When using a hydropneumatic tank set as a Dipping Tube, problems are seen in the calculated results during the transient simulation, in the Transient Results Viewer. For example the water level may unexpectedly jump up or down suddenly or generally appear unstable. Example below:
At some steps, the equations used for the dipping tube hydropneumatic tank boundary condition cannot be properly solved. In these cases, the HAMMER numerical solver switches to a special "bisection" method to generate a result, which can be less accurate. This can result in slight disturbances such as the ones illustrated above.
Enhancement # 981395 is currently being considered by our development team, to improve on the calculation method in these cases in order to produce a more stable and smooth result. In the meantime, these small disturbances should not significantly skew your overall important transient results.
Modeling Reference - Hydropneumatic Tanks