|Inside an Oregon Scientific BAR206|
Humidity and temperature sensors are easily spotted, as highlighted in the picture above. With a bit of surprise, the temperature sensor is an analogue thermocouple. I found its tip sitting on the resistor, which could be the cause of the +0.5 °C constant error, added with an insufficient air flow.
I lifted the thermocouple and began the experimental comparison with an external sensor, one next to the other, BAR206 being without back cover (higher air flow).
Much to my disappointment, but without too much surprise, BAR206 body continued measuring 0.5 °C more than the external sensor. This confirms the observation at my parents' place with other three Oregon Scientific thermometers (one internal sensor in another model, two external sensors).
So this must be a firmware error or a design issue of analog circuitry inside BAR206. Even though some solder joints look awful (look at the huge blob under the thermocouple tip), the reason must be somewhere else. I may write to Oregon Scientific, but their specs say the accuracy is +/- one °C.
From now on I will mentally deduct 0.5 °C from the internal reading.
You may ask: why bother? I am an Engineer, I love accurate measurements and, most important, I hate recurrent (design) errors.