What does the difference between static and residual pressures represent on a compound gauge?

The difference between static and residual pressures on a compound gauge represents the system's ability to maintain pressure under flow conditions. Static pressure refers to the pressure in the system when there is no flow, while residual pressure is the pressure remaining when water is being drawn from the system, usually during firefighting operations or testing.

A drop in pressure between these two measurements indicates how much pressure is lost when the system is engaged, showcasing the effects of flow and the system's resistance to that flow, which typically results from friction losses in hoses and fittings or obstructions in the system. Understanding this difference helps fire apparatus engineers gauge the effectiveness of their pumping systems and the potential impact on firefighting efforts.

The other options do not accurately define this relationship. Surge refers to pressure spikes during rapid valve operation or sudden changes in water flow, which is not directly the difference illustrated between static and residual pressures. Flow rate is a measure of the volume of water moving through the system, not a representation of pressure differences. Pressure loss relates to the overall pressure changes in the system but does not specifically explain the relationship indicated by the difference between static and residual pressures.