The graph for gas pressure, gas volume, and water inflow in the Extended Node Data do not match the results seen in Time History graph for hydropneumatic tanks.
An unexpected, linear trend is seen in the Extended Node Data tab for a hydropneumatic tank:
First, it is important to note that the results shown in the Extended Node Data tab are inside of the tank. The air pressure values listed are within the tank, and will not necessarily be the same as pressure in the Time History graph.
That said, the results should be similar. For instance, the flow in the Time History graph should generally track the water inflow results in the Extended Node Data and the hydraulic grade or pressure results in the Time History tab should generally match the pattern in Extended Node Data for air pressure.
This is related to the value entered for Report Period (Transient), in the hydropneumatic tank properties. If a value of zero is entered, the program does not store the necessary data to create the time history graphs and inaccurate data is displayed.
To resolve the issue, enter a value for Report Period (Transient) and recompute the model. Entering a value of 10 for example will mean that results for every time steps will be displayed. With this completed, accurate results will be displayed in the results.
Future versions may include improvements to prevent this situation from occurring. Reference# 565364 / 217571
Modeling Reference - Hydropneumatic Tanks