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Calibrate a hygrometer with a saturated salt test: put a damp slush of table salt in a small open cup, seal it with the hygrometer in an airtight bag or box at room temperature, and wait 8 to 12 hours. The air settles at 75.3 percent relative humidity. Compare the meter’s reading; the difference is its error at that point. Apply the meter’s calibration offset if it has one, or record the correction and apply it to readings. Repeat yearly; capacitive sensors drift about a point a year.
Why saturated salts work
A saturated solution of a salt in contact with air holds the air at a fixed relative humidity that depends on the salt and only weakly on temperature. Sodium chloride gives 75.3 percent RH at 25 °C (75.5 at 20 °C); it is the standard single-point check because table salt is in every kitchen. Other salts give other points for a two-point check.
| Salt | RH at 20 to 25 °C | Availability |
|---|---|---|
| Magnesium chloride | About 33% | Pharmacies, online |
| Potassium carbonate | About 43% | Chemical suppliers |
| Sodium chloride (table salt) | 75.3% | Kitchen |
| Potassium chloride | About 85% | Salt substitute, pharmacies |
Method
- Put two to three tablespoons of salt in a small cup or bottle cap. Add water a few drops at a time until it is a wet slush with no free liquid on top. Undissolved salt must remain; a clear solution is not saturated.
- Place the cup and the hygrometer in a zip-seal bag or a small airtight box. The sensor must not touch the salt or water.
- Seal it. Keep it at steady room temperature away from sun and drafts.
- Wait at least 8 hours; overnight is ideal.
- Read the hygrometer through the bag or immediately on opening. The reference is 75.3 percent.
- Record the error: reading minus 75.3.
Applying the result
- Meter with calibration mode: follow the manual to set the reading to 75 (some use 75.3) while still in the bag.
- Dial hygrometer with adjustment screw: turn to 75 while still sealed.
- Meter with no adjustment: write the offset on a label; add or subtract it from readings. See calibration vs adjustment.
- Error over 5 points: for anything that matters (instruments, humidors, damp investigations), replace the meter.
Two-point check
An error at 75 percent does not guarantee accuracy at 35 percent; capacitive sensors can have a gain error as well as an offset. A second test with magnesium chloride at 33 percent shows whether the error is constant. Two points allow a simple linear correction.
Things that go wrong
- Too much water: a dilute solution gives higher RH than 75. Keep it a slush.
- Leaky container: room air dilutes the reference.
- Too short: the sensor has not equilibrated. See why sensors need time to stabilize.
- Temperature swings during the test shift the equilibrium and the sensor.
- Opening the bag and waiting before reading; the sensor starts drifting toward room air within a minute.
How often
Yearly for household meters, every six months for instruments and humidors, and whenever two meters disagree by more than a few points. For selecting a meter worth calibrating, see how to choose a humidity meter. The humidity checker converts corrected readings to dew point.
Recommended Tools
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Frequently Asked Questions
What humidity does the salt test produce?
Saturated sodium chloride (table salt) holds the air above it at 75.3 percent RH at 20 to 25 °C. Magnesium chloride gives 33 percent and potassium carbonate 43 percent for a second point.
How long does the salt test take?
At least 8 hours for the air and the sensor to equilibrate; overnight is convenient. Small sealed bags settle faster than large boxes.
My hygrometer reads 70 percent in the salt test. Is it broken?
It reads 5 points low. Many meters allow an offset adjustment; otherwise note the correction and add 5 to its readings, or replace it if accuracy matters.
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