Special cases
Be prepared to seek advice - some humidity measurements need special care:
Temperature change
If measuring a gas at one temperature and using it at another, measured values may need to be converted or interpreted. In particular, relative humidity falls when temperature rises (and rises when temperature falls).
Pressure change
If measuring a gas at one pressure and using it at another, measured values may need to be converted or interpreted. Some humidity parameters change when gas is expanded (examples are: dew point and mass of water per unit volume).
Low (vacuum) pressures
Humidity measurements can be made below atmospheric pressure, but need some thought to set up and interpret.
Non-air gases
Humidity is measured in a wide variety of gases, such as fuel gas ('natural gas') and various process gases. Take care to choose a measurement method suitable for the particular gas. Beware of humidity readings displayed in units that are gas-dependent, and be ready to interpret (convert) for the gas used. Wherever units related to mass, or volume ('ppm') are displayed, they may be internally calculated with an assumption of measuring in air.
Harsh environments
Dust, vibration, condensation, airborne droplets, chemicals ... all pose challenges to most hygrometers.
Older hygrometers
Modern electronic humidity sensors have increasingly long lifespans, if treated well. If you are using a relative humidity sensor or dew-point probe that you know to be old, don't expect the best performance. Some older hygrometers may suffer from drift, slowed response, etc.
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