I use this exact burner and have had the exact same experience. I’ve learned to keep an extra tank on hand, and typically switch out about halfway through the boil, when the first tanks starts to lose momentum. I guess the upside to this arrangement is that between the two tanks, one of which is significantly fuller than the other, I don’t have to worry about running out of fuel during a boil.
To start, I will typically open up the flame pretty generously until I reach a boil and then taper back so that I can maintain a healthy boil while using a minimal amount of fuel. This being said, I’ve also learned to keep an eye on the boil, especially once one of the tanks dips below half full. On several occasions I’ve had to increase the gas flow mid-boil because, for whatever reason, the burner lost it’s ability to maintain the boil, even though the valve was not adjusted down. I’ve also noticed that at some point during a 60 minute boil, the original tank will no longer be able to maintain a boil, regardless of how much the valve is opened. The problem is corrected once I move over to the new tank.
I placed a call to NB a while back to ask about this, but did not get a definitive answer as to why this happens. As I said, my solution has been to keep two tanks on hand, and this has solved the problem, even up to 90 minute boils.
I, too, would be glad to hear from anyone out there who could explain this phenomenon…