How to Test a Hygrometer

Last Updated: January 31, 2023 Approved

If you're a cigar aficionado, you need a hygrometer to make sure you're storing your wares at the correct humidity. A hygrometer is an instrument that is used to measure humidity in cigar humidors as well as greenhouses, incubators, museums, and more.^{[1]
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Research source} To ensure that your hygrometer is functioning properly, it is best to test it prior to use, then calibrate it if necessary. The salt method is a tried and true way of testing a hygrometer for accuracy. Here's how to do it.

Steps

1
Gather supplies. To test your hygrometer using salt, you just need a few household supplies:^{[2]
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- A small food storage bag that zips
- A tiny cup or bottle cap from a 20-ounce soda
- Some table salt
- Water
2
Fill the cap with salt and add enough water to make a thick slurry. Don't add so much water that the salt dissolves; you just want it to be wet. If you add too much, use paper towels to mop up the excess.

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3
Put the cap and the hygrometer inside the baggie. Zip it up and place it somewhere out of the way, so it won't be disturbed during the test.^{[3]
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4
Wait 6 hours. During this time the hygrometer will measure the humidity inside the bag.^{[4]
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5
Read the hygrometer. If it's accurate, it should show a humidity of exactly 75 percent.^{[5]
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6
Adjust the hygrometer if necessary. If your hygrometer shows a humidity lower or higher than 75 percent, you'll need to calibrate it so that it's accurate when you use it to check the humidity of your humidor.^{[6]
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- If you have an analog hygrometer, turn the knob to adjust it to 75 percent.
- If you have a digital hygrometer, use the buttons to adjust it to 75 percent.
- If you can't adjust your hygrometer, take note of how many percentage points higher or lower than 75 percent it is. Next time you use your hygrometer, add or subtract a few percentage points for an accurate reading.

Community Q&A

Question

How do we know for sure the humidity is 75%?

Community Answer

Salty solutions will create a specific atmosphere inside of the bag because of the modified vapor pressure. Just using distilled water, for example, would give you exactly 100% RH, and a saturated lithium chloride solution would give you about 11% RH. Keep in mind that this is true only at room temperature (20 °C), the whole process is temperature-dependent. Table salt (sodium chloride) tends to be very stable in that regard (less than 1 % change between 0 °C and 30 °C).
Question

I calibrated my hygrometer for 30 hours and it read 74%. I adjusted it to 75%, but after 24 hours in my humidor, it read 66%. Is it broken or did I do something wrong?

Community Answer

It doesn't sound like it's broken. More likely is that the humidor is dry.
Question

I have 2 hygrometers. One shows the room at 25%, and the other shows it at 40%. In the salt test, using the same container, the first goes to 80% and the second goes to 75%. What explains this?

Community Answer

It tells you that second hygrometer is accurate and your room is at 40% relative humidity. It also tells you need to calibrate the first one.