Described herein are various embodiments of differential air pressure systems and methods of using and calibration such systems for individuals with impaired mobility. The differential air pressure systems may include an access assist device configured to help a mobility impaired user to stand in a pressure chamber configured to apply a positive pressure on a portion of the user's body in the sealed pressure chamber. The system may also include load sensors configured to measure the user's weight exerted inside and outside the chamber. The system may be calibrated by determining a relationship between the actual weight of the user and the pressure in the chamber, where the actual weight of the user may be measured by more than one load sensor and at least one load sensor is not in the pressure chamber.