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I'm working on a project to determine the resistivity of 1,1,1,2-Tetrafluoroethane which is estimated to be around 10^14 ohm-cm. The problem is, the most sensitive commercial conductivity sensors for liquids available only measure up to 10^10 ohm-cm. So I looked into transformer oils where a paper states the resistivity of Tetrachloroethylene to be 5.6 x 10^14 ohm-cm at 20C. I've been trying to find out how these other high resistivity liquids had their resistivities determined but can't find anything online. Was wondering if anyone here knows anything, I feel like i'm missing something like i'm overestimating how difficult this is. Would really appreciate the help, thank you.

J. Doe
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  • Maybe a large volume of the material was used and it had electrode dimensions suitable for producing a resistance or conductance measurement that was in the range of something sensible. Material constants are usually hardly ever made directly but inferred from a practical experiment. – Andy aka Mar 22 '19 at 12:21
  • @Andyaka yh you make some good points. I thought the same, about the need for large volumes to get a small conductivity cell constant but i started doubting myself after my supervisor insisted on using small volumes for the experiment (it's really damaging to the environment so they're scared of using too much). – J. Doe Mar 22 '19 at 13:11
  • @Andyaka when you say "inferred from a practical experiment" do you mean the result is found from extrapolated data? – J. Doe Mar 22 '19 at 13:12
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    This is a bulk resistivity value, if you have a sample between two fairly large area, but not that widely spaced, electrodes, the value of the resistance could be a lot lower. – Phil G Mar 22 '19 at 13:53
  • @PhilG yh its bulk resistivity, sorry forgot to mention that – J. Doe Mar 22 '19 at 22:33
  • How large of a volume would you consider acceptable? 10mL? 1L? – K H Mar 23 '19 at 21:24
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    The more narrowly and precisely you can space your electrodes, and the more voltage you can apply, and the smaller a current you can measure, the less volume of liquid you will need.

    If you can space two 1m x 1m plates exactly 1mm apart, and put a 1000V difference across them, you will need 1L of liquid and you will need to be able to measure 1 uA of current. (This is ignoring the problem of the resistance of whatever mechanism you use to space the plates and/or hold the liquid, which may be a serious practical problem as discussed in the other answer below.)

     – Glenn Willen Mar 23 '19 at 21:33
  • As far as environmental damage goes -- as long as you keep the stuff clean you can maybe reuse/recycle it. Most of this kind of stuff isn't allowed to be dumped anymore, but has to be reclaimed. – Glenn Willen Mar 23 '19 at 21:35
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    Just as a side note - Absolute, fanatical attention to purity and cleanliness will be necessary. You'll need to eliminate all traces of contaminants from your tank and plates. – WhatRoughBeast Mar 23 '19 at 21:59
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    @GlennWillen omg that is so helpful, thank you so much. Thanks to everyone else too. – J. Doe Mar 23 '19 at 23:57

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This is to elaborate on Phil G's comment.

You can measure resistance between two large Plates, keeping the gap between the plates small enough that the total resistance stays within the range of an accurate sensor.

The closer together the plates are, the more misalignment or roughness of the plates will decrease accuracy, so the smaller the volume you want to measure, the greater the machining cost.

The greater the area of the plates, the easier it will be to align them, and the harder they will be to machine. The sides of the device must have extremely high resistivity, preferably orders of magnitude above the material being measured, and I have no idea what would be available to meet that spec, so I'll just mention that because area increases exponentially with plate size and perimeter length increases linearly, the large the plate, the less resistive a border material you need for the same accuracy.

K H
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