Even if you buy a hygrometer brand new, still calibrate it. I got one from incubator warehouse and that sucker was off by 13%! I've read that the best way to calibrate is to use 1/2c salt 1/4c water and mix them in a coffee mug. Then plage the mug and your hygrometer in a large ziplock bag, and let it sit for 8-12 hours. Salt evaporates at a 75% humidity, so in the 8-12 hours it should read 75%. If it doesn't, just make a note of what you need to add or subtract from your readings to make them accurate. Best of luck!