Testing hygrometer accuracy.
You can test an hygrometer's accuracy in two common ways:
1. Wrap a damp cloth on the back of the hygrometer for 6 hours. The indicator should then read approx. 95%. If it reads lower or is off scale completely, merely adjust the indicator to read 95% using one of the methods described above.
2. Another way to check calibration uses table salt, a see-through container such as a Ziplock baggie, and a small shallow open container, like a bottle cap.
Place a teaspoon or so of salt in the shallow container and add a few drops of water - just enough to get it wet. Remember, you don't want a salt water solution, just damp salt. Place the shallow container in the baggie along with the hygrometer. (Make sure the salt does not get on the hygrometer, this may damage it).
Seal the baggie with some air trapped inside and let it sit for 6 to 8 hours. Check the hygrometer reading without opening up the baggie. It should read 75%. If not, remove the instrument from the baggie and adjust the reading to 75% with a small screwdriver.
To maintain accuracy of the instrument, it is advisable to recalibrate every six months.
You can do this with digital also but no screw driver is needed.
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