What primarily differentiates a hydraulic valve from a pneumatic valve?

• A. The only difference is that power is transmitted by a change of pressure above the actuator (diaphragm or piston). This change of pressure is transmitted through the hydraulic control tubing or by diverting a portion of water flowing through the valve.
• B. Most of the hydraulic control valves are normally open and power has to be transmitted to close these valves.
• C. The system uses electric solenoid to actuate.
• D. It is manual only.

Answer: (A)

The fundamental distinction is the working fluid and how pressure is used to move the valve actuator. A hydraulic valve uses a liquid (commonly water or oil) and transmits power by changes in fluid pressure through hydraulic lines or by diverting a portion of water flowing through the valve. That pressure then acts on a piston or diaphragm to move the valve.

This is different from pneumatic valves, which rely on compressed air (a gas that is easily compressible). The defining difference isn’t the actuation method (solenoids, manual, etc.) or the tendency to be normally open versus closed; those features can appear in both systems. They don’t establish what sets hydraulics apart from pneumatics—the key is the incompressible liquid signaling via pressure in hydraulic circuits versus compressible gas signaling in pneumatic circuits.