Introduction: SCUBA Tank Bell

I was at an event  recently where someone had the most amazing array of instruments made from recycled stuff.  The one that really captivated me was a hanging set of gas tanks that had been turned into bells, though I suppose you could call them a tuning fork (though they may not be in tune) or a gong.

Essentially a line is cut across the bottom of a gas cylinder continued about 300mm (since imperial people rarely include metric measurements, here it is back at ya) up the sides with the cut being on opposite sides on the way up.

This splits the lower end of the cylinder into two and when given a decent hit with a striker of some sort (wood is best), will ring loud and long and if made with care, in tune.

Since I had a well out of date SCUBA tank lying around I thought it absolutely vital I made one of these and at the same time, my first instructable - please forgive phone camera.

What you need:

An empty gas cylinder - scuba, oxygen etc.  This may work with a round tank but I have only done it with a long cylyndrical one.
Angle grinder and a few 2.5mm cutting discs 
Maybe a hacksaw (with a spare blade)
Dressmaker's tape measure  or builders tape measure and a piece of string
A marking pen
2 saw stools (to use as a jig)
Spirit level (if your work space is a bit uneven)
Chalk line (optional)
Other stuff 

I just noticed something about the video.  When the tank is spun rather than going wow - wow - wow, every time a cut passes, it goes wow wow - wow wow - wow wow  one wow as the cut approaches the front and one as it departs the front, reinforcing the idea that the wow doesn't happen when the cut is right in front of me.  There must be an explanation for this.

 A couple of words of caution that should be obvious, but here they are anyway:  Make sure there is no pressure in the tank you are going to use - open the valve, leave it open, maybe even remove it.  Also, if the tank has been used for any sort of flammable gas, release any pressure, remove the valve and fill it with water (then empty it) before cutting.

Step 1: Keep Things Steady

Working with anything round can be tricky so I made a simple jig to hold the tank horizontally.

I tied a pair of saw stools together with just enough gap so that the tank would sit nicely.

Once you have done that you can roll the tank easily without worrying about it crashing onto your toes.

Step 2: Measure

What we are going to do is make a cut that is a single line across the bottom and then part way up each side.

I am pretty sure it is important that the side cuts are opposite each other.  The best way to do this would be to get a dressmaker's cloth tape measure, mark a point near the bottom, measure the circumference and then mark half way round.

I am not a totally incompetent male, but I could not find the cloth tape measure anywhere and given that my wife is overseas (in an imperial measurement country) I had a good excuse to not have to ask where it was.

Instead I grabbed a piece of string, marked a circumference, folded it in half and marked the middle, then transferred that to the cylinder.

I now had my two opposite side line positions.

Step 3: Measure and Mark Some Lines

First up you need to mark a line vertically from each of your starting points.

Be sure your cylinder is plumb before marking your vertical lines.  My saw stools are not perfectly level and doing it on the ground wasn't going to work as the plumb line needed to extend past the bottom of the cylinder (ideally)

I snapped a chalk line and then used a marking pen to make the line permanent.  If you don't, when cutting you might find all the chalk vibrates off.

Once I had my two lines I marked off 10mm increments to be sure I would cut the same distance up both sides (go ahead Americans, use inches if you must). This was likely overkill, but I was making this up as I went along and wanted to be sure.

As it happened, despite my marking only going 250mm, after experimenting I ended up cutting 300mm.

Step 4: Cutting

Another word about safety before you start cutting.  

WEAR GOGGLES!  In a competition between your eye and a flying hot metal filing, see if you can guess what will win.  

WEAR HEARING PROTECTION.  If you don't you will end up like me, wearing two hearing aids by the time you are 50 years old...granted that was from music, but it is the same effect.

GLOVES are a good idea unless you like the feeling of hot metal filings hitting your wrist.  Next day you will forget and wonder where all those little red spots came from.

OK, go ahead and cut the vertical lines way down to the bottom.

Once you have gotten all the way to the bottom on both sides look how easy it is to draw a line across from one to the other.

Cut some more.

You are likely going to discover that the bottom of a gas cylinder is thick...much thicker than the fact it is very thick.  Unless you have a big grinder with a large diameter disk, you are going to discover that you haven't cut all the way through it.  I started with 100mm disks than then went to 125mm and still didn't make it all the way through right at the bottom of the side but I did manage to burn out my angle grinder.   A hacksaw did the trick finishing those thick bits.

One thing I noticed was that the metal may be under some sort of tension since when I cut through the blade jammed.  A bit of oil on it freed it up.  But I did have to go back and cut it again later as it had closed up on both sides.

Step 5: Try It Out

So, you have cut down the side, across the bottom and up the other side.  You have found it closed up and and needed to re-cut a couple of times and now you are ready to try it out.

My first attempt was just holding it and hitting the bottom against the edge of the concrete slab.  This kind of worked but  it knocked a chip off the edge of the slab and besides that it was awkward and didn't sound all that good.  I suggest tying a rope around the valve and hanging it up - that's going to be the final position so give it a go.

I could tell straight away that my first cut of 150mm was not long enough - it just didn't ring well.  Looking at the length of the cut in proportion to the cylinder  it sort of looked obvious, so I threw caution to the wind and doubled the length.

Much better but still not quite right.

A couple of things I remembered about the original one I saw.  It had a hole at the top of each cut.  After adding those with a 10mm drill it certainly seemed to sound better.

Then I noticed inside that it had quite a lot of filings and swarf stuck in there.  I used hacksaw blade to get them out, hitting every once in a while to vibrate them to the bottom.  If I had taken the valve out I could have saved myself a bit of work.

I then gave the entire outside a light grinding to get rid of all the paint.  I may try to polish it some time in the future.

When you hit it, hit right at the bottom and right on the join for best effect.  I flattened the area where the join meets the bottom to give a visual clue for a target.

How does it sound?  Well the video doesn't do it justice, but it sounds fantastic.  I kid you not when I say it rings for at least 4 minutes!  It is incredible.  Give it a hit and put your hand near the opening, feel the energy. Now hit it, give it a spin and enjoy the wow wow wow effect as the openings pass you.

The couple of people who have seen it so far have been amazed and I am sure you will be too when you make one.

Here's a serious question I don't understand and maybe someone can explain.  When it spins I would expect the wow to be loudest when the openings are directly in front of me, but it seems as though it is loudest about (maybe) a couple of degrees further on.  Check it out in the video in step 1 - as it starts to turn back after winding up it is most obvious.  

As I said I wear hearing aids so perhaps that is making things weird,  but it does seem like a real phenomenon, any ideas people?


valhallas_end made it! (author)2011-05-18

You should look up propane tank hang drums (or Hank drums) on Youtube. They are a different type design where...tabs, I guess is the best word, are cut into an old propane tank and vibrate when hit with mallets or a hand. They're similar to xylophones and the like in that respect - stiff surfaces connected at one end (embedded cantilever-type). You might enjoy making one, and there are lots of fantastic instructions online - I think it would nicely complement this tank instrument.

This is a very cool build, by the way. If I ever come across an old scuba tank, I'd like to build one. My folks' garden is filled with windchimes and gong-type instruments - I've always loved these types of things.

rimar2000 made it! (author)2011-05-18

This is wonderful!

Many (many!) years ago I read that aluminum is the most sonorous metal. At first I can't believe it, but as I had a broken Al bicicle ring (almost a circle, open 10 cm on the break), I did a try: hung it from a branch of a tree by its middle, and hit it with a stick. The sound was very, very strong! After this I thought the alum pans/pots are very noisy, and I think it is truly: Aluminum is the most sonorous metal. But a search in Google today says nothing about the matter.

Maybe you know that better than me, ¿is it so?

waynevanwijk made it! (author)2011-05-18

My thoughts are that the cut is not to let the sound out but to let what is left vibrate more freely. Or in other words, the sound is not coming from the cut out section but from the vibrations made by the outside surface of the tank

Without any knowledge of acoustics or engineering I am really only guessing when I suggest the the metal near the cut would be vibrating more than the metal in the centre of each curve.

Is there an engineer type of person out there who can confirm, deny or expand on this? If you confirm my theory as correct I will feel very clever. If you show how wrong I am I will be almost as happy to be learning something new.

Microbe made it! (author)Microbe2011-05-18

likely. Maybe widening the gap gave it a bit more space to vibrate, though I think I would have heard it buzzing had the two sides been touching.

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