The
second thermometer linked to by raskusdrotti is neither digital nor mercury. It makes use of a coiled length of spring steel, very similar to an old-fashioned watchspring or clockspring. The inner (central) end of the spiral spring is fixed to the baseplate whilst the outer end is attached to the pointer and is free to rotate. When the spring gets warmer it will expand and try to straighten and uncoil, thus moving the pointer in one direction, and conversely when it cools it will shrink and try to close in on itself, thereby moving the pointer in the opposite direction.
The drawback in the design is that it isn't terribly robust, so if it's dropped then the whole assembly is liable to rotate slightly, giving several degrees of error. I keep one in my fridge and I've dropped it many times but it's simple enough to recalibrate, using a digital thermometer as a reference - recalibration being simply a matter of rotating the 'fixed' centre mounting point - this mounting point being fixed fairly tightly but not absolutely, because it does need some freedom to move for the purpose of initial calibration and subsequent adjustment.
Without some modification, a thermometer of that design probably just isn't robust enough for your purposes.
