What you need:

Assembly step1

Fold straw or tubing in half and fasten.


Straws: The straws are almost 6" long. Pinching it in the middle makes it easier to bend. Wrap a rubber band several times around the ends to hold them together. Make sure the rubber band is at the end.

Tubing: Cut a 6" piece. Fold it in half, making a hard crease. After folding it as much as I can with my hands, I bite it to really make the fold. Otherwise it's too much of a loop to fit into the bottle. Make sure the rubber band is at the end.

Step 2

Add weight to the diver.

Pull the outside end of a paper clip out a bit. Hook the part you bent out onto the rubber band holding the straw or tube together. It is easiest if you hook it to the part of the rubber band in between the straws. The crossed out way is not so good because the paper clip can fall off easily.

It is hard to say how many paper clips you will need. The idea is to get the diver to be almost all the way submerged, but not quite. The tall cup of water comes in handy here. Occasionally, the sharp bending of the straw will have caused an invisible crack. This will allow air to leak out and the diver will sink no matter what. It happens to one or two percent of the kids in groups I work with. We simply make it part of the lesson and use another straw.

Step 3

Fill the bottle with water and test the diver.

Fill the bottle all the way to the top. It is best to avoid extremes in water temperature. Put the diver in. You might have to use your free hand to help get the paper clips past the bottle neck.

Twist the cap on. I once provided some unintentional comic relief when I forgot this during a demonstration. When I squeezed the bottle, the resultant splash caught me right in the face! Ideally, the diver will sink when you squeeze and rise when you let go. Often, however, the diver benefits from some fine tuning. And if the bottle gets droped on the floor, the diver will stay sunk. See below for instructions to get it working.

Cool Variations of Divers

Professor E (aka Paul Eisenzimmer) passed on a great tip. You can replace the lid of the diver bottle with a "fizz keeper," which are available at grocery stores. Ostensibly these tiny air pumps that screw on are supposed to create high pressure inside the bottle so less carbon dioxide fizzes out of solution. However, we know that their highest calling is to pressurize Cartesian divers!

And here is an amazing tip from Eric Knispel, a science teacher at the John Burroughs School in Montana:

"Hi Slater, Wanted to share a favorite demo with you about Divers. After my 10th graders make their own as a lab activity using plastic pipettes (I ask them for 3 different colors diving in a specific order-blue, yellow, no color) I bring out my glass bottle diver."

"Using a glass bottle with flat sides (Italian dressing or whiskey bottle) I fill it completely full of water and add a diver that just barely floats. Squeezing the glass sides will give just enough pressure to flex the glass and sink the diver. It is very sensitive to temperature so on the days I find it already sunk, I squeeze the NARROW sides of the bottle and make the diver float again. (flexing the bottle into an oval increases the volume and reduces the pressure on the diver allowing it to float). The students don't believe it at first, assuming it is the heat from my hands etc."

"The divers are made from disposable pipettes, cut the stem off, glue on a few small metal nuts from the hardware store, fill with water until just floating, seal the ends with glue to keep colored water inside or leave open if no color is desired."

"PS: Mustard and ketchup packets from fast food places are the simplest and easiest divers I have ever come across. Put packet in plastic bottle of water and squeeze!" (editor's note: this is true as long as there is a little air trapped in the packet)

You can see a beautiful activity related to density that Eric has his students do if you click here. It is layers of different concentrations of saline solution (salt and water mixture). Because each concentration is differently colored, it forms a work of art!

And here is spinning variation from engineer and dad Harry Shuttleworth of the U.K. Hoping to have a picture of it soon. I've tried it and it works beautifully! I wish I'd thought of the idea.

"A few years ago I made some cartesian divers for the kids, similar to those also on your site. I also used 2l pet bottles, but I made the diver from the nozzle from a gunge gun (silicone etc), with the tip blocked, and weighted down with a 8mm nut and bolt. To get the effect of a wizards head under the cone "hat" I had dipped the bolt in a bath of hot melt glue to get a nice blob (and drew a face on). I then put small holes through the side of the hat to allow the water in."

"For additional fun, on one of them I set up the holes to be tangential, so it would spin as it pushed the water out when rising."

IF THE DIVER DOES NOT DIVE: Try squeezing the bottle harder. If that doesn't do it, add more paper clips. You can break a paper clip in half (with scissors and/or bending) and really fine tune the weight. If you are using the clear tube instead of the straw, it might get stuck where the bottle narrows at the top. Usually, just shaking the top of the bottle a little solves this problem. If it continues to be a problem, you could add a second tightly wrapped rubber band so the diver is too narrow to get caught anymore.

IF THE DIVER SINKS BUT DOES NOT RISE: Did the bottle drop? If so, see the following paragraph. Try removing one paper clip, or replace it with half a paper clip. As noted above, in rare cases a straw could be cracked and leaking air. Replace.

When the bottle is dropped, it invariably stays sunk. The pressure shock has allowed too much water to get into the diver. You must remove the diver from the bottle and shake out the water. Then it will work again. See step 4 for an easy hook for fishing out divers.

Step 4

Make a hook out of straws paper clips and tape.

Straighten out two paper clips. Place them mostly inside a straw, with just 1/2" sticking out. Now bend the straw and the wire inside the straw, 1/2" from the edge. You have to tape two more straws onto the first to reach to the bottom. Because the water bends the light waves (refraction) it is a bit of a challenge to see exactly where the diver is to hook it. Happy fishing!


Notice that when you squeeze the bottle, the air bubble trapped in the diver compresses and the water level goes up. To explore the Cartesian diver some more, click here.


For explanations, activities and cool links related to Cartesian divers click here or on the picture.

Back to the Cartesian diver introduction page.

Back to the science toymaker home page.

I'd like to know how this project goes for you. I'm happy to answer questions about it. Feedback from you is an important way for me to know what works and what needs clarification.
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