Pressure ratio is the density ratio divided by horsepower.

• A. True
• B. False
• C. Depends on units
• D. Never defined

Answer: (B)

Pressure ratio is simply the ratio of the outlet pressure to the inlet pressure across a device, a dimensionless measure of how much pressure the component adds. The density ratio ρ2/ρ1 is related to how density changes across the same device and, for an ideal gas, ties to both pressure and temperature via ρ = P/(R T). So ρ2/ρ1 can be expressed as (P2/P1) × (T1/T2). Horsepower, on the other hand, is the rate at which shaft work is delivered to the fluid, connected to mass flow and specific work, not a divisor that yields pressure ratio. Dividing a density ratio by horsepower yields a quantity with incompatible units and meaning for defining pressure boost. Therefore, the statement is not correct: pressure ratio is defined by pressures, not by density ratio divided by horsepower.