Hygrometer Salt Test

You cannot check a hygrometer against another unverified hygrometer — that only measures their disagreement. You need an environment whose humidity is known from physics, and a saturated salt solution provides exactly that: a slurry of plain table salt and distilled water pins the air above it at 75.3% RH at normal room temperature, within about half a point, regardless of quantities. Whatever your hygrometer reads in that sealed space, the true value is 75%. The difference is the instrument’s error.

This is a laboratory-standard reference, not a folk method — which is why it is worth doing before a new unit goes into service, including units sold as pre-calibrated, since factory calibration predates being thrown around in shipping.

Making the slurry

1. 1Fill a bottle cap or shallow dish with plain table salt.
2. 2Add distilled water a few drops at a time and stir until the salt is thoroughly damp but not dissolved — the texture of wet sand. Standing water means too much; add salt.
3. 3Undissolved crystals must remain visible. It is the saturated solution in contact with solid salt that pins 75%, not salt water.
4. 4Put the cap and the hygrometer into an airtight transparent container so that neither the sensor nor its case touches the slurry — salt water corrodes sensors.
5. 5Seal it completely and leave it somewhere with a stable temperature, out of sunlight. A slow leak toward room humidity is the most common cause of a failed test.

How often to re-test

• —New instrument: always, before first use.
• —Digital in service: every 6 months, plus after any battery change, drop, seasoning cycle at 80%+ RH, or move to a different climate.
• —Analog in service: every 3–6 months. The mechanisms wander faster.
• —Regardless of schedule: any reading that stops changing, any sudden multi-point jump, or disagreement beyond 2–3% with a paired instrument.

The full reasoning, including how digital and analog units drift differently and what to do with a unit that has no adjustment, is in calibrating a hygrometer. If you are choosing an instrument in the first place, what to look for argues that calibratability matters more than the accuracy spec.