The sight glass makes it very easy to keep track of how much liquor you have used during the sparge. I used a standard 1/2 inch boiler fill valve shown to the left. This valve and sight glass is used on hot water boilers used to heat buildings.
I didn't really want the valve, just a fitting for the sight glass. Seems they don't exist. What to do? Replace the valve 'guts' with a dial thermometer.
This was simple. Remove the original valve stem. Now you need to find a rubber stopper (cork) with a hole that the thermometer probe will slide through and is small enough to fit inside the valve body. Now slide the backing nut over the thermometer stem, place the rubber stopper over the stem and assemble to the valve body. When you tighten the backing nut it will compress the stopper and form a water tight seal.
It is attatched to the liquor tun with a 1/2 inch coupling welded to the side of the tun as shown. You can also see a thermocouple well that contains the temperature sensor for measuring the liquor temperature. Check the yellow pages under boiler repair for a sight glass parts source.