SAFE, SIMPLE, RELIABLE and PROVEN OVER THE YEARS as evidenced by the US ARMY ABERDEEN PROVING GROUNDS using the BALL-IN-THE-WALL® and also, with the US ARMY using the BALL-IN THE-WALL® for HOSPITAL CONSTRUCTION ICRA:

“Among the many safety aspects of performing construction within a hospital, work areas are equipped with “the ball in the wall” to visually show the negative pressure maintained in sealed work areas. As air is sucked out of these construction areas and filtered, an orange ball rolls in or out of view inside a clear domed portion of each room’s temporary ventilation airway, based on positive or negative pressure in that sealed area.”

https://www.army.mil/article/176819/glwach_breaks_ground_for_more_ground_breaking_technology–