Quick answer: A hygrometer you have never calibrated is a guess. The saturated salt test pins your meter against a known reference point near 75% RH in about 8 to 12 hours and costs nothing but table salt. Note the offset, apply it to every reading, and recheck every 6 to 12 months, ideally just before heating season.
Put two case hygrometers side by side and there is a fair chance they disagree by five points or more. One of them is lying, possibly both, and since your entire humidity routine hangs on that number, an evening of calibration is the best free upgrade in instrument care.
Why meters drift
Consumer humidity sensors typically leave the factory rated around plus or minus 3 to 5 percent, and they age: exposure to very dry or very damp air, temperature swings and plain time all push the reading. Analog dial meters drift even more and often arrive far off. None of this makes a meter useless. It makes an unverified meter useless, a distinction our hygrometer buying guide draws when choosing what to trust in the first place.
The salt test, step by step
- Make the slush. Put a few tablespoons of plain table salt in a bottle cap or shot glass and add a few drops of water, just enough for wet sand. Salt must remain undissolved; a slush, not a solution.
- Seal it with the meter. Place cap and hygrometer together in a clear airtight container or a well-zipped freezer bag, meter face visible, salt not touching the meter.
- Park it somewhere stable. Room temperature, away from sun, vents and the kitchen. Temperature swings during the test smear the result.
- Wait 8 to 12 hours. The sealed air settles at the reference point: saturated sodium chloride holds close to 75% RH at room temperature, 75.3% at 25 °C, which is why the salt test is the classic home calibration.
- Read through the container, then do the math. Reading minus 75 is your offset. A meter showing 72 reads 3 low, so a case reading of 46 really means about 49. Write the offset on tape stuck to the meter's back. If it has a calibration screw or reset, adjust; most digital units just carry the offset.
| Salt test shows | Offset | What to do |
|---|---|---|
| 73–77 | Within 2 points | Trust it as-is; recheck in a year |
| 70–72 or 78–80 | 3–5 points | Note and apply the offset, recheck in 6 months |
| Beyond that, or unstable | Over 5 points | Replace the meter; a wandering sensor cannot be offset away |
Cross-checks that keep you honest
Calibration at 75% tells you about one point on the scale, and your case lives near 49, so run cheap cross-checks in the range you actually care about. An RH indicator card in the same case should agree with the corrected meter within a band's width. And a practical mid-range check: seal the meter in a small airtight box with a fresh Humidi-Cure® 49% pack for a day, and the corrected reading should settle at 49 within about 2 points, since that is the setpoint the two-way fiber holds. Meter, card and pack agreeing is as close to certainty as home methods get.
The drift schedule
Calibrate a new meter before trusting it, then recheck every 6 to 12 months. The smart calendar slots are just before heating season in the US, Canada and Korea, when the number starts mattering daily, and after any flight, drop or battery change. Players in Singapore and coastal Australia should also check after long stretches at high RH, which ages sensors faster. The meter's whole job is guarding the 45–52% sweet spot, and packs about 3 months into their service, with case airtightness the biggest variable, get judged by that same corrected number, as our pack lifespan guide explains.
FAQ
How accurate is the salt test really?
Done patiently, with a true slush, a good seal and stable temperature, it lands within a point or two of the reference, which is tighter than the sensors it is testing.
Can I calibrate an analog dial hygrometer?
If it has an adjustment screw, yes: run the test, then set the needle to 75. If not, tape the offset to the case, or retire it in favor of a digital unit.
Why does my case read different from the room?
Because the case is doing its job. A closed case with a two-way pack is a microclimate, and the whole point is that it does not match the room.
What number should the case show?
Anywhere in 45–55% is healthy, and a pack-managed case should sit near 49 once the corrected meter and the card agree.
Give the meter something to guard: Humidi-Cure® 49% Humidity Pack, 4-pack of 60 g packs, $8.99, no liquid or gel, food-contact safe plant fiber, from the instrument collection.
Stores and repair benches: calibration makes a great counter service. Wholesale program or info@atmosiscience.com.
Meter reading something weird?
Describe the readings and the case and we will help you figure out whether it is the meter, the case or the climate.
