Grow science · 14
Calibrate your hygrometer
Every humidity target is only as good as the meter that reads it.
Why it matters
Cheap digital hygrometers are often 3–5 % off, sometimes more, and they drift over time. At 26 °C, a 5 % error in humidity shifts your VPD by 0.17 kPa. That’s a third of the whole veg range.
The salt test
A saturated table-salt solution holds the air above it at about 75 % relative humidity, almost independent of room temperature. You need:
- a bottle cap or small dish
- plain table salt (no anti-caking additives if possible)
- a few drops of water
- a zip bag or airtight box big enough for the hygrometer
Step by step
- Fill the cap with salt and add water drop by drop until it’s a wet slurry. Wet, but the salt must not fully dissolve.
- Put the cap and the hygrometer into the bag or box and seal it. Don’t let them touch.
- Keep it at a stable room temperature, out of the sun and away from the grow light.
- Wait 8–12 hours, then read the meter without opening the bag.
Read the result
| Meter shows | Meaning | Correction |
|---|---|---|
| 75 % | Accurate | none |
| 79 % | Reads 4 % high | subtract 4 % |
| 71 % | Reads 4 % low | add 4 % |
Many meters have a calibration offset in their settings. Otherwise write the correction on the back with a marker. Test again every few months.
Want a second point? Magnesium chloride does the same at about 33 %. That’s useful because many sensors are off by different amounts at high and low humidity.
Where to put it
- At canopy height, not on the floor or near the tent roof.
- Shaded from the lamp. Direct light heats the sensor and lowers the reading.
- Away from the humidifier, dehumidifier and exhaust opening.
Now your numbers are real: ideal humidity by stage.
Questions growers ask
How do I calibrate a hygrometer?
Seal it in a bag or box with a cap of wet table salt (a slurry, not fully dissolved) for 8–12 hours. The air in the bag settles at about 75 % humidity. The difference to your reading is the error.
How accurate are cheap hygrometers?
Often 3–5 % off and sometimes more, and they drift over time. Test them every few months.
Why is my hygrometer reading too low under the grow light?
Direct light heats the sensor. Warmer air at the sensor means lower relative humidity. Shade the hygrometer and place it at canopy height.
Can I calibrate at a low humidity too?
Yes. Magnesium chloride does the same trick at about 33 %. Many sensors are off by different amounts at high and low humidity, so two points tell you more.