How do you read a manometer for differential pressure and what does it indicate in a hydraulic system?

• A. Differential pressure equals the height difference divided by fluid density
• B. Differential pressure equals the height difference times fluid density
• C. Differential pressure equals the height difference times fluid density times g; indicates pressure differences driving flow or losses across a device
• D. Differential pressure equals velocity head

Answer: (C)

Reading a manometer comes down to converting the vertical height difference of the manometer fluid into a pressure difference. The pressure difference between the two points equals the weight of the fluid column per unit area, so ΔP = ρ g h, where ρ is the density of the manometer fluid, g is gravity, and h is the height difference. This gives the differential pressure in pascals. In a hydraulic system, that ΔP represents the driving force for flow or the losses across a component—more height difference means a larger pressure drop, indicating greater resistance or energy losses. The reading uses the fluid’s density and gravity; it’s not simply height or density alone, and it isn’t equal to a velocity head (which would be ½ρv^2).