepts in the provided options do not align with the definition of fixture units. For example, the maximum pressure a fixture can handle pertains more to the structural integrity and design of the fixture rather than its demand on the plumbing system. The value of a plumbing system installation is generally more about cost and economic aspects rather than usage demand. Lastly, the size of a plumbing pipe relates to the physical dimensions required to transport water rather than the measurement of fixture demand itself.