I use Coopers fermentors not carboys and the probe was originally place on the upper part of where the wort is (above liquid level of cooler).
I started using Starsan for several reasons - using water for a long period of time in the cooler caused some mold looking spots, water gets dirty, smelly. I have a few gallons of Starsan at all times, I test the ph level of it and so far it hasnt lost its strenght. If it does I sometimes add a little bit of it and have been using it for 2+ months.
I've had an infection which I have back traced to the outbreak of moldish looking slime in the cooler. Better safe than sorry.
Old school or the new, doesnt mean a thing if your heart's not true!
Primary: 04/29 Pomegranate Nectarine Apfelwein, 05/23 Petite Saison
Kegs drinking: AHS Midnight Wheaten Stout 03/18 - 04/14 - 04/28,
Kegs conditioning: 01/29 - 04/29 Cranberry Apfelwein, 04/29-05/23 Petite Saison
RIP: 11/07-11/28-01/29-05/04-11/17 Flanders Brown Ale