Humidity
Humidity is the presence of water vapour in air or other gas. Water vapour is the gaseous form of water, and can be thought of much like any other gas. Humidity can be expressed in relative terms (i.e. relative humidity) or in various absolute terms (such as vapour pressure, dew point, and concentration).
Air humidity directly affects the moisture content of materials. The presence or absence of water influences a vast range of physical, chemical and biological processes. Structures may deform or lose strength when their water content changes. Water is a key agent in corrosion, and is critical to the formation and structure of chemicals from simple salts upwards. Bacterial, fungal and all higher plant and animal life forms incorporate water and depend on finely balanced interactions with water. Measurement of humidity is of widespread importance, and can be critical to product quality, reliability and lifespan.
NPL has a range of facilities for humidity calibration and measurement in-house or off-site. There is also a programme of research and development of calibration methods for humidity and related measurements.
Humidity research
Water Vapour Flux
NPL has collaborated with suppliers and users of instruments to develop improved calibration of sensors for measuring water vapour loss from skin (known as TEWL - transepidermal water loss).
Humidity in industrial conditions
NPL is developing facilities to calibrate hygrometers in industrial process gas conditions.
Moisture in materials
NPL is beginning new work on measurement of moisture content in materials.
Air Temperature
Relative humidity varies strongly with air temperature. NPL is investigating the scope for improved calibration of air temperature sensors.
- NPL calibrates air temperature sensors - both those incorporated within relative humidity instruments, and thermometers used purely for air temperature measurement.
- NPL can calibrate dew-point measuring hygrometers against an NPL primary standard or a transfer standard in the range -90 °C to +90 °C.
- NPL offers a mobile service to calibrate or characterise the temperature and humidity performance of climatic chambers.
- Calibrations can be provided for capsules, ampoules or other suitable artefacts that use salt solutions or other chemical substances to generate stable values of humidity.
- NPL calibrates humidity generators by comparison against transfer standards, or directly against NPL's own primary standard generators.
- A two day course on humidity measurement and calibration covering dew point, relative humidity and many other humidity quantities, it will concentrate on methods of measurement which are of greatest technological relevance to attendees.
- NPL can calibrate humidity instrumentation of practically all types, including relative humidity sensors, dew-point hygrometers, psychrometers, spectrometers, humidity generators, climatic chambers and others.
- NPL has established a facility for studying the moisture content of materials, now available for measurement services, consultancies and instrument assessments.
- Relative humidity is derived from dew point and air temperature. NPL's standards and calibration service span a range from 2 %rh to 98 %rh at air temperatures from –40 °C to +100 °C.
- NPL frost-point standards and services extend well into the trace moisture range below 1 mmol/mol (i.e. below 1 ppm corresponding nominally to frost points below –75 °C).