Your humidifier and your hygrometer disagree, and the humidifier is usually the one that is wrong

The sensor inside a humidifier sits in the wettest air in the room, because the machine put it there.

It is mounted in the housing, often within 30 cm of the mist outlet, reading air the appliance has just added water to.

A separate meter two metres away is reading the room instead. Both are right about different air.

Assume the appliance is measuring its own output until proved otherwise

A built-in humidistat exists to stop the tank running dry into an already damp room. It was never designed as a room instrument.

Manufacturers know this, which is why the better models let you site a remote sensor or pair a separate one.

The cure costs a few pounds. A separate digital hygrometer is the cheapest instrument in the house.

Move the meter, then wait, because most disagreements are placement

Put the meter at roughly 1 to 1.5 m above the floor.

Keep it away from the mist plume, away from a radiator, and away from an external wall.

An external wall in a British house in winter runs several degrees colder than the room, and cold surfaces read high.

Kitchens and bathrooms run as their own microclimates, so neither is a fair test of the house.

Then leave it alone for 20 to 30 minutes.

Capacitive humidity sensors respond slowly. A reading taken after two minutes is largely a reading of the last room the meter was in.

Relative humidity is relative to temperature, and that is not a figure of speech

Air holds roughly 6 to 7% more water vapour for every degree Celsius it warms.

So an unchanged amount of moisture reads as a different percentage at different room temperatures.

Nothing was added or removed between those three bars. Only the thermometer moved.

That is why an unheated hallway reads damp while the sitting room reads dry on the same February afternoon.

A cold corner is not imagining its damp; in relative terms the air really is wetter there.

Which also explains the winter complaint about dry air. Cold outdoor air brought inside and heated by 10°C loses roughly half its relative humidity on the way.

The salt test tells you which instrument to believe

Saturated table salt holds the air above it at 75% relative humidity at normal room temperature. That is a physical constant rather than a calibration you have to trust.

1. Half fill a bottle cap with ordinary table salt and add a few drops of water — damp, not dissolved.
2. Seal the cap and the meter together in a freezer bag or a lidded jar.
3. Leave both somewhere with a steady temperature for 8 to 12 hours.
4. Read the meter through the bag without opening it.

Anything from 72 to 78% is a pass.

Consumer sensors carry a stated tolerance of ±3 to ±5 percentage points and will not do better than that.

Further out than 78 or below 72 is an offset. Note it and subtract it, or replace a meter that costs less than a filter.

A reading that swings by a few points is the humidistat working

A humidifier stops at its target and restarts three to five points below it, to keep the fan from short-cycling.

So a display cycling between 45 and 50% is behaving exactly as designed.

Judge the machine on where that swing is centred, not on either end of it.

The figure worth acting on is the one from a properly placed meter, read across a whole evening rather than a single moment.

If you want the sensor argument settled in the appliance itself, the Levoit Classic 300S review covers how its humidistat holds a set point.