Incubating eggs..how do u calabrate a thermometer?

[erijn5]

I have 5 (yes 5) thermometers sitting on my table. 2 say 68 degrees, 1 says 74, 1 says 72, 1 says 70.....

These are the mercury thermos...I remember someone said to put them in a baggy with salt and ice I thought but I'm not sure.

Does anyone know...how much ice, salt, HOW long do you leave them for to get them calabrated?

Thanks!