In a common ethanol thermometer, the space above the alcohol column is filled with nitrogen. It also contains varying amounts of the alcohol in the gas phase.
At high temperatures, the gas phase volume decreases because the liquid expands. As a consequence, the pressure rises (lower volume, higher temperature, more ethanol in the gas phase because of the increased vapor pressure). Does this pressure increase contribute to the non-linearity of the measurement, i.e. position of the meniscus vs. temperature?
Even without a possible pressure effect, the rise of the liquid is non-linear with temperature because of the non-linear expansion coefficient of ethanol. I am wondering whether the pressure effect significantly contributes to the non-linearity. I am also wondering whether the expansion chamber (at the tip of the thermometer) helps to prevent excessively high pressures.