I have a Taylor stainless thermometer that I took apart and installed an insulated, stranded 14ga. auto electric wire on about 20 years ago. Chlorine fades the color in the insulation but it is pretty much a permanent solution. Eventually the insulation will crack, but at pool temps it lasts much longer than it does in an engine, where it is good for at least 30 years. The exposure of copper to the water is infinitesimal (only at the end), and is contained within the thermometer.