Can you make your own salt test to calibrate a hygrometer?

No, not one that’s accurate. 

You’re probably watched videos online that show someone filling a bottle cap with salt water to calibrate a hygrometer. Salt is the key but not in an at-home science experiment. 

Here’s why the bottle cap test can’t give you a precise relative humidity (RH) reading. After you fill a bottle cap with salt water, what’s the RH? Your RH readings will vary for your homemade solution, depending on its salt to water ratio.

“People try the salt test because table salt is accessible,” explained Rob Gagner, the host of Boveda’s  Box Press podcast.

“Who doesn’t have salt in their house?” Rob continued. “Salt does naturally regulate humidity—that’s the basis of Boveda and why there’s a saltwater-based solution in our packs. But Boveda uses specific salts in specific amounts to achieve specific humidity levels.”

To myth bust do-it-yourself hygrometer calibration, the cigar podcast host ran salt tests in the Boveda Innovation Center by: 

• Mixing distilled water with salt
• Filling three bottle caps with the salt-water solution
• Finding the RH reading of each sample using an $8,000 water activity machine:
  – SAMPLE 1 = 72% RH
  – SAMPLE 2 = 75% RH
  – SAMPLE 3 = 78% RH

“The readings were all over the place—up to a 5-point RH difference,” Rob said. “And that’s by testing salt-water samples with an $8,000 piece of laboratory equipment.”

A +/- 5% in RH makes a huge difference for cigars, which ideally should be stored between 65 and 72% RH. To make sure you maintain perfect RH for cigars, your hygrometer needs to read right. (Hygrometers—even digital ones—are notorious for falling out of calibration.) 

To get accurate readings, run a salt test on your hygrometer using the Boveda One-Step Hygrometer Calibration Kit. Boveda’s professional-level hygrometer test is accurate to +/- 2%. 

During manufacturing, Boveda goes through a quality control check using that $8,000 machine to make sure the run reads 75% RH. Therefore, the Boveda Calibration Kit is the exact opposite of a homemade salt test— quality controlled and precise. 

And unlike a bottle cap, Boveda’s solution is sealed in a leakproof membrane—so there’s no mess. The calibration takes just 24 hours and there’s nothing more for you to add—except a hygrometer.  

“If you want the accuracy of a lab hygrometer to gauge RH, use what an actual lab would use,” Rob concluded. 

Order a calibration kit for humidors here.