Which statement best describes the fluids used in hydraulic versus pneumatic systems?

• A. Hydraulic fluid is incompressible; pneumatic fluid is compressible.
• B. Hydraulic fluid is more viscous; pneumatic is less viscous.
• C. Both fluids are incompressible.
• D. Pneumatic fluid is metallic.

Answer: (A)

The key idea is how the fluid responds to pressure. Hydraulics use liquids that are essentially incompressible under normal operating pressures, so a small change in pressure transfers force very predictably through the fluid and into the actuators. Pneumatic systems use gases, which are highly compressible, so the same pressure change leads to a noticeable volume change and energy is stored in the gas as it compresses. That fundamental difference is why describing hydraulics as incompressible and pneumatics as compressible is the best summary of how these systems operate. The other statements miss this central behavior: viscosity varies with fluids but isn’t the defining contrast here, both aren’t described as incompressible, and pneumatic fluids aren’t metallic.