How to calibrate a hygrometer
How to calibrate a hygrometer
Seal the hygrometer in an airtight bag with either a Boveda 75% calibration pack or a bottle cap of damp salt, leave it 24 hours at room temperature, then compare the reading with 75%. The difference is your offset: adjust the hygrometer if it allows it, or subtract that figure from every future reading.
- Both methods target exactly 75% relative humidity
- Leave it a full 24 hours — readings taken at 6 or 12 hours are not settled
- A new digital hygrometer is commonly 3-5% out of the box
- Recalibrate every 6-12 months; digital sensors drift
The salt test, step by step
Put roughly half a teaspoon of ordinary table salt in a bottle cap or small dish. Add water a few drops at a time until the salt is damp and slushy but not floating — you want wet sand, not soup.
Put the cap and the hygrometer in a zip-lock bag or an airtight container, without the hygrometer touching the salt. Seal it.
Leave it somewhere with a stable temperature for 24 hours. Do not open it to check.
A saturated salt solution holds the air above it at 75% relative humidity. Whatever your hygrometer reads after 24 hours, the gap between that and 75% is its error. A reading of 71% means it reads 4% low, so a later reading of 65% is really 69%.
The Boveda calibration kit
A Boveda one-step calibration kit is a sealed 75% pack and a bag. Put the hygrometer in with the pack, seal, wait 24 hours, read. Same principle, no mess, and it removes the main way people get the salt test wrong — too much water.
It is the method worth using if you own several hygrometers, since one kit does them all.
Adjusting versus offsetting
Some digital hygrometers have a calibration button or a recessed screw. If yours does, adjust it until it reads 75% while still in the bag.
Most cheap units do not. In that case write the offset on a piece of tape and stick it to the back. A hygrometer that reads 4% low is perfectly usable as long as you remember it reads 4% low.
Analogue needle hygrometers, especially the ones bundled with inexpensive humidors, are often 10% or more out and drift badly. If your readings never seem to make sense, the instrument is the most likely culprit.
Related questions
The variations of this question people search for most.
How long does it take to calibrate a hygrometer?
24 hours at a stable room temperature, whether you use the salt test or a Boveda calibration pack. Shorter tests give false readings because the air inside the bag has not reached equilibrium. Do not open the bag to look partway through.
How do you calibrate a Govee hygrometer?
The same way as any other digital hygrometer: seal it in a bag with a Boveda 75% pack or damp salt for 24 hours. Govee units let you set a calibration offset in the app, so enter the difference between the reading and 75% rather than adjusting the hardware.
How often should I calibrate a hygrometer?
Every six to twelve months. Digital sensors drift slowly, and a hygrometer reading 5% high will have you running a humidor at 60% while believing it is at 65%, which dries cigars out over months without any obvious cause.
Why does my hygrometer read 75% after calibration?
Because it is still in the calibration bag, or was only just taken out. A hygrometer takes a couple of hours to settle to a new environment. Put it back in the humidor, close the lid, and read it the next day.
How do you calibrate a digital hygrometer for a humidor?
Identically to any other: seal it in an airtight bag with a Boveda 75% pack or damp salt for 24 hours, then compare the reading with 75%. Most digital units for humidors have a calibration button or an app offset; if yours has neither, note the error and subtract it from future readings.
How do you calibrate an analogue or dial hygrometer?
Same 24-hour salt or Boveda test, but adjustment is mechanical: most dial hygrometers have a small screw on the back that rotates the needle. Turn it until the needle reads 75% while the unit is still sealed in the bag. Dial hygrometers drift more than digital ones, so check twice a year.