Adapting a Shop Vac

for Vacuum Hold Down

Based on some recommendations I had seen I purchased a ShopVac in Lowes. It was recommended because of it's capacity and ability to pull a vacuum without burning out it's motor. Most vacuums rely on airflow to cool the motor. Try to pull a vacuum with them and there's no airflow. The motors overheat. Specifically, it's the 5 Gal, 5.5 Hp, 587-24-11 model and it's only available in Lowe's. According to the performance tab on that web page it's capable of pulling 59 inches of water. That translates into 2.13 psi of vacuum.

It comes with a limited number of accessories, so my first problem was adapting it's 1.5inch hose to couple to standard PVC pipe. Along the way I picked up a cheap vacuum gage so I could measure the actual performance in my system

That's a Slip Joint extension, found in the plumbing section,
chosem because it's a friction fit to the inside of the hose.
It's shortened to match the depth of the end of the hose.

The threads are ground off until the end of the slip joint just
fits inside the end of a standard 1 1/2 inch pipe coupling.
Then it's glued in place.

I threaded a barbed hose coupling into a hole in the end of
a piece of 1 1/2 inch pvc pipe. The coupling cuts it's own
threads. A piece of clear poly tubing and some pipe clamps
links the gage to the pipe. A pvc cap terminates the pipe
for testing.

With the Shop Vac connected to my gage, using a clean filter, I got readings of -12 Kpa or 3.5 inches of Hg (mercury). Those translate into about 1.72 psi. Not surprising at the end of a 12 ft hose. Joints were taped with duct tape for testing.

To determine what would happen if there was a leak in the system, I put a number of 1/4 inch holes into the PVC pipe.
Here's what happened to the pressure readings.

# holes kpa inhg psi
0 12 3.5 1.73
2 10 3 1.46
4 8 2.5 1.19
6 7 2.25 1.06
8 6 2 0.93