Well today I conducted an experiment to try and determine the nature of the issue I'm having with the plastic sight "gauge" (Polar Ware doesn't call it a Sight Glass) cracking and to find out if It's possible to boil 11 gallons of liquid and NOT allow the heat at the sight gauge to exceed the 285 degree warning posted by Polar Ware.
First, I trimmed the shield I installed under the sight gauge so I was only shielding the sight gauge and about a half inch on both sides. I wanted to eliminate, or minimize, the possibility of ununiform metal expansion being the cause - or contributor - to my sight gauge cracking.
Next, I calculated that I would need slightly less than 50% of my 72,000 BTU burner to heat the liquid to boiling point. I figured 35,000 BTU should be sufficient.
So, I configured a makeshift plug over the lower sight gauge hole that allowed me to fill the kettle without leaking. Mission accomplished. I filled the kettle to approximately 11 to 11-1/2 gallons of water at 101 degrees F, fired up the burner and began measuring temperature at various locations and my finding were quite remarkable.
I decided to start my test at an even lower BTU output just to see how the kettle and burner would perform. The temperature directly above the sight gauge shield was averaging 145 degrees F and remained steady throughout the entire 80 minute test. However, the "spill-by" temperature around the remainder of the kettle averaged over 485 degrees F. My digital thermometer didn't go any higher.
The results of my boil: The water temperature raised 38 degrees in the first 20 minutes, 27 degrees in the second 20 minutes after I dialed the head down a bit to approximately 1/3 capacity, 24 degrees in the next 20 minutes and only 17 degrees in the final 20 minutes. Now, this test was conducted with the lid on. I was finally able to achieve a boil after 1 hour and 20 minutes @ roughly 33% burner capacity. However, when I removed the lid, the boil went away. Put the lid back on and the boil recovered.
So, what I learned is that more than 24,000 BTU's of energy is required to heat 11 gallons of water to boiling point. Also, the energy loss when the lid was removed was equal to the energy used to heat the water. Which is why I lost the boil when the lid was removed.
The most remarkable find was that even with the burner dialed down to 33% capacity (24,000 BTU) the spill-by heat was in excess tha 485 degrees F!!! That's over 200 degrees higher than the manufacturer warns about!!!
So, exactly what is the intended use of this Polar Ware Boil Kettle with a sight "gauge"?? Perhaps they intended for it to be used to make soup.
Bottom line is If you're looking for a boil kettle with a sight glass for brewing beer, look at someone other than Polar Ware.....unless of coarse you intend on making a nice broth or consomme and your heat source is no hotter than a couple candles.