How to Calibrate a Hygrometer

cigar humidorsThe proper calibration of hygrometers is important since it could be showing a correct relatively humidity range while in fact it is not resulting to drying your precious cigars.

Although, hygrometers should be pre-calibrated when they came of the manufacturing plants, normal shocks resulting from packing, transporting or shipping can cause the unit to be off by a few percentage points.

First thing you need to do after you bought  your hygrometer is to determine if it is calibrated or not. You could do the table salt test to see if the hygrometer is showing off the right readings.

Get a teaspoon of salt and place it in a bottle cap. Moisten the salt a bit but don’t dissolve it. Use distilled water and not tap water. Tap water may contain residues that can affect the relative humidity in the sealed environment.

Place the cap of salt and the hygrometer in a sealed plastic container. Some air might be trapped inside once  you seal the plastic, don’t remove the air. Wait until the environment inside the plastic stabilizes. After a few hours or so, the hygrometer needle should be pointing exactly at 75%. If it is not, then the hygrometer needs calibrating. Take not of the actual reading.

The reason lies on the salt. When water is added to salt and placed in a sealed environment, it will maintain exactly 75% humidity. Calibration is therefore necessary if the reading falls below or above the 75% mark.

To calibrate an ordinary hygrometer, you just need to turn the screw located at the back. After taking note of the actual measurement you can adjust the hygrometer accordingly. If the hygrometer after the salt test measures 70%, all you need to do is turn the screw 5 points higher until the needle reaches the 75% mark. If the actual humidity measurement is 85%, you need to turn the screw until the needle falls back 10% points back to 75%.

Aside from the table salt test, one could still tell if you hygrometer is a little off with its readings. Get your hygrometer and get a piece of cloth. Moisten the cloth and wrap it around the hygrometer. The hygrometer after an hour should be at 96% relative humidity. Adjust the screw accordingly.

If, however, you have a non-adjustable hygrometer, the best way to get the correct measurement every time is to get the difference in the measured percentage points. If for example the salt test revealed a 78% reading, knowing that the reading should be 75% gives you the chance to note down how many points the hygrometer is off. You then add the percentage points difference to get the right relative humidity.

Remember, though, that before making any measurements with your hygrometer, please wait for at least two hours. The two-hour wait is often enough time for any hygrometer to adjust to its present environment and measure the correct relative humidity.

Also, we recommend calibrating your hygrometer once or twice every year.

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