World's Largest Wheatstone Bridge
in [Basics]
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From: Bret Cahill on 3 May 2010 22:39 > >>> 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
From: Michael A. Terrell on 3 May 2010 22:41 "krw(a)att.bizzzzzzzzzzzz" wrote: > > On Mon, 03 May 2010 17:29:32 -0700, John Larkin > <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > >On Mon, 3 May 2010 09:46:39 -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 > >> > > > >Differential GPS and laser rangefinders make a lot more sense. > > Sense? You're talking to Cahill, here. AKA: The world's biggest loaded dummy. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
From: Bret Cahill on 3 May 2010 22:48 > > 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 > > What you are trying to make is a strain gauge. This scheme wont work because > the gauge has to be attached to the substrate along it's whole length not > just strung up like a power line. But attaching it to the earth in any > meaningful way over distance would be next to impossible. Tie it down every few feet. > Yes, you can > compensate for the temperature coefficient of resistance but how do you > compensate for the change in length due to temperature, coefficient of > expansion, a very different animal. If you know the coefficient of expansion then you can correct for it, either mechanically or later in the calculations. > I can't see a strain gauge being a > solution for seismic motions. Is there any other way to measure very low frequency / very small displacements? > The best way is a laser interferometer for small displacements and GPS or > gross measurements. Only two points need to be attached to the earth with > these schemes. What's the accuracy? Bret Cahill
From: John Larkin on 3 May 2010 22:52 On Mon, 3 May 2010 19:39:58 -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. Knowing the rate of creep has zero useful predictive value. John
From: Bret Cahill on 3 May 2010 22:51
> >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 |