The Dippy Bird

Meet Bob

I bought my bird from American Science and Surplus. Here is an image from the catalog:-

I see my bird as a red robin; and because he is always bobbin' I call him Bob.

History

The original United States Patent, 2402463, was filed by Miles V Sullivan, 18 June 1946. Here is the patent. You can follow the links to: drawing, description, claims.

How does it work?

Assume that you have your Dippy Bird working; that is, it is periodically dipping its beak into a tumbler of water. If the weather is hot and dry it could be doing so as freqently as once every 10 seconds; under less extreme summer conditions (Melbourne) it could do so every 30 to 40 seconds.

Note that the neck (say 2 cc) of the bird is a thin tube which extends from the head (say 12 cc) into the body (say 25 cc) of the bird.

Now observe closely the sequence of events:-

The bird is almost erect and we see the liquid rising in the neck towards the head.
When the liquid has risen far enough the bird dips its head towards the water.
As the bird dips, the lower end of the neck becomes exposed above the surface of the liquid in the body, and a bubble rises through the neck from the body to the head, thus lowering the level of liquid in the neck, and thus allowing the head to rise again. [During the dipping of the head the beak becomes immersed in the water.]
The bird oscillates several times.
Now go back to 1.

Why does it work?

If you are both observant and curious you should now be asking, "Why does the liquid rise in the neck?" Clearly it must do so because the difference between the gas pressures in the body and head is more than sufficient to support the column of liquid. The gas is vapour from the liquid.

Let's talk about vapour. Run some cold water into a bath, and leave the bathroom closed for some time. When you return you will find it more humid inside the bathroom than outside it. While you have been outside, some of the more energetic of the water molecules have been leaving the water and joining molecules of Oxygen and Nitogen in the air, eventually achieving a balance betweeen water molecules leaving the water and water molecules returning to the water.

The water molecules in the air contribute to the total pressure of air plus water vapour. This contribution, known as the saturation vapour pressure (svp), increases with increase in temperature.

Before the balance is achieved the water suffers a nett loss from its more enegetic molecules; hence the mean energy of its molecules decreases and manifests itself in a drop in temperature of the water. [Think of the canvas water cooler, and the Coolgardie safe.]

Let's return to our question: "Why does the liquid rise in the neck?". Clearly, the vapour pressure in the body must exceed the vapour pressure in the head. What could produce this difference in pressure? The head is, like a Coolgardie safe, constantly losing the more energetic of its water molecules from the moistened cover around the head. Hence the svp in the cooler head is less that the svp in the body. At the moment in the cycle when the bubble of vapor rises through the neck the two pressures equalise, at a value between the two svps.

We can now summarise. We may regard the body as being maintained at ambient room temperature, and the head as being maintained at a lower temperature. At the moment of equalisation of pressures:-

The vapour pressure in the body is less than the svp, and liquid begins transforming to vapour thus increasing the vapour pressure.
The vapour pressure in the head exceeds the svp, and vapour begins condensing to liquid thus decreasing the vapour pressure.
The resulting difference in vapour pressure causes the liquid to rise in the neck.

What is the liquid in the Dippy Bird?

Sir William Thompson, Lord Kelvin (1824-1907), said:-
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced it to the stage of science.

It should be clear from our understanding of the operation of the Dippy Bird that we would prefer a liquid with a high sensitivity of svp with respect to temperature; that is, for a given temperature difference between head and body we want a large difference in svp. We can consult tables of svp for various liquids over the range, say, 20 C to 30 C and discover:-

Liquid	20 C	30 c	dP/dT
Water	17.4 mm Hg	31.5 mm Hg	1.4 mm Hg per C
Methylene Chloride	349 mm Hg	500 mm Hg	15 mm Hg per C
Freon 11			27 mm Hg per C

The table shows clearly that, in the range from 20 C to 30 C, the svp of Methylene Chloride is about 10 times more senstive than that of water to a change in temperature. Further, Freon 11 (trichlorfluoromethane, CCl₃F) is almost twice as sensitive as Methylene Chloride.

The liquid in my Dippy Bird is Methylene Chloride (CH₂Cl₂). Note that a Dippy Bird is very fragile and that Methylene chloride is corrosive (an ingredient of paint strippers).

We can use the above table to estimate the temperature difference between head and body. Observe that the liquid in Dippy Bird is being lifted about 40 mm in each cycle. Use the densities of Methylene Chloride (1.32 gm/cc at 25 C) and Mercury (13.6 gm/cc) to convert mm Hg to mm CH₂Cl₂ and you will arrive at a difference in temperature of about 0.2 C.

Putting the Dippy Bird to work

What power could we get from a Dippy Bird? Suppose that in each cycle some 4 gm of liquid is lifted from the lower neck and body of the bird to the upper neck and head of the bird, thus undergoing a mean change in height of some 6 cm, implying an increase in energy of some 2.4 millijoules. If the period of the cycle is 30 seconds then the rate of work done in lifting the liquid is about 80 microwatts. The Dippy Bird dissipates this potential energy in the oscillations which occur in every cycle.We could hope to use some of this power to produce useful work by coupling the bird to a suitable mechanism.

Don Rathjen, at the Exploratorium, has extracted power of about 1 microwatt from the Dippy Bird by attaching it to a windlass and using it to raise paper clips. Here is a copy of Don Rathjen's plan of his 'duck-powered' engine:-

See Richard B. Murrow's A Simple Heat Engine of Possible Utility in Primitive Environments (Rand Corporation, August 1966).

The Dippy Bird's underlying principle is also that of Wally Minto's Wheel (local copy). Details. See also:-

Quickenden T. I. and Teoh, K. G. "A study of the Minto engine - a liquid piston heat engine for converting low grade heat to mechanical energy" J. Solar Energy Engineering, (accepted for publication 2003).
Wundersames' Minto Wheel.
The Minto Wheel at Maxwells_Demon Yahoo Group

Detailed theory of the Dippy Bird

Begin with an article by Lil M Ng and Yvonne S Ng on The Thermodynamics of the Drinking Bird Toy, Physics Education, 28(1993), 320-324.

Now see an article by J. Gueimez and others on Experiments With the Drinking Bird, American Journal of Physics,71(2003), 1257-1263.

Finally explore the 'waterless' Dippy Bird in an article by J. Gueimez and others on Experiments With a Sunbird, American Journal of Physics,71(2003), 1264-1267.

Dippy Bird on the WWW (selected non-technical pages)

The BBC has an entertaining essay on The Amazing Drinking Bird. It gives some values for latent heat of vapourisation:-

Water = 2250 kJ/kg

Ethanol = 880 kJ/kg

Methylene chloride = 406 kJ/kg

Mercury = 281 kJ/kg
Freon-11, a working substance for refrigeration is used in some birds.

At scitoys.com you can see how to dispense with the water.

The Dingo's Breakfast Club has a comprehensive site for The Happy Drinking Bird . This is perhaps the most comprehensive site - exploring the carnot cycle of the lower chamber, the carnot cycle of the upper chamber, and the chaotic behaviour of the oscillations.

Wundersames has the Trinkenden Ente and other things (including the Minto Wheel).

For a comprehensive guide to variety and availability see backstreet. It also has a good simple explanation.

Maxwells_Demon Yahoo Group gives links to the Dippy Bird and other topics (including Minto's Wheel).

Physics World's The Funny Drinking Bird.

Enter "drinking bird" in google to find more such pages, as I have done.

When the red, red robin comes bob, bob bobbin' along

Here is the musical score of When the red, red robin comes bob, bob bobbin' along.