tiny hotdog compressor doubles as a vacuum source?

[Bob Hammond]

Hi, the institution where I work was selling off items that weren't needed, and they offered this battleship quality hotdog 2 gal. compressor (continuous duty 1.25 CFM @ 90 PSI, tank is 17l x 6d ") for $15. I didn't argue the price.

The manufacturer states that this type of diaphragm pump is suitable for both vacuum and compression work. I was thinking about fitting a Tee onto the intake port, with one arm going to the vacuum line, and the other arm to a vacuum sensing switch.
In operation to convert from compression to vacuum operation, I would open the tank's exhaust and bleeder valves fully, and then plug the power cord into a receptacle box that also contained the vacuum sensing switch.

Is this a reasonable idea, and where would I find a suitable vacuum switch for the 1/3HP motor? Are air filters a good idea to protect the valves?

[David King]

Bob,
Check the specs on the Gast website. You might find that it's limited in the vacuum generated and won't pull what you need for what you were hoping to do with it.
Joe Woodworker has a decent vac switch that can adjust from 1.5 to 28"Hg. I have one and it's never given any trouble even on my 220V rotary vane Gast pump.
http://www.veneersupplies.com/products/ ... 8-NPT.html

[Bob Hammond]

Thanks, I'll check the sources that you mention. I guess that there are reasons why pumps are built primarily for compression or vacuum work, but not necessarily for both. But I know that Gast makes high-quality products, and they support them with useful documentation and parts, going back for many years.

Even if I can't use it for vacuum work, I think this little beauty will fit neatly under/in a yet-to-be-built workbench, and be good for dust clearance and for a brad-or-bigger- nailer for birdhouses.

[Bob Hammond]

This is going to work out very well. Vacuum clamping of bridges should be easy, and maybe plate braces too. And I'' give a whirl at vacuum chucking on the lathe too.

A reading of the joewoodworker and Gast websites shows that the Gast pump on this compressor is the single cylinder version of the twin-cylinder pump that joewoodworker sells, and so it is rated at 2.5 cfm for vacuum work instead of 5.5 cfm. So it will take a little (twice?) longer to pull the vacuum. To set it up, I only need to install the vacuum sensing switch and wire it between the positive pressure switch and the cord's plug.

In vacuum mode, I'll open the tank bleeder petcock and connect the vacuum line to the pump inlet port. The vacuum switch will allow electric current to flow through to the positive pressure switch until the vacuum (negative) pressure is achieved. Since the the tank bleeder petcock is open, the tank pressure will never reach the cutoff pressure of the positive pressure switch and so electric current will flow to the motor until the vacuum switch cuts out.

In compressor mode, the tank's bleeder petcock will be closed, and the pump inlet port will be left open to air. The vacuum switch will never reach the negative cutoff pressure, and the positive pressure switch will operate normally.

And it'll all fit easily under the workbench, and is very quiet in operation. Nifty!

[Jason Rodgers]

Fifteen bucks?!

[David King]

Bob,
What's the maximum vacuum this pump will develop? The CFM is pretty meaningless unless you are emptying a big tank first. CFMs all drop considerably as you pull over 20"Hg. As long as your clamping gaskets don't leak you'll hardly notice a difference between pumps as the total volume of your hoses and fixtures is tiny anyway.

[Bob Hammond]

David, I measured it at 26.5" Hg with a reliable gauge that was only screwed in finger-tight. And yes, in use it's all about controlling leaks.

Jason, yup -- $15. Big institutions such as a university or hospital have limited space to store excess equipment or furniture. If an item is not requisitioned by another department within 3-6 months, the property office arranges sales before filling dumpsters (which cost money too). If such an institution is near you, then you should find the 'materials management' or 'property" office and give them a call because they don't want to see useful items wasted either. Last summer I got two very nice teak-veneer 30" x 84" bookshelves for $15 each that only needed cleaning and a rubdown with Watco oil. In this deal, I also got a second small diaphragm pump made by Gomco for a bedside aspirator for $5, and I bought just for the vacuum gauge and valve assembly. The Gomco pump will go to my daughter who'll use it for airbrushing.

[Bob Hammond]

Hmm, I just thought of one more part that will be necessary -- a 3-way MAC valve that unloads the motor when it cuts out, since the motor can not start under a load.

A complete list and prices are available at joewoodworker.com

[Jason Rodgers]

Check out the plans on joewoodworker and make sure you have all the right parts. The vacuum controller and MAC are key components, but there is also a check valve in there, and the filter is a good idea, too.

I will check with my local colleges and hospitals for such liquidations. Great tip!

[David King]

Bob,

Don't underestimate that Gomco pump, it will make a much better vacuum pump than this one as it should reach nearly 30"Hg no problem whereas what you have here would make a much better air brush pump in my opinion.

[Bob Hammond]

David, hey, when it came time to cart my treasures home, my parking lot was a block away, and I could only carry one to my car! I'll fetch the Gomco tomorrow and give it a careful workup.

Jason, a lot of what you'll see at these institutional sales is junk. When I went to the sale, I was looking for a crank-up bedside 'over-the-bed' table. I was thinking that it would be good for working on the 383/2bbl engine of a gorgeous totally stock '66 Chrysler Newport (FIL's legacy to me). Under the torsion-bar-supported hood, it's a huge space, and I don't like to lay tools on the engine or fenders where they or I might get lost.

I'd post a picture of the '66 if was allowable ... 2-door hardtop, medium green with jet-black vinyl top and interior, as a good mechanical and very socially respectable daily-driver, and restoreable too. If there was only enough time...

[Jason Rodgers]

After I walked away from that last post, I asked my wife if they have a materials management department at Kaiser Permanente where she works. She says, "Oh yeah, we have an internal website with all the inventory and an annual public sale." Needless to say, I'll be checking that out.

Post your car in the jam. My little bro builds hotrods, and I for one like looking at pretty cars.

[David King]

Hey Jason,

You could have me tag along to the next KP sale possibly?
Freightliner and Techtronics also have regular public blowouts if you're in the right loop to find out about them.

[Bob Hammond]

David, the Gomco pump is actually manufactured by Gast, and the spec sheet predicts 26" Hg vac, with a lower airflow capacity in both compressor and vacuum modes. But it certainly will be useful. It's mounted on a base with a recess for a collection jar. I think I could make a nice little rack-tree for the airbrush handpiece(s) and jars of paint.

Jason, I got wind of an upcoming renovation, and if I can get myself organized and recruit some help, I could score sturdy benchwork and wall cabinets -- probably 50-100 running feet of it in excellent condition. I hope it's towards the end of summer. Maybe I could recruit some college students.