Not to continue off subject but if you use the ice water or boiling water described in this link you have calibrated your thermometer for those temperatures. 100 degrees is almost in the middle of those temperatures and thermometers can be way off even if they are calibrated using those methods. Calibrate using ice water and your thermometer is accurate around 32 degrees. Same for boiling it will be accurate around 212 degrees. Medical under-the-tongue thermometers are required to be accurate within .3 degrees at 100F when purchased. I have calibrated many of my thermometers using
both ice method and boiling method and found them still off by 1.5+ degrees at 100F (compared to the calibrated medical thermometer). That is a big difference! I highly recommend the clay method I described above.
Here is the link to the original post I found:
https://www.backyardchickens.com/t/130840/calibrating-thermometers-the-easy-way
John