Different readings on HF vacuum set-up.

[ElMostro]

I have a Harbor Freight pot set up for pressure and vacuum. When I vacuum the empty pot I reach 29+ inches but when I place a glass jar with Plexi-acetone mix and blanks in the pot I can only reach about 22 inches of vacuum. It still works since the blanks are absorbing the mixture and sinking in the mixture but I cannot explain why the pot won't reach 29+ inches like when it's empty. What am I missing?
Note: I can apply vacuum to the empty pot and it will hold 27+ inches over night and hold 70 PSI over night with pressure so I don't think leaks are the problem.

 

[Monty]

The acetone is evaporating from the mix. You will not reach and hold the 29+ inches until all the acetone evaporates.

 

[Fred]

Mannie is definitely correct. The vapor from the acetone is adding to the atmosphere within the pot and until it is boiled off the internal vacuum pressure will always be lower than what can be reached with an empty pot. []

 

[ElMostro]

Thanks for educating me Gents.