Calibrating your humidor hygrometer: the salt test

Why calibrate your hygrometer

An uncalibrated hygrometer can be off by 5-10 percentage points. If it reads 70% but the actual humidity is 60%, your cigars are drying out without you noticing. If it reads 70% but the actual level is 80%, mold is inevitable.

Calibration should be done at least once a year, or every time you replace the battery. Analog (spring) hygrometers are the least accurate and need more frequent calibration. Digital ones are more stable but can still drift over time.

The principle of the saturated salt solution

A saturated salt solution — salt in which no more can dissolve — creates a microenvironment with a relative humidity of exactly 75% at room temperature. It is a physical constant: saturated sodium chloride always maintains 75% RH in a sealed environment, regardless of quantity (as long as there is visible solid salt alongside the solution).

This makes the salt test the most affordable and accurate calibration method available at home.

Step-by-step procedure

1. Prepare the salt: pour table salt (NaCl) into a small bottle cap. Add 4-5 drops of water. Mix: it should become a damp paste, not liquid. If you can still see salt grains, it's perfect.
2. Seal: place the cap and the hygrometer in a ziplock bag or a plastic container with a lid. The hygrometer must not touch the salt or the water.
3. Wait 24 hours: at stable room temperature (18-24°C). Do not open during the waiting period.
4. Read: after 24 hours, read the hygrometer without opening the container. It should read 75%.
5. Adjust: if it reads differently (e.g., 72% or 78%), use the adjustment screw (found on analog hygrometers) to bring it to 75%. If it is digital without adjustment capability, note the offset (e.g., -3%) and add it mentally to future readings.

Analog vs digital hygrometers

Analog (spring): affordable, aesthetically pleasing, but with typical accuracy of ±5-8%. They should be calibrated twice a year. Most have an adjustment screw on the back.

Digital (electronic): more accurate (±2-3%), more stable over time. Some lack manual adjustment: in that case, note the reading error after the salt test and correct mentally.

Human hair hygrometers: the oldest type, slow to respond (30-60 minutes to stabilize). More accurate than spring analogs but require annual calibration.

How often to calibrate

• Analog hygrometer: every 6 months
• Digital hygrometer: every 12 months
• After replacing the battery: always
• After a shock or fall: always
• If you notice anomalous readings (sudden changes with no apparent cause): immediately

Signs your hygrometer is inaccurate

If your cigars feel different to the touch than the hygrometer suggests — dry to the touch but the hygrometer reads 70%, or spongy but the hygrometer reads 65% — the hygrometer is likely out of calibration. Touch is a good secondary indicator: a properly stored cigar has a slight give under thumb pressure — it is neither stiff as wood nor soft as a sponge.