ZBasic Forum

... is a standing wave on the surface of a resonating body of water - building a wave in a bathtub, for instance, until the water sloshes over the edge. In particular, harbors driven by distant seismic activity and winds are known to have experienced seiches that built waves so high as to leave boats on land around the harbor.

Breakwaters and other devices intended to shelter can instead increase the resonance of a harbor. Design features (like smaller resonators) are specifically constructed in some man-made harbors to control resonances, just like in a music hall. I was surprised to learn that harbors actually have a quantifiable Q-factor just like an LC circuit, and reverberation time just like a room - obvious in retrospect, but it was a revelation to me.

This arose as I one day watched my swimming pool surface slowly rise and fall about a centimeter over many seconds. There was a small breeze but not enough to raise a long wave on a 40' pool, I thought. The long period on the pool appears to be about five seconds or so while the 15' width seems to cycle about twice as quickly and was more complex.

I'm wondering where these waves originate and whether the pool constitutes a useable detector of something. I'm thinking about how and where to sense the level change, perhaps in the corners (the pool is rectangular) and, somehow, in the center. This could probably be done capacitively, resistively (float/pot) or optically. What might you do?

GTBecker wrote:I'm wondering where these waves originate and whether the pool constitutes a useable detector of something

According to Wikipedia, they are caused by wind and/or atmospheric pressure variations interacting with gravity, resulting in standing waves.

How about a ping pong ball with an accelerometer and a Bluetooth transmitter to shore?

JC

DocJC wrote:How about a ping pong ball with an accelerometer and a Bluetooth transmitter to shore?

JC

The problem is to differentiate a standing wave from any other wave or combination of waves. Especially when the standing wave is likely to have a long period and have other waves on top of it.

I guess that this requires fourier analysis of the height of the water. You are looking for a long period peak, but not so long as to be a tidal peak.

An instrumented buoy with high resolution elevation measurements could do it. I don't think an accelerometer will be good enough for long period waves.

If you have the ability to hang an arm over the water, then you could use a toilet bowl float type of arrangement. This would use much less esoteric parts, but might be inconvenient or impossible to do.

If you have a big enough pool, you might be able to detect gravity waves.... Otherwise, I am not sure that a swimming pool would generate interesting data about what is happening in the world.

-Tony

Yes, I agree. An accelerometer won't do it for standing waves, just for traveling waves.

It would seem, also, that a single sensor will also not be sufficient. To detect a Standing Wave you would ideally like to detect its peak and trough, giving you its amplitude and period.

If I was serious about the project I'd probably do it optically, with a camera aimed so as to see the waves against the wall, and an appropriately painted wall to maximize one's ability to distinguish the wave's surface.

If you were to start the project with just a couple of sensors, then floats would be an option, as would ultrasound. I believe I just saw an advert today for mm precision ultrasound modules.

JC

Acoustic sounds good, although the Maxsonar EZ-1 I have ranges from 6 to 254 inches; one inch resolution won't suit this app. Even a 1mm-resolution sensor won't provide much detail in a 1-cm wave.

Capacitive measurement, similar to what I tried in water tank level sensing, should work but the sensor area, a disk or plate above the water, might be an issue since the varying capacitance is quite small.

Optical solutions could include reflecting or refracting a laser at the surface, somehow measuring the resulting spot position on a virtual scale.

Some experimentation required.

GTBecker wrote:

Inspired by the capacitive measurement, I had an idea, and I don't know if it would work.

Instead of having the capacitive traces on the outside of a water tank, could you have them on the inside of an air-filled pipe with the pipe partially submersed in the water? It is sort of an inside-out version of the water tank approach.

Would it have enough resolution to distinguish a low frequency, low amplitude change in water height?

Another approach would be to use two parallel resistive wires dunked in the water. The resistance of the two would be a function of the conductivity of the water and the height of the water. I suppose that both could vary, so you would need a second sensor that is 100% immersed that measures the conductivity alone.

I guess this would not work well in seawater where sea life can grow on the wires as time passes. Maybe Nichrome wires would work OK. I suppose they would not be prone to corrosion (?), but probably algae will be happy to grow on them. Maybe an intermitent electrical pulse through the wires could make sea life unhappy enough to leave the wires alone, without being enough to kill fish and swimmers.

-Tony

You know, in this instance maybe two simple resistive dip wires would work; the chlorinated pool water certainly is conductive. Although there's no marine growth to worry about, electrolysis will likely still be a concern.

I just went out and dipped the probes of a multimeter in the pool. At first touch with the probes about an inch apart, the apparent water resistance [76.5F, 7.65pH, 1760ppm TDS, as it happens] was ~10k; with ~1cm submerged, it was ~7k initially, climbing. At ~2cm submerged the initial resistance was ~5k. If a pair of 1"-spaced, 4"-long wires are half-submerged, I'll guess that the static resistance will be initially something like ~4k - minus maybe 0.5k per centimeter of level change.

In earlier work I learned that electrolysis can quickly coat a submerged probe's surface and effectively insulate it - pretty quickly, in fact; the apparent water resistance can increase before one's eyes if a DC current is passing through the probes, as it is in simple Ohmeters. Using AC to effectively measure the resistance - an AC-coupled squarewave is easy - avoids that.

Hmmm. Might could do that.

GTBecker wrote:You know, in this instance maybe two simple resistive dip wires would work; ... was ~10k; with ~1cm submerged, it was ~7k initially, climbing. At ~2cm submerged the resistance was ~5k. If a pair of 1"-spaced, 4"-long wires are half-submerged, I'll guess that the static resistance will be something like ~4k - minus maybe 0.5k per centimeter of level change.

Actually, on second thought, the wires probably do not need to have any specific resistance. Any wire suitable for prolonged contact with water would probably do. The current (electron flux) across the wires would be proportional to the voltage across the wires, as well as the conductivity of the water and the surface area of the wires.

If you used two identical sets of wires, one set partially submberged and the other fully submerged, then the only variable is the amount of sumberged wire.

By taking the ratio of the resistance of the partially submerged wires divided by the resistance of the fully submerged wires, I think you can determine the amount the partially submerged wires are in the water.

Metal strips might be better than small round wires....

-Tony


ADDENDUM:

Each probe coould get voltage thru a totem pole of transistors. The left and right totem poles would have complementary inputs. In this manner, you can achieve alternating current between the probes. To determine the resistance between the probes, you would have to measure the current running between the probes. There would be little current between the probas and it would be a little hard to measure

Maybe another technique would be to use two transistors in a current mirror arrangement so that there is a constant current between the probes. Then by measuring the voltage between them, the resistance can be inferred.