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While browsing for something unrelated, I came upon this notion in an "ask the experts"- type archive, see [link]. Said expert's answer was "I don't know." - which set my halfbaked idea detector a-ringing!
The 'bakery's only explorations into hydrophones seem to have been of the ever-popular piezoelectric
kind. [link2]
A "solid state" hydrophone could be constructed of insulating material, with a thin section allowing entry of the saltwater to serve as conducting media between two electrodes. This could have strong advantage should large arrays of hydrophones be required (as for underwater submarine detection, etc.)
As noted in [link1], the excitation current should be a/c and of fairly high frequency to avoid changing the salinity in question.
Does salt water (not very compressible) change salinity with change in pressure sufficiently? The hydrophone need not have extraordinary signal/noise ratio to be useful...
saltwater hydrophone question
http://www.madsci.o...062077177.Ph.q.html
can you sense a sound wave in (salinated) water by electrical resistance [csea, Nov 22 2004]
Drowning alarm
http://www.halfbake...ea/Drowning_20alarm
nice reference to u/w radio propagation [csea, Nov 22 2004]
The equation of state for sea water
http://stommel.tamu...k1/book/node66.html
[UnaBubba, Nov 22 2004]
Soundpressure level calculator
http://www.sengpiel...ator-soundlevel.htm
ref1 [Ling, Nov 23 2004]
properties of seawater (conductivity)
http://web.deu.edu....toprak/ani4081.html
ref2 [Ling, Nov 23 2004]
Annotation:
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The salt settles to the bottom of the ocean, assuming the water is not constantly disturbed, slowly, as do methane and some other hydrocarbon products, along with manganese, etc. |
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I'm having trouble understanding why the hydrophone needs to work on electrolyte concentration rather than pressure waves. |
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I see that have not been sufficiently clear. It's not a static electrolyte concentration I'm trying to measure, clearly that does change with depth and a host of other variables. |
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What I'm after is an instantaneous (well, maybe just up to a few kHz) increase in salinity as a measure of an acoustic waveform. It need not be large, as it can be effectively amplified electronically. |
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>I'm having trouble understanding why the hydrophone needs to work on electrolyte concentration rather than pressure waves. |
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In a word: simplicity. Conventional hydrophones have to go to great lengths to waterproof and protect sensitive innards. This one would use the surrounding saltwater itself as the transducer. |
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The density of seawater is measured most often by deriving a value from separate measurements of pressure, temperature, and salinity and the use of the International Equation of State of Seawater (an empirically derived formula [link]). Pressure may also be measured using a strain-guage suspended in the water column; before this method was available, pressure was derived by noting the difference in the temperature of 2 thermometers, one of which was exposed to the ambient pressure and one that was not. |
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I don't think the titration of salts and electrolytes would vary sufficiently to make a difference, for your device. Apart from that, wave motion affects rates of dissolution of solids, including electrolytes. It could act as a source of power, using electrolysis, but I can't figure out how you could take advantage of the extremely limited compressibility of water to ensure detection of changes in electrolyte solution, without factoring in turbidity, movement, temperature, localised diffusion differences and a raft of other variables, rendering your equipment very expensive or largely useless. |
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Nor would the salts increase in concentration, if the water was compressed. There would be the same amount of water / plankton / salt / etc., packed into an infinitessimally smaller volume. |
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Add one diver to that recipe....I'd better watch out for your boat. |
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Boat? We don't need no steenking boat! |
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<clipped> Conductivity increases by the same amount with a salinity increase of 0.01, a temperature increase of 0.01°C, and a depth (ie pressure) increase of 20 m.<unclipped>
If a sound wave has a pressure level of, let's say, 60dB then this is the equivalent of 0.000002m difference in depth. But 20m pressure change has the same effect as 0.01C, so 60dB SPL will change the conductivity of seawater the same amount as a 0.000000001C change in temperature.
In conclusion, I'm afraid the noise to signal ratio is impossibly high. |
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[ling]
Thanks, just what I was looking for! |
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Oh, well, another idea bites the salt... |
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Looks like all your statistics are now just "static". I appreciate your effort: it was interesting, for me at least. |
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Thanks, [Ling]. I'm used to blind ends in science, and am learning about similar ones in society! |
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