Simple Rubens' Tube

Intro

A Rubens' tube lets you visualize music in fire. What could be cooler? Often used a tool for teaching physics, this propane powered fire display can make an eye-catching piece for your next chamber music concert or house party.

A few things to know before playing with fire:

• Leaks are bad. The main thing we try to avoid in the design of this Rubens tube is to let propane out any place we don't want to let it out. It should only be exiting through the holes on the top of the tube, nowhere else.

• This is a low pressure system. That means it runs at atmospheric pressure. DO NOT USE ANY PART OF THIS SYSTEM with high pressure propane, and do not attach an accumulator!

• Do not replace any parts in this design unless you know what you're doing.

• DO NOT USE RUBBER or PLASTIC parts. Rubber degrades with propane. That is why we use a nitrile glove on the end instead of a rubber glove, and vinyl electrical tape instead of rubber electrical hose. We also have to use gas rated hose - any old rubber or vinyl hose won't do.

• A regulator is extremely important and legally required. Do not remove any of the valves from the design. In fact, if you do modify the design, you might consider adding an extra ball valve closer to the propane source.

• HAVE A FIRE EXTINGUISHER ON HAND WHEN USING THIS DEVICE.

• Do not leave the Rubens tube unattended. If you see the flames go out, turn the propane off at the source immediately.

Find the parts

If you're like me and you're not a plumber or a mechanical engineer, you probably don't know a close nipple from a bushing, or an iron pipe fitting from a flare fitting. This interactive list of parts with pictures on McMaster Carr's site here was helpful to me to figure out what I needed and how it would fit together.

I recommend drawing a diagram. Here is mine:

Note that even large hardware stores like Home Depot probably won't have all the parts you need. I ended up having to add a few adapter parts because the only needle valve I could find was 1/2 inch. Obviously this is not ideal, because every threaded part you add increases the risk that propane could escape from the threads.

Tip for shopping at Home Depot (note, Home Depot did not sponsor this project, although that would have been great):

Download the Home Depot app and look up your parts in the app before you go. This will give you a handy chart of which aisles you need to visit. Even doing this, it took us over an hour to find all the parts we needed.

Make the tube

Take a piece of tape and run it down the length of the metal tube. You'll notice that there are striations, or seams, that run down the metal. If you follow one of these, you should get a straight line. Starting eight inches from one end, draw a dot every half inch using a ruler and a sharpie. You should end up with 100 or so dots.

You may also want to make an "X" on the side with the 8 extra inches - this will be the side that we connect to our speaker (I mixed this up and almost burnt my speaker)!

Use a hammer and nail or punch to punch a dot where the drill will start.

Using a drillpress with a 1/16th inch drill bit, cut holes in the metal. My friend Ryan had the good idea to set up a jig using on of the adapter mounts that will later become the stand. This keeps the tube from rolling as you drill it. Make sure you keep the metal tube flat as you drill it, otherwise the drillbit will wander. Our holes weren't perfectly in line, but they still looked pretty good.

We actually used a 1/32nd inch drill bit, and the holes ended up a little too small. If you make them too big, you can always run some foil tape down the tube and try varying the width by punching holes in the tape with a pin or pen tip.

Make the stand

The stand consists of two foot long 2"x4"s, each with two adjustable leveling feet in the bottom and an adjustable wood adapter to hold the metal tube on each side.

The wood adapters have two crown bolts that come up from underneath and prevent the adapter from sitting flush with with the board

Mark where they would sit and drill two shallow, 1/2" holes (or however wide the end of your crown bolts are) in the top of the board, 3 1/2" apart, so that the adapter can sit flush with the board.

Use two 2 1/2" screws to screw each wood adapter onto the edge of the board. I just screwed the top part.

Screw the adapters on over the fence post/tube until the tube is held tightly against the wood.

Connect the threaded parts

First connect your parts in the order that you want them without tape, to make sure that they all fit together.

I am using a half inch needle valve, since I couldn't find a quarter inch on in any of the local hardware stores.

Using pipe thread sealant tape

Taping the threaded parts well is extremely important, since this is what prevents gas leaks.

Hold the beginning of the tape with your thumb as you wrap it, trying to stretch the tape tightly over threading of the connectors. You'll want one side of the tape to *just* overlap the end of the nipple, but not overlap the opening of the nipple.

Tape three times around.

You always want the tape going the opposite direction as the direction you're screwing. So if you're screwing a male fitting clockwise into a female fitting, run the tape counter clockwise around the male fitting so that the tape's 'tail' overlaps the other tape and holds it on as you screw.

Use two wrenches. One is to hold the part in place that you're tightening to. The other is to turn the part that you're tightening. Tighten until you can't go any tighter. If you see a lot of yellow tape coming out of the female part, the tape has gotten scrunched up. You may need to redo your tape and try again.

If you mess up, remove any old tape before re-taping.

Take your time and get this really tight and well done. If you test it and there are leaks, you'll have to retape it.

Connect the hose to the barb valves

Never use barb valves on high pressure propane projects! We're only using barb valves here as this project runs on a low pressure system.

Slide your hose clamps onto your hose. Push the barbed end of the barb valves into the hose. Run your hottest tap water over the end your hose (if your tap water doesn't get more than lukewarm, you may want to heat up some water until it hurts to touch).

The hot water will expand the hose and make it easier to push the barb valves in. Using a flathead screwdriver, tighten the hose clamps clamps over the barb valves while the hose is still hot. It will be nearly impossible to get the hose out once you've done this.

Cut the cap

Turn your cap upside down. Ours had some letters inside, so it was easy to see where the exact center of the cap was based on the lettering. Setting it on the drillpress, the hole below the bit provided a place for it to rest and it settled directly into the center. Using the drillpress, hold the cap tightly with a gloved hand and drill a pilot hole using a 1/16th inch drill bit. Now switch drill bits and drill a quarter inch hole. Finally, finish by drilling a 1/2 inch hole.

By gradually going up in size, you'll prevent too much torque from ripping the cap out of your hand.

Try 'screwing' the 1/4" MIP side of the barb valve into the cap. If it fits, great! If not, try using a reamer to slightly increase the width. You can use it on a drill if you go very slowly, or try using it by hand.

Epoxy the parts

Using alcohol and a cloth, clean the cap and the nipple. This is an important step, as it removes any grease from the metal so that the epoxy will stick correctly.

Mix a tiny bit of quick-dry JB Weld on a piece of old cardboard with a toothpick until it turns a nice dark grey-brown color. The quick dry kind sets in six minutes, and by three minutes you can't spread it, so only mix what you need right now. You don't need to use very much. Make sure that the area between the cap and the nipple is completely covered on both sides.

Connect the parts to the tube

Add the nitrile glove to one end of the tube and clamp it on. Make it tight, but not drum tight, as you'll want the sounds waves to be able to travel. Also, be careful not to make a hole in the glove when you connect it.

Run vinyl (not rubber - propane degrades rubber!) tape around the other end of the tube. Do four turns, starting about an 2 inches in, and ending on the edge of the tube. Place the cap over the tape on the end, and run at least 3 turns around the cap and the tube, sealing the cap to the tube.

Test the Rubens tube

Fill a spray bottle with a cup of water and half a teaspoon of detergent. Shake it up until it bubbles.

Screw the regulator onto the propane tank. Open the propane.

Spray the area before the ball valve with the soapy water solution and wait a minute. If you don't see any bubbles, open the ball valve and spray the area between the ball valve and the needle valve with soapy water. Spray the area in front of the needle valve with soapy water and open the needle valve. Spray the taped cap as well.

If you see any bubbles, immediately turn off the propane from the tank.

Unscrew and retape any connections where bubble showed up. DO NOT USE YOUR RUBENS TUBE UNTIL THERE ARE NO BUBBLES!

Note: any bubbles will be pretty obvious. We didn't have the regulator connected to our flare fitting tight enough on the first try, and we got some bubbles.

Connect the speaker

Connect your speaker to the end of your tube that has the nitrile glove. Wrap the adhesive weather stripping from the glove to the speaker cone. Depending on the speaker cone size, you may need to use a funnel or create your own paper maché cone to attach it.

Since this is Hackster, we're using a piezo speaker attached to an arduino to generate tones. You can also add a normal speaker. I even tried hooking up my electric violin. I found that using low notes, like those from a subwoofer, had the most dramatic effect.

Anyway, I made a basic circuit to test this using a rotary potentiometer that controls a buzzer. As you turn the pot, it changes the tone so you can test different tones and see how they affect the fire.

Here's the schematic:

Here's the basic code:

byte potPin = A0; 
int val = 0; 
void setup() { 
   Serial.begin(9600);   
} 
void loop() { 
  val = analogRead(potPin); 
   Serial.println(val); 
 if (val > 0) {     
   tone(8,val/30); 
 }  
 else { 
   noTone(8); 
 } 
 delay(500); 
}  

Fire it up!

The Rubens tube can produce dangerous carbon monoxide, so only use it outdoors. It works best in no wind.

Open the propane first. Open the needle valve all the way. Finally, hold a lighter over the first hole in the tube and open the ball valve. The fire should spread all the way down the tube on its own. If it doesn't, use your lighter to light the other holes. Your holes may be a bit too far apart or too small. Close the needle valve partially until the flame is about one inch tall along the tube.

If the needle valve is all the way open and the flame still hasn't ignited for more than 90 seconds, close everything and re-assess.

Check to see if your nitrile glove has a hole in it, and make sure your needle valve is working.

Calibrating

Things to test:

• Distance of the speaker to the end of the tube

• Width of the holes (try running aluminum tape over the holes and punching new holes with a pin)

• Volume

• Pitch of the notes

• Tightness of the nitrile glove

More videos of the Rubens tube in action coming soon!