VAV boxes can also be classified as pressure independent and pressure dependent. A pressure independent VAV measures cfm and will maintain the proper airflow regardless of the box inlet static pressure provided by the main air handling unit. A pressure dependent VAV does not measure airflow. The cfm will change depending on the inlet static pressure. VAV systems are also usually designed with a diversity factor which means that the main air handler design airflow is less than the sum of the total airflow of all the VAV's. This is a common design because not all of the VAV's in a building will be in full cooling or maximum cfm all at once. There are three ways that a VAV can be controlled; pheumatic, electric, or Automated Control Systems (ACS). Pneumatic control systems are becoming obsolete. The VAV damper is opened and closed by a controller sending air pressure to an actuator hooked to the VAV damper. Electric simply sends a signal from the thermostat in volts to an electric motor connected to the VAV damper. ACS works the same as electric except there is a main computer set up in the building that gets information from all the VAV's and air handlers displaying it in text and graphics form. The possibilities are endless with automated controls and is truly a huge leap forward in the HVAC industry.