When you get a probe type thermometer, the instructions with it will probably require a larger hole in the outer pipe with a bushing to prevent sensing he outer wall pipe temperature. Install as instructed.
The object is keeping the chimney flue above 250*f to the top when smoke is present. Below this critical temperature, water vapor from combustion condenses on chimney flue and pipe walls allowing smoke particles to stick. This forms creosote.
Your magnetic pipe thermometer is for single wall pipe. It will read surface temperature which is about 1/2 the internal flue temperature. Notice the safe low temperature zone is close to 250*f. This is actually 500* internal, assumed to cool back down to 250* at top. This is only a guide since all venting systems cool differently. The probe thermometer will give you actual flue gas temp. Then allow for cooling as gases rise to top. (as bholler stated above, about 350*f considering cooling of your particular chimney.
The object is keeping the chimney flue above 250*f to the top when smoke is present. Below this critical temperature, water vapor from combustion condenses on chimney flue and pipe walls allowing smoke particles to stick. This forms creosote.
Your magnetic pipe thermometer is for single wall pipe. It will read surface temperature which is about 1/2 the internal flue temperature. Notice the safe low temperature zone is close to 250*f. This is actually 500* internal, assumed to cool back down to 250* at top. This is only a guide since all venting systems cool differently. The probe thermometer will give you actual flue gas temp. Then allow for cooling as gases rise to top. (as bholler stated above, about 350*f considering cooling of your particular chimney.