Humidity

How a hygrometer works

Every digital hygrometer sold today uses a capacitive polymer sensor. A very thin water-absorbing film sits between two electrodes, and as it takes up or releases moisture its dielectric constant changes, which changes the capacitance of the tiny capacitor formed by the electrodes. The instrument measures that capacitance and converts it to a percentage. It is fast, stable and small enough to sit alongside a temperature sensor on the same chip.

Traditional analogue hygrometers work mechanically instead. A moisture-sensitive element, historically animal hair and now usually a synthetic fibre or paper coil, changes length or curls as it absorbs water, and that movement drives the needle. They are decorative and need occasional recalibration, but they need no batteries.

Dew point, and why it matters more than humidity

Dew point is the temperature to which air must be cooled before its moisture starts to condense. It is a far more useful figure than relative humidity because it does not move when the temperature does — it describes the actual moisture content of the air.

The practical use is simple. If the dew point of the air in a room is 14°C and any surface in that room is colder than 14°C, water will form on it. That is condensation, and that is where mould starts. Comparing dew point against wall and window temperatures tells you whether you have a ventilation problem or an insulation problem, which are fixed in entirely different ways.

What indoor humidity should be

For comfort and health, 40 to 60 per cent relative humidity is the accepted target band. Below about 30 per cent you get static, dry skin and irritated airways, and timber and stringed instruments start to shrink. Above about 65 per cent sustained is where mould and dust mites become a genuine problem.

Different jobs need different targets. Museums and archives typically hold 45 to 55 per cent with minimal variation. Wine storage wants 60 to 70 per cent. Greenhouses run much higher by design, but need control to avoid fungal disease. Server rooms and workshops want the low end.

Accuracy, and what to expect

A good consumer hygrometer is accurate to around plus or minus 3 to 5 per cent RH in the middle of its range, and noticeably worse at the extremes below 20 and above 80 per cent. This is normal and it is why two instruments side by side rarely agree exactly. If you need a reading you can defend, look for a stated accuracy figure and a calibration certificate rather than a bigger number of decimal places.

Recording humidity over time

A live display cannot tell you what happened at three in the morning, and in damp investigations the overnight hours are the ones that matter. A logging instrument such as the TFA KlimaLogg Pro Thermo-Hygrometer with Data Logger records at set intervals so you can plot the whole week, and the wider Humidity Loggers and Data Loggers ranges cover multi-channel and download-to-PC instruments.

What we recommend

For a straightforward room reading, the Technoline WS9450 Temperature & Humidity Station and Technoline WS9430 Indoor Temperature/Humidity Station both do the job cheaply. To compare several rooms at once, the Bresser ClimaTemp Thermo-Hygrometer 6-piece Set gives you six matched units. The Bresser Thermo-Hygrometer with Ventilation Recommendation goes a step further and tells you when to open a window, and the Mobile Alerts MA10230 Indoor Climate Station sends readings to your phone so you can watch an empty property remotely. If you would rather have something decorative, the Weather House Hygrometer & Thermometer is the classic mechanical version.

Related measurements

Humidity is almost always read alongside temperature, and the combined instruments are in Temperature & Humidity and Hygrometers. Because damp is partly a ventilation question, Air Quality Monitors are worth a look. For outdoor humidity as part of a full sensor suite see Home Weather Stations, and for the other elements see the Elements index.