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This is good use for a scope.

Discussion in 'Electronic Design' started by Jamie, May 20, 2013.

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  1. Jamie

    Jamie Guest

    Although I have done this many times myself, I thought this guy
    did very well in this demo.

    Years ago I had coax going up to the roof and it was in the walls
    to make things look pretty on the out side.. I had a DC short and
    I wasn't going to tear out all the walls to find it. I shot a
    pulse up it, did a rough calculation and found it on the second
    floor. It turns out the electric company had put in new wire on the
    outside and a strap fastener poked into the wire. I got with in
    foot where it was.

    But over the years of doing this I've always question this practice.
    Can we rely on velocity being a constant? I know where I work currently
    we make many different communication cables and one the factors is
    chemistry change in the dialectic with age, especially with foams. This
    also effects the impedance.

    I know recently I made an inquiry here to see if any actually uses that
    method. I was going to implement it in a cable debugger tool but then I
    realized the velocity isn't a constant due to inconsistent geometry.

    Just something to think about I guess.

  2. tm

    tm Guest

    What's really neat and I find amazing is sending an optical pulse down and
    back 50+ miles through a glass fiber. It is amazing how clear that glass is.

    With some fiber, the twist in the bundles adds measurable length to the
    physical cable that one must account for when looking for a fault.

  3. Jamie

    Jamie Guest

    We have HP TDR's at work in the lab, both optical and electrical.
    But at home, I do not have one, although I have been offered one that
    is no longer being used in the lab, maybe I should add that to my
    collection here at home :)

  4. Jamie

    Jamie Guest

    You would think that however, we've tested products that have been in
    storage (samples) from years ago and found the Vfactor to change, due
    to the dialectic aging. It does not happen to all of the compounds used,
    just a couple that had been used for coax cables and such.. With foam,
    the Z changes and some grades will actually shrink in size..

    It gets real bad when you check your cable and all you have is wire
    in the center with flatten braid and nothing but soup between.

  5. Jamie

    Jamie Guest

    Really? I can only assume you are referring to fiber that is still
    sitting on the roll with no body bindings on it?

    You're talking 264K feet of fiber or, half that for reflection time,
    but still. 132k ?

    we run like 1 mile lengths bunched with a bundle of them and a TDR
    shows losses.

  6. tm

    tm Guest

    No, I am saying it is a 50 mile long, installed cable with 216 single mode
    fibers. The OTDR pulse travels 50 miles out and 50 miles back and you can
    see every splice in the return waveform.

    The fiber cable is installed in 2 inch buried conduits with access pull
    boxes every 1000 feet or so.

  7. Jamie

    Jamie Guest

    We have HV bridge fault analyzers that seem to work very well for
    locating shorts and opens.. But looking for a way to do it with out HV
    is always a plus.

    In any case, it can find the defect with in 4-5 feet on lengths up in
    the 10's of thousands of feet! The method is archaic, but it works.
    There is a 10 turn pot that you set to null the meter. This meter is in
    the bridge circuit. You note the reading, then reverse the test leads
    and do this again.. This gives you the % of the real length to find it.

    If you are dealing with an open, the process is the same, except now
    you need to apply HV to form a bridge.

  8. miso

    miso Guest

    There are fiber optic perimeter detection systems using TDR. They can
    sense when a buried cable is driven over or a cable on a fence is jiggled.
  9. Guest

    I believe it read somewhere that the railways use (or work on) using a
    fiber optic
    cable along the rail to detect where trains are on the track by
    measuring the distance to where the fiber is bend by the weight of the

  10. josephkk

    josephkk Guest

    If you have heard of Sprint the telecom carrier? That is a division of
    Southern Pacific Railroad leasing some of their excess FO capacity.

  11. miso

    miso Guest

    Running fiber optic cable along railroad right of way makes sense since
    the railroads seem to have cart blanche on their right of way. I don't
    know if they have an additional fiber optic sensor related to the tracks.

    The power companies have been running fiber in ground wires for years,
    but it is probably dark.

    It would be interesting to know just how much dark fiber there is and
    who owns it, but I suspect such a document doesn't exist.
  12. josephkk

    josephkk Guest

    From my field experience, a lot of it is 80 to 90% dark. There are some
    cases where 50 to 75% is lit though.

  13. Sylvia Else

    Sylvia Else Guest

    I can't see why geometry would have much effect on the overall
    propagation delay. If one's concerned about the wave velocity, one need
    only perform the experiment from both ends. That gives the total
    propagation time for the cable, and thus the proportion of the cable
    traversed to the fault, provided, of course, that there's only one.

  14. Jamie

    Jamie Guest

    It has a lot to do with it.

    Power cables made with materials that are soft tend to not hold their
    wall thickness as they are twisted and bunched. Sending a fast raise
    pulse down the cable results in random readings from one length to
    If it were cables like molded twins or coax, you could then use that
    method, however, even molded twin laying on top of each other on a spool
    can cause that reading to be random.

    Of course we are talking about lengths of 1000 feet or more.

    After I thought of the idea originally, I then remembered that we
    tried experimenting with a TDR on these types of cables and it wasn't
    consistent enough to use. It works great on small signal twisted pairs
    and such.. Those types of cables must maintain their physical structure
    though out.

    So it boils down to using a wheatstone bridge with HV if opens are to
    be found.

  15. Charlie E.

    Charlie E. Guest

    Many years ago, when I was working on toll roads, they were trying to
    make a truck scale using fibers embedded in a rubber strip in the
    road. After spending quite a few bucks (including a test installation
    on the Autobahn) they finally figured out that the environment was too
    variable to get reliable measurements. Temperature, water, snow,
    rubber aging, they all changed the response significantly...

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