It’s drinking the water!
July 23, 2011 4 Comments
I have a list of life goals, and somewhere on that list is ‘use a drinking bird toy to solve a problem’. I leave it to you to judge what insight this offers into my character.
(This site has moved to http://www.flashingleds.net, go there if you have comments or questions)
I have not yet crossed this item off the list, but naturally in order to get underway I should first at least own a dippy bird. It turns out they can be had from DealExtreme for a very reasonable $4, so I’m now marginally closer to achieving a life goal. I don’t have an application in mind just at the moment; the classic is pushing a button repeatedly or triggering a motion sensor but to my immeasurable displeasure neither of those represent a problem for me right now. Since I haven’t yet identified a clever use, today we’re going to talk about a completely stupid one: a drinking bird as a temperature sensor.
The dippy bird is a popular physics toy, because it demonstrates a lot of interesting thermodynamic principles (actually that’s not entirely true; there’s no such thing as an interesting thermodynamic principle). There is no shortage of explanations online about how the toy works, but to recap:
- The bird consists of two chambers (body and head) connected by a tube; the body is filled with a volatile solvent.
- The head is covered in felt; when the felt is wet the head is cooled through evaporation.
- This lowers the pressure in the head and draws liquid up the neck
- The bird becomes top heavy and tips over, which empties the liquid back into the lower chamber. Cycle restarts.
The traditional glass of water just serves to keep the felt wet; the bird will continue to dip without it until the head dries.
As mentioned, there is a lot of physics happening in this bird, but as an experimental physicist I have no intention of invoking specific evaporation enthalpies or endoreversible heat engines. What I see when I look at this toy is:
Dip period = f(temperature, humidity, wind)
where f is some function containing thermodynamics voodoo. I don’t particularly care about predicting f, I just want to know whether there is a predictable relationship between dipping period and the ambient temperature. This would form the basis of our temperature sensor. I figure I should be able to just take some measurements and work something out empirically.
I set up the bird next to my desk for a few days with some data logging electronics. I’m measuring three quantities: ambient temperature with a DS18B20 temperature probe, relative humidity with an HH10D capacitive sensor and dipping period with a photo-interrupter ripped out of an old CD-ROM drive. (I tacked a piece of business card onto the body to trip the photointerrupter). Since this is indoors, we can eliminate wind as a variable.
Science in action. Humidity sensor not shown
With a glass of water to keep the felt wet, the bird is happy to keep drinking for a very long time. I left this apparatus running for about 2 days, after which I got sick of it distracting me and dismantled it. This length of time constituted something like 5000 drinking cycles. The data is available here if you want to play with it, but allow me to plot it out:
Let me first note that the relative humidity (blue trace) is not calibrated (the I2C on that unit appeared to be broken, so I had to just use typical values), though I would still expect it to capture trends in the humidity.
The temperature (green trace) varies periodically over 24 hours as one might expect, but the overall variation is quite small since this is a climate controlled office (19.5 ± 1) °C.
The dipping period (red trace) is extremely noisy, and over small timescales quite unpredictable. I’ve marked two points of significance on the plot above. At some point I opened the window next to the bird, and with the rapid changes in temperature, airflow and humidity the bird went completely nuts (also reflected in the temperature). The second point happened when I topped up the glass the bird was drinking from, I’m guessing this temporarily spiked the evaporation rate.
Looking at the long-term trends in the data I can see something of a correlation between drinking period and ambient temperature (which is what we wanted), but the humidity doesn’t look like it’s having much of a bearing. To better demonstrate correlations, we can plot variables against each other:
From these we can see that there’s a clear (albeit slightly messy) negative linear relationship between the dipping period and ambient temperature. This is exactly what we would expect; as the temperature goes up the evaporative cooling is enhanced, so the drinking process goes faster. The second plot (against humidity) is a complete mess with no discernible relationship. Not quite what I would have expected, but oh well – for the present purposes it’s a good thing.
To bring things back to the idea of a temperature sensor: by fitting a line to the first plot above we get a formula for converting dipping period into temperature. How good is it?
Pretty good! And by pretty good, I mean “worst case error of 0.5 degrees in an operating range of 2 degrees, with a response time of approximately 15 minutes”.
Which is to say, barely passable. But hey, cut it some slack, it’s based on a frickkin’ dippy bird.
A dippy bird!
Is your heart made of stone?
Incidentally I did try to repeat the data collection at home where lately the temperature has been hanging around 14°C. Evaporation is then so slow that it takes the bird about 10 minutes to drink; so not ideal for a sensor.
Lest you suppose that I am alone in my ambitions, some kind of weirdo grasping desperately to the dream of a drinking bird toy serving a higher purpose, allow me to offer you some perspective from the scientific literature.
- Ralph Lorenz carried out a quite similar experiment to this in 2006, published in the ever entertaining American Journal of Physics (you should read it, it’s a great paper). As we’ve seen in the preceding plots, the dipping period is very erratic on small time scales. Ralph suggested using this as a seed for random number generation
- Ralph also noted that the water in the glass evaporates 10x faster when the bird is drinking it, and hence suggested that drinking birds might be useful as humidifiers.
- Don Rathjen of the Exploratorium discussed hooking up a ratchet to the bird to create a mighty 1μW engine.
- R. Murrow in a 1966 report proposed using dippy birds to irrigate crops near the Nile. I am not capable of making something like that up.
To conclude: I have successfully made a temperature sensor out of a drinking bird, but it’s not a very good one. But let’s not be so negative about it. Let’s summarize this the semiconductor industry way: with an overblown press release from the marketing department.
SOME DUDE ON THE INTERNET LAUNCHES INDUSTRY’S MOST CHARMING TEMPERATURE SENSOR FEATURING BEST-IN-CLASS DIPPY BIRD DENSITY
Jul. 24, 2011 – The FlashingLEDs blog, an industry leader in mildly amusing electronics project documentation today introduced a new model of temperature sensor, the FLED538I-AAF-$XQ12. The product features an innovative drinking bird toy topology in a compact 20 cm3 package. FlashingLEDs’ new sensor is designed for the demanding needs of process monitoring in stable, climate controlled indoor environments.
“FlashingLEDs has one of the industry’s broadest portfolios of stupid crap, and to meet expanding customer needs we are adding this amazing drinking bird transducer concept,” noted Craig, director of Marketing and chief architect of the drivel on FlashingLEDs. “This revolutionary new paradigm of temperature sensing puts more dippy birds in the field than any other technology on the market today, and we feel that this is an important new direction for the industry.”