If doing so also eliminates the weight change, then perhaps the weight change was really caused by air jets from moving parts or from high-voltage all along. If high temperatures are involved, then rising plumes of hot air can create forces. If the AG force from your device is small, and if it rises slowly with time and then decreases slowly when power is removed, then suspect bouyancy forces and rising hot air. To eliminate this problem, operate your device within a vacuum chamber. Or, if your device is meant to create a directional thrust, then rotate your device so that all of the expected force is sideways.
Electrostatic forces caused by high voltage can masquerade as small weight changes.
This will eliminate any electrostatic forces between the bag and the outside world. Don't trust scales, especially if your device is vibrating. Some scales are nonlinear, and will convert any vibrations into a false reading of directional force. Better to suspend your device from a pendulum, orient it so that it produces sideways thrust, then measure any deflection from vertical. Or, if your device only produces changes in weight, then build a balance beam, suspend your device from one end of the beam, and attach a counterweight to the other end.