A Proportion-Air closed-loop pressure assembly, like the QB2, can be used to simulate atmospheric changes in fuselage fatigue testing. Large volumes of air at low pressure need to be added and exhausted quickly and pressure needs to be extremely repeatable for this type of application.

The standard accuracy is ±0.2% and repeatability is ±0.02% of full scale calibration. In this application venting low pressure to atmosphere could be very time consuming, due to the small differential. To speed up exhaust time Proportion-Air modified the proportional regulator so that the customer had access to the exhaust valve drive signal.

The customer also added large, electrically-actuated dump valves and a time delay relay. The valve drive signal was then used to actuate the large dump valves to get back to sea level pressure very quickly. The speed of each fuselage test was improved by 45% when exhaust flow was increased. In the application drawing a RM-Series Volume Booster and a DS-Series Transducer   assembly was used to further streamline the process.