I have a problem with a vacuum-pump test.
I was showing my son the behavior of water when it is vacuumed with a 2-stage pump.
So, I pored a glass of water in a drinking glass bottle and attached the vacuum to it.

There was also an electronic vacuum gage connected to it to let him show what happens with vacuum pressure if you stops th vacuum and the pressure rise to the corresponding saturation pressure and remains then stable.

We started the pump - vacuum hose of 3/8 - and the 5 cm (2 inches) water in the bottle started to boil really heavy after +/- 20 seconds.
Waw.

But during 2 minutes, you saw that the boiling became lesser and lesser and after 2, 3 minutes the boiling stopped completely although the vacuum pressure was still the same.
We were then well below the saturation pressure of water at 20 °C.

We changed oil in the pump- it was new oil the 1st time - but same result.

So we let it ran for 2 hours and there was perhaps every 4 to 5 minutes one bubble coming out of the water. But the level didn't drop at all.
We also tried to heat the bottle but this gave no result.

We repeated the test with 2 other pumps and always the same bad result.

The first heavy boiling we saw was probably the enclosed gasses that evaporated. But as soon those were gone, the boiling process stopped.

Why is the process stopping at a sudden? I can make a small movie of this but for the moment, it doesn't work at all like I had hoped.

So my son made of course the right observation: if there's water in the system, then it isn't removed by the pump

PS is it gage or gauge pressure?