How does a hygrometer work?

Capacitive — what is in almost everything

The dominant mechanism, and the one behind essentially every modern reading.

A hygroscopic polymer film, typically under a micrometre thick, sits between two electrodes. That sandwich is a capacitor, and its capacitance depends on the dielectric constant of what is between the plates.

Water has a very high dielectric constant — around 80, against roughly 3 for the dry polymer. So as the film absorbs water from the air, the capacitance climbs steeply and measurably. The device reads that capacitance and maps it to a percentage through a factory calibration curve.

Two things make it the winner: the film is thin enough to reach equilibrium with the air in seconds, and the response is close to linear in relative humidity across most of the range.

How it fails: the film is altered over time by contaminants, solvents and repeated condensation. The capacitance still changes with humidity, but no longer along the curve the calibration assumes — so the reading drifts, usually by a few percent a year.

Resistive — the cheap cousin

Same absorption idea, different electrical property. A salt or conductive polymer changes its electrical resistance as it takes up water.

Cheaper to make and adequate for switching a fan on. Less stable, more temperature-dependent, and generally the sensor behind a device that disagrees with its neighbour by ten points.

Hair tension — the mechanical original

Human and horse hair are hygroscopic, and absorbed water pushes the keratin chains apart. A hair lengthens by around 2.5% between bone dry and saturated — a fraction of a millimetre over a typical strand.

The whole instrument is the amplification of that. A bundle of hairs under light tension pulls on a lever train that multiplies the movement into a pointer sweeping a dial. De Saussure built the first practical one in 1783 and the design was still in service two centuries later.

How it fails: hair loses elasticity, takes a permanent set, and responds ever more sluggishly. An old dial hygrometer is usually reading low and slow. Traditionally they were "regenerated" by wrapping them in a wet cloth for half an hour, which is a real procedure and a partial fix.

Psychrometer — thermodynamics instead of a material

Two matched thermometers, one with a wetted cotton wick over the bulb, with air moving across both — either by whirling the assembly on a handle or with a small fan.

Water evaporating from the wick takes latent heat with it, cooling that thermometer. How far it cools depends on how readily the surrounding air accepts more water: in dry air, a lot; in saturated air, not at all. The wet-bulb depression — the gap between the two readings — gives the humidity.

The elegance is that nothing is being calibrated. It is a thermodynamic relationship, which is why psychrometers remained the reference for so long and why the instrument still matters.

How it fails: a dirty or dried-out wick, insufficient airflow, or the two thermometers not actually matching each other.

Chilled mirror — measuring the definition

A polished metal mirror is cooled, usually by a thermoelectric element, while an optical sensor watches its reflectivity. The moment condensation begins to scatter the light, the mirror's temperature is recorded — and that temperature is the dew point.

This is the only common instrument that measures a humidity quantity directly rather than inferring it, and it is the reference the others are calibrated against, at around ±0.2°C.

How it fails: contamination. Salt or dust on the mirror lets water condense at the wrong temperature, so the mirror needs periodic cleaning and good instruments include an automatic cycle for it.

Which is in yours

Whichever you have, the salt test will tell you how far off it currently is — which for anything that has been on a wall for a few years is usually more than its specification claims.