A hygrometer is calibrated using the salt test: damp salt in a sealed container creates a stable 75% RH environment, letting you adjust the dial or apply a correction factor.
For the full breakdown, see our best Hygrometer For Greenhouse guide.
A quick calibration check fixes that.
What the Salt Test Does
The salt test leverages a simple physical principle: This creates a known reference environment. By placing your hygrometer inside that space, you can read its error and either adjust the mechanism or mentally record the offset for future readings.
How to Calibrate a Hygrometer With the Salt Test
- Prepare the salt paste. Put two tablespoons of regular table salt into a small cup or bottle cap. Add just a few drops of water — stir until the salt is damp and paste-like, not fully dissolved. Too much water destroys the 75% environment.
- Seal everything in a container. Place the damp salt and your hygrometer inside an airtight zipper bag or food container. Keep the hygrometer dry and away from direct contact with the salt paste.
- Wait 6 to 12 hours. Let the sealed bag sit undisturbed at room temperature. Most manufacturers, including INKBIRD, recommend at least 6–8 hours for stabilization. Leaving it overnight is the safest bet for accurate results.
- Read the hygrometer without opening the bag. If possible, read the value through the bag. The stable reading should be 75% relative humidity. If it matches, your device is accurate.
- Adjust or record the offset. If your hygrometer has an adjustment screw (common on analog models), use a small screwdriver to turn the dial to 75%. For digital units, check for a calibration button. If there is no adjustment, simply record the error — for example, if it reads 70%, add 5% to every future reading.
| Test Condition | Expected Reading | Action Required |
|---|---|---|
| Stable salt-paste environment | 75% RH | No adjustment needed; your gauge is spot on. |
| Stable salt-paste environment | 70% RH (reads low) | Adjust dial upward or apply a +5% correction to all readings. |
| Stable salt-paste environment | 80% RH (reads high) | Adjust dial downward or apply a -5% correction to all readings. |
| Poor seal or excess water | Fluctuating or erratic | Redo the test with a proper airtight seal and damp paste, not a puddle. |
Professional Calibration Methods
For most hobbyists and growers, Fluke’s professional calibration guide is a helpful reference for understanding the limits of field methods.
FAQs
How long does the salt test take?
Leaving it overnight is the most reliable approach, as opening the container too early can invalidate the reading and force you to start over.
What if my hygrometer doesn’t have an adjustment screw or button?
For example, if it reads 70% in the 75% salt test, add 5% to all future readings.
Can I calibrate an analog hygrometer without salt?
References & Sources
- INKBIRD. “How to Calibrate a Hygrometer.” Describes the saturated salt-solution method for digital hygrometers and the steam-check for analog models.
- Fluke. “How to Accurately Calibrate a Hygrometer.” Professional guidance on humidity-chamber calibration using distilled water and controlled temperature ramps.