Why Calibration Matters
A hygrometer reading of 70% RH only means something if the hygrometer is accurate. Most analog hygrometers ship from the factory without calibration and have accuracy tolerances of ±5–10%. A digital hygrometer from a reputable manufacturer is typically accurate to ±1–2% out of the box, but can still drift over time or ship slightly off.
The salt test creates a controlled, chemically defined environment at a known humidity level — 75% RH — and tests your hygrometer's reading against that known value. The difference between what the hygrometer reads and 75% is its offset.
Method 1: The Salt Test
The salt test exploits a well-established chemical property: a saturated solution of sodium chloride (table salt) in contact with air in a sealed environment will maintain exactly 75% relative humidity at room temperature. This is a physical constant — used in laboratory calibration of precision instruments.
What You Need
- Table salt (standard iodized or non-iodized — either works)
- Distilled water
- A small dish, bottle cap, or shot glass
- A large ziplock bag or sealed container
- Your hygrometer
- 8–12 hours
The Procedure
1. Place a small amount of salt in the dish — a tablespoon is sufficient.
2. Add just enough distilled water to make the salt wet but not dissolved. The correct consistency is damp salt with no standing water above the salt surface. This is the critical step — the system only works correctly with saturated salt, not dissolved salt.
3. Place the salt dish and your hygrometer together in the sealed bag or container. Close it completely.
4. Wait 8–12 hours without opening the bag. The environment inside will stabilize at 75% RH.
5. Read your hygrometer without opening the bag — look through the plastic, or open quickly and read immediately.
6. Note the reading. The difference between your reading and 75% is your offset.
Interpreting Your Results
| Hygrometer Reads | Actual Offset | Action |
|---|---|---|
| 75% | None — accurate | No adjustment needed |
| 70% | Reads 5% low | Add 5% to all future readings |
| 80% | Reads 5% high | Subtract 5% from all future readings |
| Off by more than ±10% | Significantly inaccurate | Apply offset, or consider replacement |
Adjusting Analog Hygrometers
Most quality analog hygrometers have a small adjustment screw on the back — typically accessible with a small flathead or Phillips screwdriver. After completing the salt test with the bag still sealed, use the screwdriver to adjust the needle to read exactly 75%. This physically recalibrates the instrument rather than requiring you to remember and apply an offset.
Not all analog hygrometers have this adjustment screw. If yours doesn't, the offset method is your only option. Digital hygrometers typically cannot be physically adjusted, but many have a calibration offset function in the settings menu — check your manual for this feature.
Method 2: Boveda Calibration Packs
Boveda produces calibration packs specifically designed for hygrometer calibration — pre-packaged saturated salt solutions sealed in a Boveda membrane, calibrated to a specific RH level (75% is most common). The procedure is identical to the salt test, but more convenient. No salt preparation, no water measurement, no risk of getting the salt-to-water ratio wrong.
The trade-off is cost — a Boveda calibration pack is a one-time-use purchase, whereas the salt test uses materials you probably already have. For a single calibration, either method works. If you're calibrating multiple hygrometers simultaneously, the salt test is more economical.
How Often to Calibrate
- Digital hygrometers: Once on initial setup, then annually or when readings seem inconsistent with what you're observing in the humidor.
- Analog hygrometers: Every 6 months. Analog sensors drift more consistently than digital sensors.
- After any significant event: A hygrometer that's been dropped, stored in extreme conditions, or exposed to water should be recalibrated before being trusted again.
- When something seems wrong: If your humidor seems drier than expected despite normal humidification, calibrating your hygrometer is the first diagnostic step.
Practical Notes
Temperature during calibration matters. The 75% RH of the salt test is accurate at approximately 68–77°F (20–25°C). If your home runs significantly outside this range during calibration, the result may be slightly off. Calibrate at the temperature you normally store cigars.
Don't rush the process. 8 hours is the minimum for reliable results. 12 hours is better. The sensor needs time to fully equilibrate — reading at 4 hours often produces a result that's still moving toward its final value.
Recalibration doesn't fix mechanical damage. A hygrometer that's been dropped or has bent components may read inconsistently even after calibration. If your readings are erratic — jumping around rather than slowly drifting — the sensor may be damaged and calibration won't help.