Joe, Copper is fine if it is 'inert' like a length of pipework... but as soon as you start applying any sort of voltage to it (even if it's micro-volts) it causes electrolysis which speeds up it's demise. I'm not talking about the total destruction of your probe into a heap of debris at the bottom of your tank... think about it... if you are measuring Capacitance or Resistance, the smallest change of just the surface of your probe makes massive changes in your measurements rendering the probe useless - that can happen after only a few days of service!!