Buying and Placing a Hygrometer (Including the Test That Tells You If It's Lying)

Every article here eventually says the same thing: measure your room. This is the one that explains how, because “get a hygrometer” turns out to hide three decisions that all affect whether the number you read means anything.

What to buy

You do not need an expensive one. A digital hygrometer in the ten-to-twenty-dollar range is adequate for this job, and the specification that matters is accuracy, usually quoted as ±2% to ±5% RH. Anything quoting no accuracy figure at all is a guess in a plastic case.

Two features genuinely worth having:

• Min/max memory. It records the highest and lowest readings since you last cleared it, which is how you discover that your room swings twelve points during a set without standing there watching it.
• A temperature reading too. Temperature moves your cure as well, and you may as well see both numbers on one screen.

Analogue dial hygrometers — the ones that look like a small barometer — drift badly and are tedious to recalibrate. Skip them.

Where to put it

This is the part that ruins most readings.

Put it at working height, where the client’s face actually is. Not on a shelf by the door, not on the reception desk, not on the wall behind you. Air near the floor, near the ceiling, and near a doorway are all measurably different from the air six inches above a reclined client’s eyes, and that last one is the only air your adhesive ever touches.

Keep it away from anything that makes weather. A hygrometer sitting next to a humidifier reads the humidifier, not the room. Same for a vent, a window, a radiator, or a fan — see what your HVAC is doing.

Leave it in place. Moving it between readings means you’re comparing two different rooms.

Give it twenty minutes to settle after you first put it somewhere. These sensors do not respond instantly, which is also why a reading taken thirty seconds after you walk in is not your room.

The test that tells you if it’s lying

A cheap hygrometer that reads eight points high is worse than no hygrometer, because you will confidently act on a wrong number. There is a standard way to check, it costs nothing, and it takes a day.

The salt test:

1. Put about two teaspoons of table salt in a small dish or bottle cap.
2. Add a few drops of water — just enough to dampen it. Do not dissolve it. You want wet salt, not salt water.
3. Put the dish and the hygrometer together in a sealed zip-top bag, without the salt touching the device.
4. Leave it somewhere with a stable temperature, ideally around 70 °F, for at least 6–8 hours; 12–24 is better.
5. Read it without opening the bag.

A saturated salt solution holds the air above it at 75% RH. So a correct hygrometer reads 75.

If yours reads 71, it runs four points low — either adjust it if it has a calibration function, or write “+4” on a piece of tape and stick it to the back. Knowing the offset is as good as fixing it.

Re-test once a year, and any time a reading suddenly stops matching what the room feels like.

What to do with the number

Read it before your first set, every day, for a week, and write it down. One reading tells you almost nothing; seven tells you which band you actually work in and how much your room moves.

Then take it to the 45–60% rule, or run it through the Matcher if you’d rather have it interpreted for you.

And if it turns out you’re sitting comfortably in the window already, that’s a genuinely good result. It means your retention problems are somewhere else, and you’ve just eliminated the biggest variable for the price of a takeaway coffee.

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Related: The 45–60% rule → · Your HVAC and your adhesive → · Retention troubleshooting →