What Is a Humidity Sensor and How Accurate Can It Be

Relative, not absolute

Humidity on a consumer or light-industrial instrument is relative humidity: the water in the air as a percentage of what that air could hold at its current temperature. Because the denominator moves with temperature, the same air reads differently in a cold room and a warm one. This is why a dew point figure, which our consoles derive alongside RH, is often the more useful number when you are comparing two spaces.

How the element works

The sensing element is normally capacitive: a polymer film between two electrodes absorbs water vapour, which changes the dielectric constant and therefore the capacitance. The electronics convert that to a percentage. It is a good, cheap, repeatable technique, and it is also why humidity sensors dislike condensation, dust and solvent vapour — all of which change the film.

What the tolerance covers

A ±3% figure is a tolerance across a stated range under stated conditions, not a guarantee at every point. At the extremes — below 10% or above 95% RH — almost every capacitive element degrades, which is why our outdoor range is stated as 1% to 99% at ±3% and the indoor one as 10% to 99% at ±5%. Read the range and the tolerance together; either alone is meaningless.

Drift and calibration

Capacitive elements drift with age and with exposure. On our consoles humidity can be calibrated manually against a reference, which is the practical answer for a customer who has an instrument that now reads three points high. For anything where the number is evidence rather than information, plan on periodic comparison against a calibrated reference.

Where you place it matters more than the spec

A sensor on an outside wall, above a radiator or in direct sun reads the wall, the radiator or the sun. Most complaints about humidity accuracy that reach us turn out to be placement, not the element.