Here are the results on the oven test that I did. I assume that I did this right and of course my oven could be off slightly, but I put four thermometers in the oven and set it at 400 degrees. I also bought two Condar flue thermometers (older style black).
Woodstock thermometer (provided with stove): 440˚
Vogelzang Model TG-01: 500˚
Condar Flue thermometer Model 3-19 (first one): 1100˚
Condar Flue thermometer Model 3-19 (second one): 900˚
This is quite a descrepancy between thermometers. The Woodstock seems most accurate and Vogelzang is close.
The huge difference in the Condar units could be how they were packaged (the indicator arm is on the outside of the circular gauge and was not really protected in the box so well). It could also be that they are only meant to measure from the tip of the probe and heating up the surface throws off the reading. I would like to think that a reputable company such as Condar would not sell an inaccurate product when safety is an issue.
For clarification, one of the Condar thermometers was damaged from handling/packaging. The indicator arm was bent a little. This probably explains the difference between both units.
FWIW, I installed the other one and it seemed to work great. I did not exceed 550 degrees on my flu temp according to this thermometer. It seems that it is meant to measure only from the probe; any attempt to test the whole unit in the oven seems to throw off the measurements.
Also, the Vogelzang surface thermometer seemed to indicate that you will get a reading approximately half of what is actually in the flu with double-wall stove pipe. It ran almost exactly half the temperature as the internal probe thermometer (Condar) throughout the burn cycle.
Mike