Basically the sight glass is simply a tube connected near the bottom of the kettle with the top end open to the atmosphere. The liquid in the tube will rise to the same level as the liquid inside the kettle. You can then mark the sight glass tube with volume gradations so that you can tell how much liquid is in the kettle at a glance. There are a number of ways to construct a sight glass. The simplest is to use a weldless fitting and a section of clear or translucenct tubing. A more sophisticated arrangement would use a polycarbonate rigid tube with a compression fitting. You could also have a coupling welded to the keg to make the connection. I don't use one myself as I find it fairly easy just to measure the liquid level with an aluminum yardstick. I use 0.8 gallons per inch above the false bottom. There's 1/2 gallon additional liquid below the FB, but that's a constant and easy to account for. I also know that the seam at the midpoint of the keg is the 7.5 gallon level. This is handy to know during the boil as I frequently do six gallon batches and begin the typical boil with 7.5 gal.
