''Stop,`` said the mama fishie, ``or you will get lost''

[austin_dern]

The Underwater World aquarium on Singapore's Sentosa Island, which is a huge tank featuring a plexiglass tube underneath so that you can stand on the moving sidewalk or crowd onto the too narrow fixed portion and stare up underneath rays of various kind that lay on the tube, has put in a new feature. This is as though I didn't already miss the place enough and want to return, despite going on average once a year when I actually lived a very short bus ride away. Well, the new trick is an interesting one: they've put radio frequency identification microchips into some of the fish. When they swim past a sensor, a touch-screen panel displays the species and personal name and whatever other information may be relevant about the fish. That might diminish some of the traditional zoo and aquarium fun of looking at a storybook-type illustration of a creature and trying to locate one that looks convincingly like it, but it would help convey information more efficiently and avoid pointless arguments between people who disagree about which drawing more closely matches the fish that doesn't look quite enough like any of the illustrations. Though deep down I think they did it just for the fun of talking about ``fish and chips''.

The thing that made me suspicious of the news report when I first noticed it was that I didn't think radio waves worked underwater. This is why submarines would have to surface, exposing them to considerable risk, to receive new orders: if they're deep underground the light just can't reach them. But then I remembered, bypassing everything I learned as a physics major, back to high school and that radio waves don't penetrate water well, but can at the right frequencies make it a couple of feet in. And for this application radio waves getting a few feet in is just fine. Until I remembered that I was starting to spin out new flights of fancy thinking of sound-based ways to try the radio frequency identification-style trick, and none of them looked likely to work.

The tagged species according to one article include ``arapaima, alligator gar, flower ray, pacu, redtail catfish, shovelnose catfish, and walking catfish'', and they're considering tagging the sharks. Underwater World marketing manager Peter Chew claims they're the first aquarium to tag exhibit fishes with microchips this way. People can also buy pet toy fish, name them, and put the names in reader range to see the pet names on the displays.

Trivia: On 3 June 1956, a total of 1,095 airplanes took off in the United States, carrying 136,823 passengers. The day was documented by Life magazine. Source: Naked Airport, Alastair Gordon.

Currently Reading: Last And First Men, Olaf Stapledon.

Comments

[lederhosen.livejournal.com]

I didn't think radio waves worked underwater. This is why submarines would have to surface, exposing them to considerable risk, to receive new orders: if they're deep underground the light just can't reach them.

VLF can reach a depth of about 20 metres, and I don't think there's a significant depth limit on ELF transmission - although both these systems have some major drawbacks that would make them useless for gish.

[austin-dern.livejournal.com]

That is a lot deeper than I remembered, although since I was thinking back twenty years and just groping for the qualitative feeling that's not such a bad pick.

[chefmongoose.livejournal.com]

The Ripley's Aquarium in Gatlinburg, Tennessee also has the tank with the plexiglas underneath, and the similar moving sidewalk and just-a-wheelchair-wide non-moving path behind. Given my usual "Eh. A fish." reaction to most aquariums, I'm on the sidewalk.

Well, the new trick is an interesting one: they've put radio frequency identification microchips into some of the fish.

I wonder how small they've gotten those to do that with insignificant harm to the fish. Especially given that they're putting some back into tanks with sharks at some point.

--Chiaroscuro

[austin-dern.livejournal.com]

Good question. I suppose they can't have the tags crazy-glued onto the fish either.

It's a really good aquarium, at least here. Very hard to resist standing still and looking up even as people nudge into you.

[captpackrat.livejournal.com]

Those little loss-prevention tags that set off the alarms when you try to leave the store with something you paid for are magneto-acoustic. They use a magnetic field which causes the tag to vibrate, which generates an ultrasonic tone. The tone is picked up by a receiver which sets off the alarm.

They are dumb tags, however, and provide no data, just "Hey, someone's leaving the store with me!"

I suppose that could be useful to prevent people from smuggling the fish home in their pockets.

[austin-dern.livejournal.com]

I know you think you're making a joke, but there is a fish ``petting zoo'' up top of it, and given a decent sack you could probably make off with something.

[orv.livejournal.com]

I used to know someone who had a radio-controlled submarine that was good to a couple feet, so I can see this working.

Trivia: Water's RF absorbtion peaks at about 22 GHz. A lot of people assume it's at 2.4 GHz, because that's the frequency microwave ovens use, but the heating effect microwaves use is not really direct absorbtion, and at any rate you wouldn't want all the energy to be absorbed by the surface of the food leaving none to penetrate to the center.

[austin-dern.livejournal.com]

I had thought the microwave frequency was set so as to induce a rotation mode, too, rather than vibrating ones, but I have to admit the precise details have been vague in my mind. (I'm also none too sure what a wave guide is exactly except it's important somewhere down the line.)

[orv.livejournal.com]

Yup. The water molecule is highly dipolar and rotates to try to align itself to the constantly-changing electromagnetic field. At least that's my understanding.

A waveguide is essentially just a hollow metal tube. They can be square, rectangular, or circular. Above the cutoff frequency (usually the frequency where the diameter is 1/2 wavelength) it acts like a transmission line, with the signal bouncing off the walls as it propagates. (If this sounds like how a fiber optic cable works, that's because it is.) The loss is lower than for coaxial cable because there's no dielectric. Coax gets impractically lossy at microwave frequencies -- you can easily lose half the input power in just a dozen feet of cable.

Antennas for waveguides are almost comically simple. The most common are horn antennas which are just a section of a cone or pyramid that matches the waveguide's impedance to the impedance of free space. Often this is used at the focal point of a dish to feed it. Sometimes the end of the waveguide is just left open to form an antenna. The popular "Pringle can" and "juice can" WiFi antenna designs are just short waveguides with one open end.

[austin-dern.livejournal.com]

Maybe that's what's throwing me, in that it seems like a wave guide ought to be more than just ... you know ... whatever it is you happen to have around. It doesn't seem like there's anything to do anything. Maybe I need some experience playing with them with the 350-experiments-in-one kits .