What device allows fluid to flow in one direction while blocking flow in the opposite direction?

The device that allows fluid to flow in one direction while blocking flow in the opposite direction is known as a check valve. Check valves are crucial in hydraulic and pneumatic systems because they prevent backflow, ensuring that the fluid flows correctly through the system. This functionality is essential for maintaining the efficiency and safety of the systems.

In situations where directional control of fluid flow is needed, check valves automatically open to allow flow in the desired direction and close to prevent any reverse flow when the fluid pressure drops. This mechanism helps maintain pressure and prevents damage to equipment or contamination of the fluid system by unwanted backflow.

The other options are not designed specifically for this purpose. A stop valve is used to completely shut off the flow of fluid, while a pressure regulator controls the fluid's pressure without necessarily managing direction. A flow meter measures the flow rate of fluids but does not affect the direction of flow. Thus, the check valve is uniquely suited to the requirement of allowing one-directional fluid flow while blocking the reverse direction.