I thought it might be beneficial to show how I calibrate my boards. It does not need to be this complicated but it does work.
I have had good luck with this setup for calibration at room temperature. I mount the board to the 3 leg mount (won't rock) and hold in place with a low tension rubber band. I put it in a zip lock bag, zip it shut with the microSD card and a short power lead sticking out. I freeze the whole assembly for a hour.
When it's been an hour I quickly take out the board and put it in the foam lined box plugged into a short extension lead. Close the box and put it in a place that will not be susceptible to any vibration or magnetic changes. The box is made of particle board which is heavy, and the mounting board has weight so that the relaxing of the plastic bag as it warms won't create movement.
It takes about 1:15 to 1:30 to reach a stable temperature at normal room temperature. It my not work if the ambient temperature is too low.