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The reason for the maximum capacitance spec is to limit stored energy that could cause a spark when that energy is released all at once.  There are tables and equations that show how much capacitance is allowed for any given voltage.

You should therefore use your CS value.  According to your description, it includes all possible capacitance that the cable adds to any conductor.  The conductor to conductor capacitance is already included.  I'm basing this on what you said.  It's your job to read the cable spec carefully to make sure this is really true.

The way to find the maximum allowed cable length is to look at the C<sub>o</sub> parameter of your barrier, and subtract the C<sub>i</sub> parameter of your sensor in the propane tank.  That's how much capacitance the cable can add before violating the IS rules.

For example, let's say your barrier limits its output voltage to 10 V, which means its C<sub>o</sub> spec is probably 2.75 &micro;F.  If your sensor adds 2.0 &micro;F (high for a passive sensor, but possible for a powered sensor), then the cable has a budget of 750 nF max.  That divided by your 200 pF/m spec means the cable can't exceed 3.75 km.  You'll have other issues with a cable that long.