From what I've found from my research, the probe or surface thermometer must be placed at least 18" (45.72 cm) above the top
of the stove. There is no indication on the maximum but I think it cannot be placed too far from the stove.
Data from SBI Heating Accessories for a probe thermometer:
- 100 - 400°F: Temperature too low. Fire may be dying out. Causes creosote accumulation, smoke or soot.
- 400 - 900°F: Operating efficiently.
- 900°F +: Possible overheating. These temperatures may be reached for a brief period during initial firing. Not recommended for prolonged periods.
Magnetic surface thermometer on a HEPA stove: make sure that the temperature does not exceed 475F (246C).
Source: Drolet Stove Builder International inc. I have no offical data for the minimum. I have seen between 270F and 475F somewhere.