How to check if your hygrometer is accurate (the salt test)
Disclosure: Humid Home helps you match replacement humidifier filters. Some Shop links go to retail product pages. This is general guidance drawn from EPA, National Bureau of Standards (NBS) and sensor-maker (Ruuvi) material.
Why the reading can be wrong
EPA notes that hygrometers are available at hardware stores and that you should not humidify above 50 percent relative humidity. A meter that is several points off can push you past that, or leave the house dryer than you think. How to place and read a meter for winter targets stays on winter indoor humidity. Here’s how to check whether your meter reads true.
Why damp salt sits near 75%
A sealed space over a saturated sodium chloride (table salt) solution is a humidity fixed point. Greenspan (1977), in the Journal of Research of the National Bureau of Standards, reports equilibrium relative humidity values for saturated aqueous salt solutions. For sodium chloride, Table 2 lists about 75.3% RH at 25 °C (75.29 ± 0.12). That is why a salt slurry in a closed bag or jar is used as a home check: after enough time, the air inside should sit near 75%, and you can compare your hygrometer to that.
Step by step: the salt test
These steps follow the common household method described by sensor maker Ruuvi (salt, a little water, airtight container). Greenspan’s published table is the source for the 75% figure.
- Put table salt in a small open cup or bottle cap. Add a little water so the salt is damp (a slurry), not a clear solution.
- Place that cup inside a sealable bag or jar with your hygrometer. Keep the salt from touching the sensor.
- Seal it and leave it undisturbed. Ruuvi says to wait about 24 hours.
- Read the hygrometer. Near 75% means it is close. Note any offset (for example, it reads 70% when the chamber should be near 75%).
Keep the setup at a steady room temperature. Greenspan’s tables are temperature-dependent; the familiar ~75% figure is the room-temperature fixed point, not a guarantee at every temperature.
If it’s off
- If your meter has a calibration or offset setting, set it so the sealed-bag reading matches about 75%, then recheck.
- If it does not adjust, write down how far it is from 75 and correct for it when you read the room. Example: if the bag reads 70%, add about 5 points to later room readings.
- If the offset is large or keeps drifting, replace the meter.
How often to recheck
Ruuvi recommends calibrating a humidity sensor at least once a year, and more often in harsh or high-humidity use. A simple habit: check again before each heating season, when you start running a humidifier, so the winter target on winter indoor humidity is based on a meter you have verified. Condensation on glass is still the stop sign: humidifier window condensation.
Quick checklist
- Damp salt slurry in a small cup (not touching the sensor)
- Sealed bag or jar for about 24 hours
- Compare to ~75% RH (Greenspan / NBS fixed point)
- Adjust the meter or write down the offset
- Recheck before the heating season
More: winter indoor humidity · humidifier window condensation · whole-house vs portable · all tips