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- #21

OP

Specific Conductivity = Conductivity * Cell Constant (K)

Cell Constant = distance / Area

ASSUMING they use the units:

[µS/cm] = [µS] * Cell Constant

the cell constant needs to have the unit [1/cm]

Calculating the expected resistance in a few conditions:

fresh water | salt water | |

500-2000 µS/cm | 10.000-50.000 µS/cm | |

K = 0.1 cm^-1 | 200 -50 Ohm | 10 - 2 Ohm |

K = 1 cm^-1 | 2000 - 500 Ohm | 100 - 20 Ohm |

K = 10 cm^-1 | 20k - 5k Ohm | 1000 - 200 Ohm |

I can still measure a few hundred Ohm with a 1k reference resistor.

Downside is, of course, that you dump about 1mA current through the water, which sounds dangerous. For humans about 20mA become dangerous, not sure how it is for fish

I will therefore aim at the 1k-100k range.

So we should be looking for probes with cell constants of 10/cm for fresh water or 100/cm for salt water, then you measure something like a few kOhm and dump a maximum of 0.1 mA through the water, which sounds a lot better.

I had a quick try with my audio jack and measured a resistance of several 10kOhm in water with something like 4000µS/cm, which sounds like a cell constant of roughly 40-200. Would be funny if that works out