Casio Thermometer Digital-Analog
Nice hidden Function in this watch: 99
I will play with all the function normally when watch is new, just to squeeze the last out of them :-) One day I pressed down a button (I believe to remember it is the adjust button that you normally press with the thumbnail) while reading the elapsed time and realized that there is a two digit hour field displaying elapsed time up to 99 hours. Of course the manual does not mention about this and the designers probably did not tell the marketing, you know how it works :-)
By setting the thermometer about 5.5 degrees minus, you can actually measure environment temperatures quite accurately. Those minus 5.5 degrees are used to compensate the body heat of the wearer. Of course you will have to find your own threshold, an important factor is also how tight you wear your watches and whether the case back is permanently laying on your wrist skin.
The little aluminum rivet is the temperature sensor.