Cigars 101

How do you calibrate a hygrometer?

Reading Time: 2 minutes

How to Calibrate a Hygrometer

Most analog hygrometers are manufactured with a tolerance of +/- 4-5% (or more) of humidity within the range of 40-80%. Although humidity gauges are supposedly pre-set at the factory, it’s not unusual to find the calibration off once you get the unit home and placed in your humidor. If you want to see how accurate your hygrometer is, there is a simple, easy-to-do method you can use with items found right in your kitchen. All you need is some table salt, a sealed container (Tupperware™ type or ZipLock™ bag) and a plastic bottle cap.

  • Place a teaspoon of salt in the bottle cap and add a few drops of tap or distilled water to moisten it. Don’t overdo it. You don’t want to dissolve the salt. Add only enough water to dampen the salt. When water is added to common table salt, it will maintain an exact 75% humidity in a perfectly sealed environment.
  • Carefully place the salt and your hygrometer into the airtight container. (Try not to get any moistened salt on the gauge.) Check the unit to assure its current reading is somewhere in the 40% to 80% range.
  • Seal the container tightly but don’t try to remove any remaining air trapped inside. Now, wait for at least several hours until the environment has stabilized (this could take up to 6 hrs.). Do not open the container. Read the gauge’s humidity % level. It should be exactly 75%. If it is not, note the deviation as being the amount your hygrometer is out of calibration. If for example, it reads 65%, the gauge is 10% low. If it reads 80%, the gauge is 5% high.
  • Carefully remove the unit from the container/bag. Assuming your hygrometer has a calibrating screw on the back (most better ones do) take a very small flathead screwdriver and turn it slowly while watching the dial on the front. If your gauge was low by 10%, turn the screwdriver so the dial is set 10 percentage points higher than it was previously. Conversely, if your gauge was high by 5%, turn the screw in the opposite direction to set the dial 5 percentage points lower.

Your hygrometer should now be properly calibrated, which will help you maintain a healthier overall environment for your cigars. If the gauge does not appear to be moving, try blowing warm, moist air into the back of it. One other thing you can do, which is also another way to test the unit, is by wrapping the hygrometer in a warm, damp paper towel for about 30 minutes or until it reads about 80-90% +/- a few points.

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Chalky
2 years ago

Thanks. Pretty simple. I have an airtight humidor – will 50-50 water and propylene glycol keep it at the correct humidity?

Derek McDoogle
2 years ago

I did not know that you can calibrate a hygrometer by using items found in your kitchen like a teaspoon and salt. My neighbor owns a pet store and he says that he has to have the right humidity percentage to keep some of the animals alive. It would be nice to learn how to have a professional come to his store and check the hygrometer.

Douglas Benoit
2 years ago

Must this be sea salt, or can it be table salt (Iodized)?

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John Pullo
2 years ago
Reply to  Douglas Benoit

Hi Douglas – sorry for the late reply…yes, it can absolutely be table salt.

Brandon
1 year ago
Reply to  John Pullo

I just came upon this trying to figure out why my lab hygrometer calibration was taking so long to equilibrate and saw this comment. You’d probably want to avoid sea salt because it is not very pure NaCl. Each salt has a different equilibrium %RH, only NaCl is 75.29%RH at room temperature. Sea Salt can be like 7% MgCl2, which has an equilibrium %RH of 32.78%RH. Table salt is more like 97-99% NaCl so the equilibrium %RH will be much closer to 75%RH. The effect of the other salts in sea salt might be small though, I am not sure. You might want to get non-iodized table salt for this as well, the iodine is added via additional salts that might effect the equilibrium %RH also. Scientific NaCl is too expensive to justify here; the science warehouses sell a cheap version for high school classrooms, but they only guarantee 95% NaCl, and table salt is typically higher than that, so probably best to use supermarket salt.

Brian Klug
2 years ago

I have made a video on How to Calibrate a Hygrometer here: https://www.youtube.com/watch?v=aeLPRZQTA4w

John P
2 years ago
Reply to  Brian Klug

is a sealed sandwich bag adequate?

Justin Lovern
1 year ago

Would there be any difference between using tap vs. filtered water?

WRS55
1 year ago
Reply to  Justin Lovern

In calibrating the hygrometer, I am sure you won’t get too much of an off reading, but there will be much more significant impurities in tap water that can affect your humidity. I have always seen it suggested with distilled water, but If you are unable to purchase distilled water, you can always distill some yourself with a pot, a glass bowl, and some water and ice. You won’t need very much. See this video for DIY distillation https://youtu.be/-AhtVcnktOM .

Gary Korb

Gary Korb

Executive Editor

Gary Korb has been writing and editing content for CigarAdvisor.com since its debut in 2008. An avid cigar smoker for over 30 years, he has worked on the marketing side of the premium cigar business as a Sr. Copywriter, blogger, and Executive Editor of Cigar Advisor. A graduate of the Newhouse School of Public Communications at Syracuse University, prior to his career in the cigar business, Gary worked in the music and video industry as a marketer and a publicist.

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