How to Calibrate a Hygrometer: The Salt Test, Calibration Packs and What Accuracy to Expect

Every decision in drying and curing leans on a humidity reading. If your hygrometer reads a few points off, you could pull material too early, jar it too wet, or chase a problem that isn't there. Checking it takes a day and costs almost nothing.

This guide explains how accurate typical hygrometers are, how to check one with the salt test or a calibration pack, and what to do with the result.

Equipment guide only. Growing, processing and storing cannabis is legal in some places and not in others, so follow the laws where you live. Nothing on this page is medical advice.

Why you should check your hygrometer

A hygrometer measures relative humidity: how much water vapor is in the air compared with the most the air could hold at that temperature. Small differences matter in post-harvest work. The difference between commonly cited drying targets and conditions that are too damp is often only a few percentage points.

Inexpensive digital hygrometers are useful, but they are not lab instruments. Manufacturing tolerances, age, and exposure to very humid or dusty conditions can all push a sensor off. Two cheap sensors side by side often disagree, and without a reference you can't tell which one is right.

How accurate are typical hygrometers?

Check the listed accuracy on the product page, usually given as plus or minus a number of percentage points RH. Treat that as a best case. Whatever it says, testing your own unit is the only way to know.

For a wider buying comparison, see best hygrometers for post-harvest rooms.

The salt test, step by step

The salt test relies on a well-established property: air sealed above a saturated solution of table salt (sodium chloride) settles at about 75% relative humidity at normal room temperatures. That makes it a free, reliable reference point.

What you need

• Table salt (non-iodized is often recommended, though ordinary salt is commonly used)
• A small cap, bottle lid or shallow dish
• A few drops of water
• An airtight container or zip-top bag large enough to hold the hygrometer and the dish

Steps

1. Make a damp salt slurry. Fill the cap or dish with salt and add just enough water to make it wet sand. It shouldn't be a liquid; there should be undissolved salt.
2. Seal it in with the hygrometer. Put the dish and hygrometer in the container without touching, then seal it.
3. Keep the temperature steady. Leave it somewhere out of direct sun and away from heat.
4. Wait. Leave it for several hours. Many people leave it overnight or for about a day so the reading has time to settle completely.
5. Read it without opening. Note the reading through the container if you can. A reading of 75% means the sensor is accurate at that point.

Using a calibration pack instead

Some humidity pack brands sell calibration kits: a sealed pack and a bag made for testing hygrometers. They work on the same principle and are more convenient than the salt test. Follow the instructions on the kit, especially the waiting time and the stated reference value, and don't reuse a calibration pack past the point the maker recommends.

A calibration kit at a lower humidity level gives you a second reference point. Checking at two points can show whether a sensor is off by a fixed amount or whether its error changes across the range.

What to do with the result

If your hygrometer has a calibration setting

Many digital models and app-based sensors let you add an offset. Set it so the reading matches the reference (75% for the salt test). Then recheck to confirm.

If it has an adjustment screw (analog)

Many analog dials have a small screw or dial on the back. Turn it so the needle points to the reference value while still in the test conditions, or immediately after removing it.

If it can't be adjusted

Write the offset on a piece of tape on the back of the hygrometer (for example, "+4") and correct readings in your head. This works fine in practice.

Placement and good habits

• Measure where the material is. Put the sensor at the height of drying branches or inside the jar.
• Keep it out of direct airflow from fans or dehumidifier outlets.
• Allow settling time after moving a hygrometer to a new space. Many need some time to stabilize.
• Use the same unit for comparisons so you're always comparing like with like.
• Recheck occasionally, such as before each harvest, since sensors can drift.

For logging conditions over time, see temperature and humidity data loggers.

Hygrometer check routine

• Run a salt test or calibration kit before each harvest
• Record the offset on the back of each hygrometer
• Adjust the unit's calibration setting if it has one
• Test new hygrometers before trusting them
• Replace units with unstable or large errors
• Keep one well-tested unit as your reference

Why temperature matters to humidity readings

Relative humidity depends on temperature. Warm air can hold more water vapor than cool air, so if the temperature in a sealed jar or room changes, the relative humidity changes too, even if no moisture has been added or removed. That has a few practical consequences:

• Test at room temperature. Run the salt test in a stable spot, not next to a window or heater, so you're checking the sensor rather than a temperature swing.
• Let jars settle after moving them. A jar carried from a cool room to a warm one will show a shifting reading for a while.
• Compare readings at similar times of day. Rooms that warm up in the afternoon will show different humidity than the same room at night.
• Use the thermometer too. Most digital hygrometers show temperature as well, and that number helps explain changes in RH.

This is also why a hygrometer that seems to jump around isn't always faulty. Before replacing it, check whether the temperature is moving at the same time. If temperature is steady and the humidity reading still swings, the sensor itself is the likely problem.

Frequently asked questions