The best replacement for conductivity is temperature.

The best alternatives to conductivity are rated as following:
★★★★★  temperature
★★★★  resistivity
★★★★  tds
★★★★  concentration
★★★  ph
★★  salinity
★★  conductance
★  specific conductivity
★  specific conductance
★  permeability
★  hardness
★  rushing streams
  resistivity water
  resistance
  total dissolved solids
  resistivity conversion
  salinity of water
  orp
  conductivity
  thermal diffusivity
  ppm
  solution concentration
  copper
  salt concentration
  nacl
  chloride

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Speaking about conductivity, one probably shouldn't forget about metals.

Other search terms related to conductivity and their ratings are:
★★★★★ metals
★★★ fluids
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conductivity
temperature
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