How do I calibrate a thermometer?

I have a metal thermometer that I want to use in my darkroom. The temperature reading is off by about 20 degrees. Isn't there some way to adjust it by using a glass of ice water?

How does the use of a cold thermometer affect the temperature reading?

Suppose that you measure the temperature of a cup of hot tea with a cold thermometer. How does the use of a cold thermometer affect the temperature reading? Is this an example of the heisenberg’s uncertainty principle? please explain!