The key difference between air to open (ATO) and air to close (ATC) valves lies in their response to changes in air pressure supplied by the actuator:

Air to Open (ATO) Valve:

• Default State: In its normal, unpowered state, an ATO valve is held closed by a spring.
• Operation: When air pressure from the control signal increases, it pushes against the spring, forcing the valve to open progressively. As the air pressure goes higher, the valve opens wider.
• Fail-Safe Operation: In many applications, ATO valves are preferred for fail-safe operation. This means that if there's a loss of air pressure (instrument air failure), the spring mechanism automatically closes the valve. This can be crucial for safety reasons in processes where an uncontrolled flow needs to be stopped.

Air to Close (ATC) Valve:

• Default State: In its normal, unpowered state, an ATC valve is held open by a spring.
• Operation: When air pressure from the control signal increases, it pushes against the spring, forcing the valve to close progressively. As the air pressure goes higher, the valve closes tighter.
• Applications: ATC valves are used where an open position upon air pressure loss is desired. This might be necessary for processes that require continuous flow even during a temporary air pressure drop.

Here's an analogy to illustrate the difference:

• Think of ATO like a car brake pedal. When you press the pedal (apply air pressure), the car slows down (valve opens). If the brake line fails (air pressure loss), the spring mechanism automatically engages the brakes (valve closes), stopping the car.
• Think of ATC like a door closer. The door stays open normally (valve open) due to the spring. When you push the door closed (apply air pressure), it overcomes the spring tension and closes further. If the door closer malfunctions (air pressure loss), the door remains open due to the spring.

In conclusion, choosing between an ATO or ATC valve depends on the specific application and the desired fail-safe behavior in case of air pressure loss.