dubious quality of probe?
Accuracy of unit after only three uses is becoming increasingly poor, as indicated by the "primitive" but accurate lab thermometer. Can't say for sure whether the problem is with the probe or the actual thermometer device, but without investing in another probe there's no way to know for sure except for going through the bother and expense of sending the unit in. Avoid: not for brewing, and digital isn't necessarily better.
Reviewed by: EthanB