4 ms·

"In theory, practice and theory are the same. In practice, they are not."

CyberDildonics 5y ago
Do you have a reason why lookup tables don't work here or are you regurgitating this quote without evidence that it makes sense in this context?
- deleted 5y ago
  [deleted]
- willis936 5y ago
  The lookup tables were made based off of power lines in air. So much effort has been spent studying the effects but presumably no one has actually checked the magnitude of the magnetic fields around submarine power lines.
  - idealmedtech 5y ago
    I was not referring to the lookup tables in the appendix, rather known lookup tables for relative permittivity and permeability in salt water at 60Hz. If you know those relative parameters you can simply adjust the air-based calculation by multiplying by some function of the ratio of the water parameter to the air parameter (effectively cancelling out). I'm no expert though, only have some high level undergraduate fields and waves course work.
    - willis936 5y ago
      Which would be fine to do, if it was done. The relatively permeability is implicitly assumed to be 1 in the references that even mention it (17). Maybe that's a reasonable assumption, but is it? It at least warrants discussion in one of these papers.
      - jbay808 5y ago
        Have you ever tried to pick up seawater with an electromagnet?
        willis936 5y ago
        Is it really so trivial? Water is diamagnetic. Salt water is conductive. All I expect is a "yes we thought about it" in the form of measurements of ocean floor salt water magnetic permeability. If the models prove too much they could always drop a magnetometer off a boat.
        jbay808 5y ago
        I just don't understand why you think this matters. Even if the relative permeability were as high as, say, 2, it would only move the 500 uT effect radius by some tens of centimetres. But just from common sense, it must be several orders of magnitude less than that, because otherwise your compass wouldn't work near the ocean. In practice, you need a very sensitive instrument to even measure the relative permeability of water, which already means that it's completely negligible for the purposes of this discussion. Wikipedia quotes water's permeability as 0.999992. By that measure, an eighty-times stronger argument against the researchers' due diligence would be "how do they know the cable isn't actually carrying 1599 amps? Did they measure the current?"
        monocasa 5y ago
        Considering running ADSL over wet string didn't work until the water was salted there's obvious EMF considerations at play. https://www.revk.uk/2017/12/its-official-adsl-works-over-wet-string.html https://www.revk.uk/2017/12/its-official-adsl-works-over-wet...
        jbay808 5y ago
        I'm pretty sure that's due to saltwater's conductivity, not any EMF considerations. Conductivity will result in some partial cancellation of the AC magnetic field. But not enough to matter at 60 Hz, which is within the Extremely Low Frequency band used for communications with submarines. https://en.wikipedia.org/wiki/Communication_with_submarines https://en.wikipedia.org/wiki/Communication_with_submarines

Do you have a reason why lookup tables don't work here or are you regurgitating this quote without evidence that it makes sense in this context?

deleted 5y ago
[deleted]
willis936 5y ago
The lookup tables were made based off of power lines in air. So much effort has been spent studying the effects but presumably no one has actually checked the magnitude of the magnetic fields around submarine power lines.
- idealmedtech 5y ago
  I was not referring to the lookup tables in the appendix, rather known lookup tables for relative permittivity and permeability in salt water at 60Hz. If you know those relative parameters you can simply adjust the air-based calculation by multiplying by some function of the ratio of the water parameter to the air parameter (effectively cancelling out). I'm no expert though, only have some high level undergraduate fields and waves course work.
  - willis936 5y ago
    Which would be fine to do, if it was done. The relatively permeability is implicitly assumed to be 1 in the references that even mention it (17). Maybe that's a reasonable assumption, but is it? It at least warrants discussion in one of these papers.
    - jbay808 5y ago
      Have you ever tried to pick up seawater with an electromagnet?
      - willis936 5y ago
        Is it really so trivial? Water is diamagnetic. Salt water is conductive. All I expect is a "yes we thought about it" in the form of measurements of ocean floor salt water magnetic permeability. If the models prove too much they could always drop a magnetometer off a boat.
        jbay808 5y ago
        I just don't understand why you think this matters. Even if the relative permeability were as high as, say, 2, it would only move the 500 uT effect radius by some tens of centimetres. But just from common sense, it must be several orders of magnitude less than that, because otherwise your compass wouldn't work near the ocean. In practice, you need a very sensitive instrument to even measure the relative permeability of water, which already means that it's completely negligible for the purposes of this discussion. Wikipedia quotes water's permeability as 0.999992. By that measure, an eighty-times stronger argument against the researchers' due diligence would be "how do they know the cable isn't actually carrying 1599 amps? Did they measure the current?"
      - monocasa 5y ago
        Considering running ADSL over wet string didn't work until the water was salted there's obvious EMF considerations at play. https://www.revk.uk/2017/12/its-official-adsl-works-over-wet-string.html https://www.revk.uk/2017/12/its-official-adsl-works-over-wet...
        jbay808 5y ago
        I'm pretty sure that's due to saltwater's conductivity, not any EMF considerations. Conductivity will result in some partial cancellation of the AC magnetic field. But not enough to matter at 60 Hz, which is within the Extremely Low Frequency band used for communications with submarines. https://en.wikipedia.org/wiki/Communication_with_submarines https://en.wikipedia.org/wiki/Communication_with_submarines