World's Largest Wheatstone Bridge
in [Basics]
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From: Bob Masta on 4 May 2010 08:35 On Mon, 3 May 2010 21:16:15 -0700 (PDT), Bret Cahill <BretCahill(a)peoplepc.com> wrote: >> String a wire back and forth across / along a fault line to measure >> very small displacements in the earth's surface. =A0If the resistance >> and/or tensile strength needs to be higher than a common single alloy >> wire then structural steel cable could be wrapped around a insulated >> wire with a higher resistivity. =A0It could be temperature compensated >> as usual, with another wire of the same length loosely supported >> nearby in another leg of the bridge. >> >> An abandoned power line may be good to go if it is properly located. >> >> Good info sometimes comes in small displacements. >> >> Bret Cahill > > >The problem is just to the East of Laguna Salada: > >http://maps.google.com/maps?f=3Dq&hl=3Den&q=3D32.2006,-115.4625(M3.0+-+Baja= >+California,+Mexico+-+2010+May+04+02:35:27+UTC)&t=3Dh&z=3D9&iwloc=3DA > >String a thin steel wire from Aqua Matias to Cucapa returning with a >larger diameter copper wire with a resistance << than the steel. > >All those << M 1.0 movements can now be documented and studied. Not clear what's the point of the different metals. Do you mean "string" as in suspended in space? Exercise for the student: Compute the stress on the wire just due to gravity. Now imagine you actually got this to (nominally) work. How are you going to allow for the change in load due to wind, rain, and dust? What happens when birds perch on it? As it turns out, they've been using laser interferometry for just this purpose for decades. Best regards, Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!
From: Androcles on 4 May 2010 08:28 "Cwatters" <colin.wattersNOSPAM(a)TurnersOakNOSPAM.plus.com> wrote in message news:cMydnfrSg9jXk33WnZ2dnUVZ8iOdnZ2d(a)brightview.co.uk... > > "Bret Cahill" <BretCahill(a)peoplepc.com> wrote in message > news:120c41b8-59e1-438b-91f0-829eeabeb471(a)a2g2000prd.googlegroups.com... >>> >String a wire back and forth across / along a fault line to measure >>> >very small displacements in the earth's surface. If the resistance >>> >and/or tensile strength needs to be higher than a common single alloy >>> >wire then structural steel cable could be wrapped around a insulated >>> >wire with a higher resistivity. It could be temperature compensated >>> >as usual, with another wire of the same length loosely supported >>> >nearby in another leg of the bridge. >>> >>> >An abandoned power line may be good to go if it is properly located. >>> >>> >Good info sometimes comes in small displacements. >>> >>> >Bret Cahill >>> >>> Differential GPS and laser rangefinders make a lot more sense. >> >>Laser rangefinders can work over dozens to hundreds of miles in a dust >>storm or rainstorm? >> >>To the nearest few microns? >> >>Bret Cahill > > How does string work over hundreds of miles ins a dust storm? > Apparently one nails it down every yard to get dust up one's nose.
From: John Larkin on 4 May 2010 10:09 On Mon, 3 May 2010 21:02:48 -0700 (PDT), Bret Cahill <BretCahill(a)peoplepc.com> wrote: >> >> >> >>> String a wire back and forth across / along a fault line to measure >> >> >> >>> very small displacements in the earth's surface. �If the resistance >> >> >> >>> and/or tensile strength needs to be higher than a common single alloy >> >> >> >>> wire then structural steel cable could be wrapped around a insulated >> >> >> >>> wire with a higher resistivity. �It could be temperature compensated >> >> >> >>> as usual, with another wire of the same length loosely supported >> >> >> >>> nearby in another leg of the bridge. >> >> >> >>> An abandoned power line may be good to go if it is properly located. >> >> >> >>> Good info sometimes comes in small displacements. >> >> >> >>> Bret Cahill >> >> >> >> Two gps stations on both sides do the same trick >> >> >> >> > What's the smallest displacement -- not movement but actual change in >> >> >> > _distance_ between two points -- they can measure? >> >> >> >> > Bret Cahill >> >> >> >> They measure continental drift with them in cm's per year.... >> >> >> >The warning might be in microns. >> >> >> >Bret Cahill >> >> >> What warning? Faults creep all the time. >> >> >At constant speed? >> >> >If that were true all the acceleration measurements published by USGS >> >or Cal Tech on the web in real time would always be zero. >> >> >There may be some characteristic behaviour of certain faults that >> >could be highly reliable early warning info. >> >> >> Knowing the rate of creep has >> >> zero useful predictive value. >> >> >Has this been proven over long distances measuring displacements of a >> >few thousandths of an inch? >> >> >Bret Cahill >> >> Nobody is making useful earthquake predictions. > >That's the problem. > >> It's probably >> impossible. > >That may very well be somewhat true. > >> A superficial surface measurement is obviously >> insufficient to understand an immensely complex and chaotic subsurface >> 3D system. > >Maybe. Probably. > >But why leave any stone unturned when _my_ safety is at stake? > > Good point. Move to Mississippi and live in a tent and you will be in somewhat less terror of earthquakes. John
From: Bret Cahill on 4 May 2010 10:27 > >> >> >>> String a wire back and forth across / along a fault line to measure > >> >> >>> very small displacements in the earth's surface. If the resistance > >> >> >>> and/or tensile strength needs to be higher than a common single alloy > >> >> >>> wire then structural steel cable could be wrapped around a insulated > >> >> >>> wire with a higher resistivity. It could be temperature compensated > >> >> >>> as usual, with another wire of the same length loosely supported > >> >> >>> nearby in another leg of the bridge. > >> >> >>> An abandoned power line may be good to go if it is properly located. > >> >> >>> Good info sometimes comes in small displacements. > >> >> >>> Bret Cahill > >> >> >> Two gps stations on both sides do the same trick > > >> >> > What's the smallest displacement -- not movement but actual change in > >> >> > _distance_ between two points -- they can measure? > > >> >> > Bret Cahill > > >> >> They measure continental drift with them in cm's per year.... > > >> >The warning might be in microns. > > >> >Bret Cahill > > >> What warning? Faults creep all the time. > > >At constant speed? > > >If that were true all the acceleration measurements published by USGS > >or Cal Tech on the web in real time would always be zero. > > >There may be some characteristic behaviour of certain faults that > >could be highly reliable early warning info. > > >> Knowing the rate of creep has zero useful predictive value. > > >Has this been proven over long distances measuring displacements of a > >few thousandths of an inch? > > I would like to add a couple things: > > 1: How do you propose to discern signal of "a few thousandths of an inch" > from noise including whatever harmless-and-usual deviations of-noise-type > from cm-per-year scale plate movement? It's just additional data. No one suggested there is any guarantee it will be any more worthwhile than Al Gore in a dust devil. Motion detectors are good down to a few microns and millihertz but the really low frequency events are lost. > I would worry more about somewhere having fault movement of a few > millimeters within a day transferring stress to "where a big one will > come from". If that does not cause a major earthquake within a day, then > the "big quake" has fair chance of coming 20-30-plus years later. > I would also worry about "a big one" having mere few minutes of advance > warning from significant acceleration of detection of "fault slips", along > with likely a majority of these being "minor burps" as opposed to "the > brown stuff hitting the fan". > > Not that I want to discourage research in this area... The state of the art is allowing a lot of people to get killed. With a couple hours warning you could even save a lot of personal effects if not buildings and infrastructure. Bret Cahill
From: Bret Cahill on 4 May 2010 11:18
> >> String a wire back and forth across / along a fault line to measure > >> very small displacements in the earth's surface. =A0If the resistance > >> and/or tensile strength needs to be higher than a common single alloy > >> wire then structural steel cable could be wrapped around a insulated > >> wire with a higher resistivity. =A0It could be temperature compensated > >> as usual, with another wire of the same length loosely supported > >> nearby in another leg of the bridge. > > >> An abandoned power line may be good to go if it is properly located. > > >> Good info sometimes comes in small displacements. > > >> Bret Cahill > > >The problem is just to the East of Laguna Salada: > > >http://maps.google.com/maps?f=3Dq&hl=3Den&q=3D32.2006,-115.4625(M3..0+... > >+California,+Mexico+-+2010+May+04+02:35:27+UTC)&t=3Dh&z=3D9&iwloc=3DA > > >String a thin steel wire from Aqua Matias to Cucapa returning with a > >larger diameter copper wire with a resistance << than the steel. > > >All those << M 1.0 movements can now be documented and studied. > > Not clear what's the point of the different > metals. The strain gage wire must have some resistance but it'll take a lot of low resistance wire to get all the terminals in one location. No one wonders "where am I going to get 3" of wire with a low enough resistance" to wire up common thumb nail size strain gages. > Do you mean "string" as in suspended in space? > Exercise for the student: Compute the stress on > the wire just due to gravity. Prestress isn't a problem with strain gages. Just stay in the elastic region. Moreover, in this application 4 sig figs might not be necessary. Also decreasing the sag/length ratio of a catenary increases the sensitivity. A small displacement in the horizontal causes a large increase in tension and resistance. > Now imagine you actually got this to (nominally) > work. How are you going to allow for the change > in load due to wind, rain, and dust? What happens > when birds perch on it? They already get down to a few microns in the millihertz range with motion detectors. The goal here is longer warning times meaning small displacements at even lower frequencies or DC velocity. Higher frequency noise can be filtered. If necessary the wires could be under or inside PVC. > As it turns out, they've been using laser > interferometry for just this purpose for decades. The resolution needs to be one part per 100 billion. Bret Cahill |