Put all your thermometers in boiling water n see what the difference are between em? 212°F (sea level)?
It is better to check them against a mixture of water and ice. Make sure that the ice and water have equilibrated. Good thermometers will have a line showing where the liquid should reach. By using ice the temperature is not dependent on altitude.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.