"Air to open" and "air to close" refer to the action of pneumatic control valves in response to changes in the air pressure supplied to their actuator. These terms are commonly used in pneumatic control systems where compressed air is used to operate control valves.

1. Air to Open (ATO):

   • In an air to open control valve, increasing the air pressure supplied to the actuator causes the valve to open.
   • The actuator of an air to open valve is designed such that it applies force to close the valve by default. When air pressure is applied, it overcomes this default force and opens the valve.
   • A common application of air to open valves is in fail-safe systems, where loss of air pressure causes the valve to close automatically for safety reasons.

2. Air to Close (ATC):

   • In an air to close control valve, increasing the air pressure supplied to the actuator causes the valve to close.
   • The actuator of an air to close valve is designed such that it applies force to open the valve by default. When air pressure is applied, it overcomes this default force and closes the valve.
   • Air to close valves are commonly used in applications where the valve needs to remain open by default, and closing is required in response to specific control signals or emergency shutdown situations.

In summary, the distinction between air to open and air to close control valves lies in their response to changes in air pressure supplied to the actuator. Understanding the operating principle of these valves is crucial for proper selection, installation, and operation in pneumatic control systems.