Hydraulics use _____________ fluid while pneumatics use _______________ fluid.

In hydraulic systems, the primary fluid used is oil, which is characterized as non-compressible. This property allows for the efficient transfer of force through the system. The term "recoverable" also applies to hydraulic fluids, as the system can often return the fluid to a reservoir after its use, allowing it to be reused in the system.

On the other hand, pneumatic systems typically use air or gas as their working fluid. Air can be compressed, meaning it is considered compressible. In pneumatic applications, the characteristics of air allow it to expand and contract during operation, which fits the definition of a "non-recoverable" fluid since the air can change volume significantly and is often vented after use.

Thus, the combination of using recoverable non-compressible fluid for hydraulics and non-recoverable compressible fluid for pneumatics aligns with the properties and functions of these two systems. This distinction is crucial for understanding how each system operates and the types of fluids involved in their processes.