Oil Sprengel Vacuum Pump
How-To Video Link:
https://youtu.be/WczS2cb8ppA?si=sLVLvRujqTHz4dag
Greetings, Internet. Today, I would like to show you this vacuum pump I made, and how you can make one yourself.
The Sprengel Pump was invented back in 1865 by German-British chemist Hermann Sprengel, and was used to make the first incandescent light bulbs and vacuum tubes for studying electrons, until the 1900s, when mechanical pumps began to replace them.
It works by using droplets of liquid mercury to catch air particles and push them out a narrow tube.
Cody’s Lab did a good instructional video for how to build one out of glass, but I didn’t want to expose myself or anyone around me to mercury. So I tried making one using vacuum pump oil instead, which is designed to have a very low vapor pressure. I ended up bottoming-out my vacuum gauge with it, and managed to make a couple of simple gas-discharge tubes with it. So now, I would like to show you what I did and walk you through how to make one yourself.
Supplies
Pipe Manifold:
(Note: you can probably replace one of these with a 1/4" x 2" Nipple if you want to avoid having to break and polish the medicine dropper)
A paperclip or piece of wire
Flux, Acid Brush, and Soldering Wire
Locking Pliers
Graphite Block
Stand:
1x 1/2" x 1ft x 2ft Birch Plywood Project Panel
Jigsaw
Clamps
Power Drill
1/8" Drill Bit
3/8" Drill Bit
1-1/4" Hole Saw
Phillips Head Screwdriver
Pump:
4x #6 Washers
Phillips Head Screwdriver
Adjustable Wrench
Soldering Iron
Needle Nose Pliers
Wire Strippers
Oil:
1 x (Cheaper than I used) Vacuum Pump Oil, with an Actual Rating for less than 0.0001 Pa!
Drill Air Vent Hole in Reservoir Plug (Optional)
Take a ¼” brass NPT plug, and drill a hole in it.
The size of the hole isn’t really that important, just so long as air evacuated from the pump can get out through it. I used a 3/32nds drill bit.
Note that this step isn’t strictly necessary, as long as you leave the cap loose when the pump is running, and leaving the cap solid will allow you to seal the pump up so no oil spills out if you need to move it.
When drilling through metal, it’s generally better to go slow so that the bit doesn’t get too hot and starts wearing down. Large chips or curly-cues are a good sign that the bit is sharp and that you’re cutting a lot of material. When you get close to the end, try to go slow and not push the drill too hard, otherwise you might punch through suddenly and snap the drill bit off; I’ve lost more than a few that way.
Pump the drill up and down through the hole a few times to clear the burs from the edges of the hole, and if you want, you can even countersink them to clear the burs and give the hole a nice bevel.
Shorten the Capillary Tube (Alternatives Available)
Next, we’re going to need a capillary tube to drip our oil down into the outlet of the pump, catching residual air molecules in the gap as it goes. I found these little glass medicine droppers were just about perfect, except for one thing: they are just a little bit too long to fit in our pipe manifold, so we’re going to have to shorten it.
(Alternatively, you can just use a longer nipple here, as mentioned above, but I find this good practice for getting your feet wet in glassworking.)
Note about where the edge of the pipe fittings reaches the side of the medicine dropper and make a mark about a ¼” further down. This will create an air-gap for the oil to drip through. Then take a glass cutter tool and carefully score the tube at that mark all the way around. You want to apply just enough pressure that the tool actually cuts into the glass and leaves a mark, but not so hard that you shatter the tube. Try to keep the cut straight as you rotate the tool, but if the ends of the cut don’t quite match up, that’s OK.
When breaking a glass tube along a scoring line like this, what you will want to do is place both thumbs at the scoring line, and try to pull the tube apart at both ends, while simultaneously applying pressure at the thumbs. This creates tension inside the glass that makes sure it will only crack at its weakest points, in this case, along the scoring line. As you can see, the cut is not perfectly straight, but we’ll be fixing that shortly.
Try to do the next few steps somewhere you won’t catch yourself or anything else on fire, and always keep an extinguisher handy. For safety, I like to keep the blowtorch between my two hands when working glass like this, but pretend that there is an imaginary wall between them, where the blowtorch is. As long as my hands never cross that line, they won’t get caught in the flame; one time was more than enough.
Turn on your blowtorch and light it with a striker. Then put the cut end near the blue tip of the flame and roll it around until it starts to glow red or yellow hot. This causes the sharp edges of the glass to melt and become smooth, in a process called “flame polishing.” Note that glass is such a poor conductor of heat, that I can hold even a short piece of glass like this very close to the flame with my bare hand and not get burnt, as long as the glass doesn’t spend too much time in the flame.
Next, we need to flare the end of the glass tube so it won’t fall down the end of the pipe fittings. For that, I like to use an old countersink. Get the end of the glass tube glowing hot again. Then quickly remove it from the flame and gently push the countersink into the tube while it’s still hot, rotating the tube as you go. This presses the glass outward while it is hot. At this point, the flare is conical, like the countersink, but if we heat it again and rub it against a graphite block, we can make it flat, and pretty close to perpendicular with the rest of the medicine dropper. You can probably get away with using something else instead of graphite, but if you’re going to be working glass regularly, I can’t recommend one enough for how handy it is.
Once the medicine dropper has cooled, take a tube of silicone caulking, and line the flared end of the dropper with it. This is the only time I’m going to recommend you use silicone for any part of this project, because silicone isn’t perfectly airtight. However, silicone is the only easy-to-get sealant I could think of that will also be able to handle the high temperatures of upcoming steps. Next, press the caulked medicine dropper into the open end of the main pump assembly, and wipe away the excess silicone. Try to make sure that it doesn’t get into the threads of the pipe fittings.
Next, find a nail, paperclip, or straight piece of wire of appropriate size, and poke it through the end of the medicine dropper and out the other side of the barb fitting, so that the dropper sets in-line with the hole in the barb fitting. This will ensure that the oil droplets fall straight down into the hole, catching air as they go, rather than dribbling down the sides of the fitting.
Leave the silicone to set overnight, and then remove the wire. Make sure you can see straight down the tube and out the other end of the barb fitting.
Solder the Pipe Fittings Together (Alternatives Available)
Next, we’re going to seal up the pipe fittings by soldering them together.
(Note that, if you don't want to solder your pipe fittings together, you can seal up the joints with teflon tape and silicone caulking, but this may result in a lower quality vacuum as most polymers outgas, and oil will tend to leak out through the seam between the reservoir and the needle valve.)
Take an acid brush and brush some flux paste on the end of it. Then paint the flux onto the threads of your pipe fittings. Flux removes the oxide layer from the outside of the pipes so that the solder will stick to and alloy with it. I recommend using water-soluble flux, because it makes cleaning easier later. You only need a thin coating of flux on your pipes. Make sure you paint flux onto both the male and female threads of your pipe fittings.
Apply flux to all of the fittings and screw them together as shown. Get them as tight as you can but don’t worry about getting them too tight, as the solder will fill in the cracks wherever the pipes are loose. You can press the fittings against the table to make sure that all of the fittings sit straight, which will make them easier to install on the backboard later.
Do this for both the main pipe assembly and reservoir pipe assembly, but DON’T apply flux to the plug at the top of the reservoir, as we will want to be able to screw this on and off later.
Now we’re finally ready to solder.
Extend your soldering wire, and bend a hook in the end of it, so it will be easier to apply exactly where you want it. Then light your blowtorch and hold the pipe assembly with a pair of pliers to keep from burning yourself. Apply heat to the pipes until they become hot enough to melt the solder upon contact. This should take a minute or two as the entire assembly heats up. Try to apply the solder at the top of the fitting and with the female fitting downhill from the male, so that gravity pulls the molten solder down and around to the bottom, following the seam between the two fittings and filling it in. Check all around the seam to make sure that the solder has filled it in completely and that there are no gaps where air might get in. Do this for all of the seams between pipe fittings. Extend the soldering wire when it starts to get short, and reposition the pliers when you need to hold the pipe assembly at the other end. Remember to take the pipes out of the flame before applying the solder, otherwise the solder will melt and fall off, getting wasted.
(Note in the pictures that I used a ball valve with nylon gaskets that will start to melt when heat is applied for soldering. This is why I would recommend that you use another needle valve here instead of a ball valve, like I did.)
You’ll also want to keep a careful eye on where you have the flame pointing at this stage, as the heat can easily melt the plexiglass cover of the vacuum gauge if you’re not careful.
Set the pipe assemblies aside, and once they are cool, twist them together and apply solder to the final seam, where the capillary tube is installed.
Use some 90% isopropyl alcohol to try and wash out the worst of the flux residue. Then drain out the fluid and leave the pipes to dry out in a fairly warm place for a day. There must not be any water residue in the pipes when you run the pump, as it will either evaporate and contaminate the vacuum, or form bubbles sinking to the bottom of the oil, which is lighter. I suspect alcohol would be even worse, trying to dissolve the oil in any residual water.
Cut Out the Stand
Now it’s time to build the backboard. I found 12”x24”x1/2” plywood project board to be almost perfect, but you can use almost any old piece of ½” ply for this, as long as it fits the pipe assembly.
For this stand, I’m going to cut about 6” off the bottom of the project board to make a foot that will hold the stand up, but feel free to build yours any way you want.
Take a tape measure and mark on each side of the board where you want to cut it, using a ruler or carpenter’s square to draw a straight line across the board. Just make sure that you leave at least 6” or more below the pipe assembly for the electronics we will install in a little bit.
Next secure your piece of plywood and start cutting along the line with your jigsaw
Next, find the store-cut end of the backboard, to be used as the foot of your stand, and mark the half-way point on it. This should be 6” but they’re never cut perfectly, so measure 6” from both sides and find the center of both of those marks; that’s the real half-way point. Then measure the thickness of your plywood; again, not quite half an inch, and try to mark about half this value on either side of your center line. Next, make a mark above these points half the width of the foot-board you cut off, in my case, about 3”, and draw a rectangle to define the shape of the slot we will cut into the board to fit the two together. Do this for both boards. I’ve learned that it is helpful in carpentry to put hash-marks on the part of the board that you intend to cut out, so you don’t accidentally cut off the wrong part.
Secure your board again and use the jigsaw to cut out the pattern. Try to cut on the inside of the lines rather than the outside to secure a snug fit. And if it’s too tight, you can always trim more material off; you can’t put it back on.
Wait a minute; how are we going to cut that sharp corner? The jigsaw’s blade has a little give to it, so twist the saw to cut an arc towards the center of the pattern on each side, cutting out a bullet-shaped cavity. Then use the jigsaw to carefully trim away the material remaining in the corners. Do this for both boards and slide them together to see how they fit; trim away more material as necessary.
Now, for the literal heart of this machine, I bought a little 6V peristaltic pump from Adafruit. It pumps fluid through small silicone tubes by running a trio of rotating rollers over them and squeezing the fluid through. This will be perfect for our pump because the motor never even needs to actually touch the oil.
Seeing where the tubes will need to reach, pick a good spot on the backboard where the pump can be mounted. If you want to, you can separate the pump from the motor and use it as a stencil to mark where you want to mount the pump.
Next, find a holesaw that is just a little bit too small for the motor to fit inside. (I believe the one I used was 1 ¼”.) Secure the backboard again, and use the holesaw to cut out the hole you stenciled. The drill bit in the middle of the saw makes it much easier to keep the hole centered, but if you don’t have one, you can use the holesaw to cut a guide out of a piece of scrap wood first, and clamp it down over the stencil. Once the hole saw starts cutting into the wood, it helps to rock it around in little circles. And when you’ve cut a little over half way through the board, flip it over and start cutting from the other side using the center drill hole as a guide. This way you won’t tear up the wood when you finally break through.
Next, put the motor in the hole and mark where the holes on the mounting bracket are.
Find a drill bit that fits through the holes on the bracket, (I think I used a 1/8” bit), and drill through the marks you made on the backboard.
While we’re here, let’s mark where we want the on/off switch to go. This isn’t strictly necessary, but it sure is convenient. Place the switch against the backboard and do your best to trace out the edges. Then make some marks a little deeper in, matching the width and height of the backside of the switch. This is the area we will cut out, so that the switch slides into the backboard but catches on the rim.
Next, find a drill bit a little bit smaller than the width of that rectangle, and drill through the middle of it. The bit will tend to drift when you first start drilling, so take it slow and consider making an initial depression in the wood with the tip, a countersink, or smaller drill bit for it to fall into. It helps to clear the chips away so you can make sure you’re drilling inside the lines. If you cut outside the lines though, you can just shift the entire switch hole a little bit to compensate; it’s not a big deal as long as it fits. And again, drill a little more slowly when you start reaching the opposite side, so you don’t tear up the wood too much when you punch through.
Next, use the jigsaw to trim away the corners around the hole we drilled out, the same way we did for the slots in for the stand. Check to make sure the switch fits. If it doesn’t, trim a little bit more of the perimeter away until it does.
Installing the Motor and Electronics
Now let’s install the motor.
You’ll need two 4-40 bolts, matching nuts, and four washers of similar size. Thread the bolts through one washer each and then through the bracket and holes in the backboard. Then do the same for the nuts and remaining washers on the opposite side. (These ¾” long bolts were just barely long enough to catch the nuts, which makes them fiddly to try to put together with my fat fingers. But they were the only ones I could find at my nearest hardware store, so if you can find some that are longer, I’d strongly recommend it.) Use a wrench and screwdriver to tighten them down.
Next we need to add our power supply. The motor is rated for 6V, 500 mA, so I found an AC to DC wall plug on ebay with a matching voltage, and cut and stripped the wires on the opposite end. (Note: The power supply will never give more current than the load asks for, so it’s OK that our power supply is rated for a higher current than we need.)
Now, while the positive and negative terminals on the motor are already marked, it will drive the pump backwards compared to what we want, so we will need to hook up the terminals in reverse; red to black and vice versa. If you aren’t sure, you can just plug in and tap the terminals in each configuration, and see which way the pump flows. And if you get it wrong, you can always swap the hoses later; so it’s no big deal.
Take a hot soldering iron and press the wires to the terminals. It helps if there’s already a little bit of solder on them, and like the pipes, we’re trying to get the terminals and wire hot, rather than the solder itself, so that the solder will wick onto them and stick them together when it solidifies.
To complete the circuit, we will need to connect one of the switch terminals to the motor and the other to the power supply. Find an extra piece of wire, cut it just a little bit longer than you need it to be, and strip the ends. Like I said, it makes things easier if there’s already a little bit of solder on the wires, so get some electronics solder and your soldering iron, and paint a little bit of of solder onto the ends of the wire. The more surface area the better, so flatten out the ends of your wire beforehand. This will make the wire hot, so be careful picking it up when you’re done.
Do the same thing to the terminals on the switch, and then solder them together; the motor to one of the switch terminals, and the remaining switch terminal to the remaining wire on the power supply; doesn’t really matter which, but it helps to avoid short circuits if you don’t cross your wires.
If you’re worried about the electrodes being exposed like this, you can coat them in hot glue when you’re done to insulate them, and staple the power cable to the side of the stand to keep the connections from getting ripped out if you try to move it.
Mounting the Pump to the Backboard
Now it’s time to bring everything together.
Place your pipe manifold where you want it on the backboard. Then get three ½” copper pipe straps and place them where you would like them on the manifold for support. (They don’t have to be copper; you can use something cheaper if you want, but I wanted it to look nice.) Mark the holes in the straps on your backboard and use some short wood screws to hold them down. Find a drill bit that is about as wide as the center of the screws but not wider than the threads. (I think this one is about 7/64" or 1/8" wide.) That way the screw will go in with minimal resistance, but the threads will still bite into the wood and hold everything in place
Drill some pilot holes for the screws where you made the marks from the straps. Replace the pipe assembly and straps and screw them down. (These particular screws have given me trouble in the past, stripping out easily, so I would recommend tightening them with a screwdriver instead of a power drill.) They don’t need to be more than hand-tight, since the idea here is to make the whole assembly easy to remove if you need to troubleshoot it.
Now we’re ready to hook up the pump.
Take the inlet hose and attach it to the barb fitting on the bottom of the pipe assembly. For convenience, you can cut the hose to length so it’s not flopping around or letting air leak in, but make sure that there’s a little slack in it, so you can take the pipe assembly off the backboard and move it around if you need to. Do the same thing with the outlet hose and attach it to the barb fitting near the top of the reservoir. It would also be a good idea to make sure you know which hose is which before you cut them, but like I said, you can swap them later if you need to, since they are connected to the pump with little plastic barbs of their own. It can just get a little messy is all.
Slide the foot into the bottom of the stand, and we’re ready to give this thing a test!
Operating the Pump
Remove the plug at the top of the reservoir, and carefully pour in a little bit of vacuum pump oil. (It’s worth acknowledging at this point that you can’t use cooking oil, motor oil, or any other oil instead, because this particular kind of oil has been engineered to not evaporate at low pressures. It’s also worth mentioning that, if you can’t use traditional mercury in this device either, because mercury will dissolve any other metal except steel.)
Fill until it just reaches the branch in the tee-fitting; you don’t need a lot of it.
Then screw the cap back on to minimize any risk of spilling. (Like I said at the beginning, you can skip the hole and just leave it loose when it’s running so the air has a somewhere to escape.)
Close all the valves, plug it in, and flip the switch!
Once the pump is running, carefully crack open the valve under the reservoir until oil just barely starts dripping through the tube; about 1 drop per second. If the oil flows in too quickly, the reservoir may empty and let air into the manifold. If the oil flows in too slowly, or not at all, the pump won't be able to pull air out of the manifold after a certain point. The greater the vacuum the more the outside air pressure will try to push oil into the manifold, so as the pressure drops, you will need to tighten the valve to keep the oil from flowing in too quickly.
Connect the hose barb at the top of the manifold to whatever you want to evacuate and open the valve on the manifold to start pulling a vacuum on it.
Don't use this pump for sucking other liquids out of a chamber, otherwise they will mix with the vacuum pump oil and contaminate it; gasses only.
Troubleshooting
If the vacuum gauge indicates that pressure is not dropping, or only dropping to a certain point, that means there is probably a leak somewhere.
Note that the needle valves have a nut through which their dials pass, which controls how hard they are to turn. If you have a leak, try tightening these first.
If that doesn't work, carefully remove the manifold and try applying hot glue to all of the seams. Hot glue is ideal for this, because it will get sucked into any leaks while it is melted but will not bond to the manifold once it cools. It is not a permanent solution, however. Watch the vacuum gauge as you do this, to see if the vacuum improves once hot glue is applied to a particular spot.
Once the problem spots are identified, apply a sealant to them and try again; silicone will probably be your best bet, even though it's a little gas-permeable. Other polymer glues or sealants may also work; just be aware that most polymers will outgas in a vacuum and may contribute to small amounts of contamination later.
I would not recommend trying to re-solder those joints after vacuum oil has been introduced to the pump, as any oil residue may catch fire during soldering. If you really want to though, I would recommend first draining all the oil by removing the top reservoir hose from its barb fitting and emptying it into a separate container while the pump is running. It's probably better not to reuse this oil, but replace it, as dust in the air may contaminate it. Once the oil is drained, wash out the manifold with isopropyl alcohol or acetone a few times, and leave it to dry before attempting to solder the manifold again.
If the pressure is still not going down, or going down very slowly, you may be using too big a vacuum chamber. This pump can only evacuate small amounts of air at a time, so it's really only good for small vacuum chambers or vacuum tubes.
If oil isn't flowing through the pump, it might be wired up backwards. Try swapping and re-soldering the wires connected to it, or swapping the inlet and outlet hoses.
Another possibility is that the oil reservoir is plugged up, and air cannot escape, building up pressure inside the reservoir. Make sure that air can pass through the plug at the top of the reservoir.
If the pump won't turn on, there may be a problem with the wiring. Use a multimeter to check for any breaks in the circuit, and re-solder them. If that doesn't work, there may be a problem with your power plug. Use a multimeter to check that it actually outputs the voltage required by the motor and an equal or greater current rating.
What You Can Do With This Pump?
You can use this pump to pull a vacuum on a small chamber or test tube to evacuate it of air.
You can put something inside the chamber or tube to see how it reacts to vacuum.
You can put two high-voltage electrodes on either side of the tube and run a current between them. Depending on the purity of the vacuum you produce, you can ionize the residual air in the tube to create a corona discharge.
You can paint one side of the tube with a fluorescent powder and the electrons that fly past the electrodes will illuminate it, like a Crookes Tube or CRT television screen.
You can put a low voltage electrode between the two high voltage electrodes to cause or prevent electrons from flowing between them, making a vacuum tube transistor.
You can add additional fittings and valves to the end of the manifold and connect them to a tank of a different gas, allowing you to evacuate a chamber of air and then fill it with the gas from your tank.
If you use a gas like argon or neon, and electrify it in this state, you can use them to create gas discharge tubes and/or neon signs.
Etc.