They are water resistant, not water proof or water tight.
You will need a thermowell that can be mounted inside a kettle with a o-ring and nut similar to the weldless valves:
http://www.ebay.com/itm/THERMOWELL-...ltDomain_0&hash=item51812d1e6e#ht_2088wt_1064
+
http://www.bargainfittings.com/inde...duct&keyword=less&category_id=0&product_id=98
OR just buy it as a kit:
http://www.bargainfittings.com/index.php?route=product/product&product_id=183
This will allow you to slide the sensor into the thermowell where the sensor is protected.
The alternative is to buy one of these liquid tight sensors:
http://www.auberins.com/index.php?main_page=index&cPath=20_3
FYI
You will need two SSRs for a 3500W system.
I recommend you get a heat sink for each as well.
And the PT-100 sensor that they sell is super accurate, but the leads are very short.
Recommend you buy them from Mixtea, you can email her what you want and she will reply quickly with a custom kit with a huge discount. I now own three units and they work great.
http://stores.ebay.ca/MixTea
Seconded for dealing with Mixtea. Mine turned up in the post today, now I just have to finish work and get home to unwrap it!
mredge are you sure that thermowell will fit the 1/4 UNC thread on the thermocouple? looks too big to me.
TheMiller, I am going through the same issue on this thread https://www.homebrewtalk.com/f170/how-install-cheap-1-4-20-tc-probe-296468/ looks like easiers way is to buy the water-tight probes, my issue is I want to mount it in the side of a 3/4" socket which the small 1/4 UNC thread would be perfect for (if it was sealed)
The other thing is you don't need a second SSR for the other hot leg but it does make it safer, PID off = no power at all to the element. With only 1 SSR you must always remember that the element is still live but you just haven't connected the circuit - Do not let that connection be through yourself!