Vacuum Pump Weekend Project

[nosaj13]

I had some spare time this weekend and I decided to build a vacuum pump to test duel deploy altimeters. I started with a 3ft section of 4in PVC, 1 t-joint, and two threaded end caps, and 3 lids. I cut the pipe into equal sections, and glued the two sections onto either end of the t-joint. I drilled a hole on the side of the PVC wall for the wires to come out and I also drilled a hole in the cap of the t-joint.

For my pump I went and bought a hand vacuum pump from the local auto-store. With the fittings provided created my hose joint in the t-joint's cap by drilling a hole and epoxying around it.

I gave the pump a test run (with wire hole covered in hot glue). It would draw a vacuum with the first pump but in the time it took to do another pump it had lost the vacuum (as shown by the hand gauge). I decided that the problem was the threads so I put a healthy serving of grease on them and tried again. The vacuum had improved but I was still not where I wanted to be. So I covered all the joints in epoxy and gave it another try. I was still loosing the slowly loosing the vacuum.

I decided the hand-pump was the problem so I needed to find another mechanism to draw the vacuum with that would be constantly taking out the air vs. in cycles. Using the old embalming pump from the 30's I had, removed the pump’s wheel and attached a drill to the axle.

I set my altimeter bay into the chamber, threaded the wires to the outside, and hot glued the hole shut (with the wires now passing through it) shut.

On the low speed setting of the drill, counting to 10 seconds I was able to draw a significant vacuum. Once I stopped, the altimeter read apogee, and at 1200ft the second charge fired. Durring the whole process, the chamber was loosing air from the vacuum and gaining air from the joints, I just happend to have a lot more coming out than in.

In retrospect, I think it would have been easier to buy the vacuums chamber that Adept sells but I had a good time building this thing. For others trying to build a chamber, I would recommend a motorized pump so that the net loss of air is less than the net gain. Also I don’t recommend trying to draw a vacuums out of PVC pipe, because there are numerous warnings that state PVC is, "NOT FOR PRESSURE."

 

[MCS]

FWIW Harbor Frieght currently has a $80 vacuum pump which supposedly draws 75 micron (0.0014 psi)

https://www.harborfreight.com/25-cfm-vacuum-pump-98076.html
coupon code 72954515
good through 12/31


-->MCS

.

 

[billspad]

Also I don’t recommend trying to draw a vacuums out of PVC pipe, because there are numerous warnings that state PVC is, "NOT FOR PRESSURE."

There's nothing to worry about with the pressures you're using. Since it's a negative pressure the worst thing that could happen would be an implosion and you're even close to that happening. PVC drainage piping is never meant for pressure but when it's installed in a house it's tested under pressure. The real danger with PVC is putting it under pressure with a compressed gas. When PVC breaks it shatters and you really don't want to be dodging daggers. But for your purposes there is no danger. And building a vacuum chamber if much more fun than buying one.

 

[user 8716]

For others trying to build a chamber, I would recommend a motorized pump so that the net loss of air is less than the net gain. Also I don’t recommend trying to draw a vacuums out of PVC pipe, because there are numerous warnings that state PVC is, "NOT FOR PRESSURE."

You might want to check out Joe Woodworker and see the vacuum systems they build for veneering. Virtually all the reservoirs are built from PVC. My system isn't anywhere near as sophisticated as the ones they show, but my reservoir is a 3 foot section of 4" PVC, and it holds vacuum just fine. The reservoirs can be easily converted over to a vacuum chamber with a plug instead of a glued on cap.

Usually the biggest problem with vacuum systems is, as you found, leaks. Using appropriate sealing fittings and good sealing materials is a must. Thinwall tubing typically won't hold its shape and will leak around barb type fittings. Plastic fittings can leak around the molding seam, fittings thorough the PVC should be o-ring sealed, although brass NPT fittings with teflon tape will provide an acceptable seal.

Wires running thorough the vacuum chamber wall can be a challenge. And I've never had any success with hot glue on a vacuum system. They need to either go thorough a vacuum feed thru (glass sealed are typical on "real" vacuum chambers) or use sealed wires as the wires themselves will leak air. I've seen people run the wire thorough a barb fitting and section of vinyl tubing, then kink the tube and wire over to seal it. Or forget about all the leaks and just get a big ass pump that can overcome all the leaks in your chamber. FWIW on my system with a typical vacuum bagged 4" body tube I can get the bag and connections tight enough that the pump only runs every 4-5 minutes, reasonable but still somewhat leaky.

 

[cvanc]

the old embalming pump from the 30's I had


'Scuse me?!?

(hey, somebody had to ask)