Informed operation of your stove's controls shows you when it is burning at optimum temperature.
In general best operation is between 230ºF to 475ºF. This handy thermometer will show you are achieving this with the handy easy to read dial.
Approx size 6cms in diameter.
Attach magnetically to single wall flue pipe. Best placement is approx 12"/30cms above the top of the stove.