They are surprisingly accurate, actually. It's an effect of the physical properties of the metal, which we can measure quite precisely. I couldn't immediately f

They are surprisingly accurate, actually. It's an effect of the physical properties of the metal, which we can measure quite precisely. I couldn't immediately find an exact figure. Repeatability is not so great, but mostly because the mechanisms are designed as cheaply as possible. But typically it's within a couple of degrees. It turns out to not be that big of an issue because you want a few degrees of hysteresis in a thermostat.