those can be calibrated,,Got my new Brinsea spot check thermometers yesterday. What a disappointment! $20 each for some dinky little plastic. They are suppose to be accurate +/-.1°. Total BS......there is a difference of more than 1 Whole ° !
So which one is codrect. Plus you can't calibrate because it it gets below a certain temp they just read low !
run some ice cubes in a blender until you have a good cupful or more..
put he crushed ice into a glass and add just a little water,,
poke the thermometer into the crushed ice until the mercury bottoms out,, that will be Oc or 32F
no matter what the number says,, on some therms you can slide the glass tube up or down to make the mercury match 32F
otherwise just take note of what the number is, and either add or subtract the difference between it and 32F,, then add of subtract that number from 99.5 and you use that number for 99.5F.. all you need then is to make a mark and forget about the numbers..