I was going to PM but thought everyone else might benefit from the discussion.
The probe looks good, I saw where you can order custom lengths so that the tip doesn't get damaged by the flue brush.. actually I worry about penetrating the inner pipe without some way of resealing it. This is going to be (I think) 2 layer pipe with insulation between. Thought about just having the probe long enough to sit against the inner pipe without penetrating.
Problem is the large multimeter type display. That sitting up on the mantle would be an eyesore. What would be ideal is a compact display with no buttons. I don't need to know averages, hi/lo, C/F/K conversion, etc... just a numerical display showing current temp, maybe 1"x2" with a connection for the cable and preferably run on a 9v or a few AA batteries.
Ian
Ian, a probe in the pipe is going to be a problem for cleaning however you do it. Due to the flow in the pipe and the conduction of the pipe, the temps in the center are going to be higher than the edges. A tab mount style tc mounted on the iside of the pipe would give you the temp of the pipe, but not the temp of the actual gases and you'd still end up drilling a hole in both pipes. Doing that, you'd also have a problem getting the wire out of the inside of the pipe. A probe is the only option unless you wanted to mount a tab style outside the inner pipe and then you would only be measuring pipe temp. If you do use a tab mount tc on the outside of the inner pipe, use an all metal crimp style nut. Match materials, ie, no steel bolts/nuts on SS.
I would go with a 1/4" diameter probe and mount it as close to the stove as possible. Hopefully, the pipe entrance into the stove is a straight vertical drop so doesn't build up much creosote and doesn't need to be brushed.
About installing the probe. I don't know how the fire codes would address my suggestion, so use at your own risk.
Ok, the probe can be set to any depth, so whether you want to have 1" poking through the pipe or 3", it won't be a problem. I would recommend at least 1" because there is going to be some conduction back through the probe, into the fitting and into the pipe.
Now about the questionable part. I don't think you want to drill a hole through a 2 layer pipe and put a fitting on the outside. You leave yourself a path for creosote to enter into the insulation area and possibly cause a fire hazard at some point in the future. If I were asked to do this in an industrial enviroment, this is how I would do it. Cut about a 3" square hole in the outside pipe. Push the insulation aside. Drill a hole in the inner pipe and weld a SS 1/8" pipe fitting on it. Install the probe. Cut another piece of SS with a hole in the center for the remainder of the probe to stick out and weld over the 3" hole in the outer pipe. If you don't weld, you can use SS rivets but not sheetmetal screws as they will loosen. As I say though, I don't know how fire codes would view this (basically a patch on the outer pipe). I would make sure you have a very good welder weld the pipe fitting on too. It is not a place where you want a failed weld.
What I did on my stove, was welded a tc directly on the single wall pipe where the stove outlet is. I don't know the actual gas temp, but I know where the stove burns good and use it as a guage to set the draft. I like a pipe temp of about 300F as I have found I don't get much buildup at that temp. I usually run it up to 500F or so to burn it out.
As far as a reader, the only readers I use are handheld or panel mount. I haven't looked for a stand alone reader that would not look "out of place" on a fp mantle.