This sight glass is a specialty item that I made up after getting several requests to do so. It is simply a polycarbonate sight tube threaded into a stainless bushing with 1/2" NPT male threads at the bottom. One use is to make your own thermometer/sight glass setup with your own 1/2" TEE and thermometer. Of course, we also sell the complete setups with thermometers and complete hardware elsewhere on the site. This SB kit comes as part of the LTS and LTSP sight tee kits.
If you already have a 1/2" NPT coupling welded into your kettle (or plan to), this sight glass kit will give you a sight glass and open 1/2" NPT female port in a single thread-in package. You'd get this if you already have a dial thermometer that has at least a 4"-6" probe and 1/2" NPT threads on the back. Another popular use is threading in an RTD or thermocouple for use with an electric brewing system (again 4-6" long probes recommended)
While my weldless kit is the easiest solution for a completely fresh vessel, many brewers have already outfitted their vessels with couplings that have been welded in. The simple solution is a basic street elbow as shown in the picture. This kit has several options depending on what you have on your kettle already.