How to calibrate a hygrometer

Every hygrometer drifts, dials the most

A hygrometer is the gauge that reads relative humidity, and no hygrometer stays accurate on its own. The only question is how far off yours is right now.

Analog dials in cheap humidors usually run on a metal coil with a salt-treated paper strip. The coil flexes as the paper takes on water and swings the needle. Wikipedia's hygrometer entry puts their error at 10% or more. That's the difference between a healthy humidor and a moldy one.

Digital units mostly use capacitive sensors. Calibrated, they're good to about 2% between 5 and 95% RH. Uncalibrated, two to three times worse. They also drift with age and contamination. Better than a dial, yes. Trustworthy out of the box, no.

Why damp table salt reads 75%

A saturated solution of sodium chloride, meaning salt and water with undissolved crystals still in it, holds the air above it at a fixed relative humidity of about 75%. Chemistry does the work. No batteries, no brand.

The figure comes from Lewis Greenspan's 1977 compilation for the National Bureau of Standards, now NIST, which fitted over a thousand measurements of saturated salt solutions. Table salt barely moves with temperature, which is what makes it such a handy reference. Magnesium chloride sits near 33% and is the usual low point for a second check.

Values are Greenspan's, rounded to one decimal. His stated uncertainty for salt at room temperature is around 0.1 to 0.2 points. Calling it 75% is close enough for cigars.

The salt test, step by step

The salt test takes a bottle cap of salt, a few drops of distilled water, a sealable bag or container and a day of waiting.

1. Make the slurry. Fill a bottle cap or small dish with table salt. Add distilled water a few drops at a time and stir until it looks like wet sand. You want damp salt, not salty water. Undissolved crystals must remain.
2. Seal it up. Put the cap and the hygrometer in a zip bag or small airtight container. Keep the salt from touching the sensor.
3. Hold the temperature steady. Leave it in a room-temperature spot out of sun and away from vents. Swings slow the test down.
4. Wait 24 hours. Don't open it to peek. Every opening starts the clock again.
5. Read through the bag. Note the reading before you unseal anything. That number, against 75, is the offset.
6. Adjust or record. Turn the calibration screw on the back of an analog dial, use the adjust function on a digital unit, or write the offset on a sticker.

Calibration kits skip the mess

A calibration kit is a sealed humidity pack set to a known RH, usually 75%, packaged in a resealable bag that the hygrometer goes into. Same principle as the salt test, no slurry.

Boveda's instructions call for at least 24 hours in the bag at a room temperature of 65 to 75°F, then either adjusting the hygrometer to 75% or noting the difference for future readings. They advise calibrating a new hygrometer before it goes in the humidor and repeating every six months.

Two-point kits add a second, dry pack, around 32 or 33%. One point tells you how far off the gauge is at 75%. Two points tell you whether that error holds steady or grows across the range. A dial that reads 3 high at 75% and 8 high at 33% isn't simply offset. It's stretched, and a single correction won't fix it.

What do you do with the offset?

The offset is the true value minus what the hygrometer showed, and you add it to every future reading. If the test reads 71%, the offset is plus 4. A humidor reading of 66% is really about 70%.

Keep the sign straight. A gauge that reads high gets a negative offset. One that reads 79% in the salt test needs minus 4.

For two points, the correction becomes a line instead of a single number. The hygrometer calibration calculator does both: one-point offset or two-point slope, with your humidor's reading corrected on the spot. Then the 70/70 humidity calculator tells you whether the corrected number is right for your room's temperature.

When a salt test gives a strange number

A salt test that reads far off, or reads differently twice, usually points to the test setup rather than the hygrometer. Check three things before you blame the gauge.

• The salt dissolved. No crystals left means it's no longer saturated, and the humidity won't sit at 75%. Add more salt.
• The bag leaked. Room air creeping in drags the reading toward the room's humidity. Use a fresh bag or a jar with a gasket.
• The temperature moved. A test near a window or a heating vent never settles. Move it and run it again.

If the same gauge reads 10 points off in two clean tests, it's not worth correcting. Swap it.

Hygrometer calibration questions

Where the 75% salt figure comes from

1. Humidity fixed points of binary saturated aqueous solutions (Greenspan, 1977), Journal of Research of the National Bureau of Standards (NIST)
2. Hygrometer: coil and capacitive sensors, saturated salt calibration, Wikipedia
3. How do I calibrate my hygrometer?, Boveda