How awful are dial thermometers?
The answer is "even worse than I imagined"!
My BP rescue came with a viv, complete with a dial thermometer and hygrometer. I knew they were pants already so I had two proper digital thermo/hygrometers available from the start. For the record, the dial thermometer overred by 12°C/20°F, and the hygrometer underred by 45% (!!) with the digital probes were stuck on the front glass of the dials for comparison.
During my monthly clean today, I finally decided to get rid of this nonsense.
Breaking them open, I found the expected coil of bimetallic (?) metal in the thermometer. The hygrometer had the same sort of thing, but one side of the coil has paper adhered to it, presumably to absorb moisture from the air.
What DID surprise me - the coil/needle pivots were simply attached through a hole in the cardboard dial and were not attached to the plastic backing - and are free for the pivot to rotate in the card. Indeed - a sharp shock perpendicular to the needle would cause the apparent reading to permanently change by a few degrees since the pivot rotates in the card.
I guess this is a rambling way of saying: Every warning against these things is justified and then some, and I can't see how they're even suitable as a guide. They're worse than useless.
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