One interesting aspect of the ones I listed is that you force water through a small test venturi keeping the probe squeaky clean as opposed to a dangling probe in slow moving water. Calcium, coraline, detritus, etc.. can quickly foul a probe in a slow move area. A spa specialist that I met at UT said that they rarely have any troubles with the screw in style probes and rarely replace them. If you look carefully at a lot of "our" probes, they are actually listed as needing to be replaced about every 6 months.