All I ever used was an magnetic mount thermometer with a stem into the stack. While the SS stack did not hold the magnet, the stem stayed in the stack. I only used it to witness the effectiveness of my AUF and AOF on stack temp. Before the AUF AOF my stack temp ran upwards of 1200-1400F at times, after the normal reading was 800-950F,. With this I was boiling much harder, thus it showed that more combustion was in under the pans rather than up the stack. I fueled by the clock, firing every 9 minutes, where as before adding AOF AUF I fired every 7 minutes. In both cases I used a timer. By the way, with the AOF/AUF I gained between 18-25% increase in boil rate, on less wood. It saved me about 35% of the wood.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.