Why would I lose 3-5 lbs of pressure at cut off?

[crombienator]

Had a new pump installed. Maintenanced the well tank. Everything is seemingly working properly. I was doing a measurement of my gpm and noticed that my pressure gauge drops 3-5 lbs at cut off. It stabilizes there, and doesn't keep leaking. Normal? What gives?

 

[Reach4]

I hope normal. I saw something like that on mine. I decided to record my digital pressure gauge as an AVI. In it I could hear the pressure switch open, and that marks about time zero on the graph. I graphed the pressure displayed on the gauge by looking at the AVI frames. Here is the graph.

What gives? My first thought was the check valve taking a while to seat, but I now suspect that the diaphragm takes a bit of time to relax. It seems too smooth to be a check valve getting seated.

 

[Sponsor]

 

[Valveman]

The bladder in the tank could be dragging a little. After the pump shuts off the bladder relaxes and allows the tank to accect a little more volume, which lowers the pressure. It is not a normal thing to see. Normally if you have an 80 gallon tank, a loss of 4 PSI means 4 gallons of water got out somehow.