A control valve that opens more when the pressure input is increased (air to open) is called a reverse acting valve.
For the water level control system, we have assumed that a reverse acting control valve is used. This means that when the water level drops below set point, the controller must increase its output to the control valve to increase the water inflow. This type of control action is called reverse acting, because when the water level decreases, the controller output must increase. If a direct acting (air to close) control valve was used, the control action would be direct acting, as the controller output would decrease when the water level decreases.
A heating system would typically be reverse acting, as a decrease in temperature would demand an increased controller output to the heating element. A cooling system, on the other hand, would typically be direct acting because an increase in temperature would require an increased output to the cooling element.
Direct acting control: A control arrangement in which the controller output increases if the measured value rises above the set point.
Reverse acting control: A control arrangement in which the controller output increases if the measured value drops below the set point.