Semi random physics questions
 

Question 1:
I have a vacuum pump and vacuum chamber, used in my pen making. The pump will pull nearly 30 in of mercury on the gauge, and the chamber is airtight, and will hold that vacuum with no appreciable loss for several hours at least.
I submerge pen blanks in a liquid stabilizing solution (heat cured ester) and place them in the chamber. When I pull vacuum, they bubble as the air is drawn out of the wood. This can take many hours.

If I leave the pump running, the gauge stays steady, and the blanks keep bubbling. If I shut off the pump and close the valve to the chamber, the valve remains steady (no loss of vacuum) but the blanks no longer bubble.
Why?

Question 2:
I have developed an allergic reaction to thickened CA glue. While curing, it gives off formaldehyde, which gives symptoms similar to a bad head cold. In order to avoid this, I am building a fume hood, similar to those seen in chemistry labs.
The US dept. of health has conveniently posted the required specs for these online. I have purchased materials, and am in the process of construction. The total length of the piping to outside is about 4', and the duct is 4" diameter.

The working part of the hood is an inline fan that pulls 190CFM. Does is make any difference if the fan is in the start, middle or end of the run? If so, why?

For the 1st question I believe what is happening is that for the given vacuum level all of the gas has can come out of solution has come out. So even though it is being held at a vacuum relative to atmospheric pressure it is at equilibrium, you would need to bring it to a higher vacuum level (like what you see when the pump is running) to pull more gas out.

As for the 2nd question, it depends. The fan should have specs that list the differences in flow when used with an exhaust or intake tube. I would imagine most of the time the best flow is going to be with the fan at the end of the tube so that the boundary layer has been more established after whatever happened at the inlet of the tube. What is most important is having either the inlet or exhausts be nice and straight. Wherever you can find space for it where it doesn't have a bend right before or after the fan is going to be the best performance.

Our fume hoods here are at the very end. They also have 180* turn down at the end so water doesn't come in. Plus a couple bends. Make sure the room you are in is open too, you wouldn't believe how much a closed door can reduce cfm across a fan.