Positioning Stack Temperature Monitoring Probe on Wood-fired Arch

We have changed to a forced air arch with an air tight door and we would like to be able to monitor and adjust the firing process bit more closely. I have reviewed a lot of the posts on monitoring stack temperature on wood fired evaporators and there is good information on temperatures. I would like to get a better idea of where a probe should be mounted on the stack. How important is the location? Woodstoves using dial-type thermometers generally say on the stove pipe, 18" above the stove. So before I starting drilling holes in a new stainless steel stack, it would be good to hear from some other producers. The base of the stack is 3' wide, tapering to a 14" diameter round stack. My plan is to mount the 6" type K thermocouple probe at the point where the round stack meets the base, thinking that this would be the most consistent point to check temperatures. This would be about 3' above the top of the steel plates and rails which support the pans on the top of the arch. Appreciate any comments on this.

Barkley,
Sounds like you have a plan. I think your location would be OK. I have one of the wood stove magnet type and have it (or will have it mounted) above my rear hood so I can see it when I am firing. It is about 5 feet above the base base of the pans.
I like to think of these as indicators only. My old pans ran about 600-700 degrees F.
The new pans and old stack ran around 400 F. Now I will see what my rebuilt arch, new stack and new thermometer will say when we fire it this spring.
They are indicators that you need more wood or are too hot. I did sometimes stop firing on my old pans when the temp got too high.
Its just another thing to keep a eye on and learn more about how your rig runs!
Did you have a probe before you went to forced air? That would have been a good reference.
Regards,
Chris

Thanks Chris for your thoughts on this. I did not have a probe in the old unit since you literally could see the fire through the space between the arch doors. It was an original early 1900's Small Brothers Lightening arch complete with lightening bolts on the castings. My Dad used to joke that those lightening bolts made it boil faster. What I am hoping to be able to do is to try to keep a relatively constant boil and not just rely on watching the rate of boiling in the pans although that is important. I fully agree with you that you have to watch several indicators. A neighbour uses a timer to make sure he adds wood at a regular time interval (about every 15 minutes) and I will likely experiment with that too. So I went ahead and installed the probe and ran a test fire using some RO permeate water I had saved for a final rinse before winter freeze up. It was a low fire for about 30 minutes since the refractory is all new and the advice is to gradually condition it. I then fired for a couple of hours and towards the end I did increase the fan speed and had both the syrup and flue pans at a gentle boil. The probe worked well and was very responsive as the intensity of the fire was increased or decreased due to amount of wood and fan speed so I am happy with it. I had the temperature up to 500 degrees at one point before dialing the speed down. Looking forward to trying this out with actual sap in the spring.

Barkley,
Nice documentation of your testing. Yes the egg timer is the only timed event that happens in syrup making! Think about that for a little bit. I also use a egg timer for firing the arch, we run about 8 minutes. consistent firing = more consistent boiling!
The stack temp will be something that will be a visual indicator but wood is tough to bring up or down quick so the egg timer system works well.

The King Arch had $ signs on the door. Folks ask what they were for and I would tell them that is the end you throw your wallet in!
Keep boiling!
Regards,
Chris