Humidity
Our work comprises the following projects:
- Provision of humidity standards
- Development of humidity standards and traceability for industrial conditions
- Development of capability for measurement of moisture in materials
Other projects:
Water vapour flux
NPL has worked with a group of suppliers and users of instruments to develop an improved method of calibrating sensors for measuring transepidermal water loss (TEWL).
Skin water loss is an indicator of skin barrier function, and therefore skin health. Measurements of TEWL can be used to detect damage (often invisible) as a result of exposure to solvents, for example. TEWL is measured in dermatological research, workplace health monitoring, cosmetics research, and defence, among others.
However, without consistent calibration, TEWL instruments can disagree dramatically (disparities of up to a factor 2 have been seen), rendering comparisons between research studies extremely difficult. The calibration method, developed collaboratively at NPL, is based on evaporation of a measured droplet of water, in wells of different depths, to produce a range of water vapour fluxes. Calibration by this method improves agreement between instruments, and reduces the achievable measurement uncertainty to a few percent.
Humidity in industrial conditions
NPL is developing facilities to calibrate hygrometers in industrial process gas conditions.
Although humidity calibrations are usually performed in air at atmospheric pressure, industrial processes often involve other gases, and at other pressures. In 'non-ideal' gases the behaviour of water vapour is less well known than in air. The more different a gas is from standard air, the more its thermal and transport properties will differ, with the possibility that sensors will perform differently. If so, how applicable are humidity calibrations performed under 'standard conditions'?
NPL is constructing and validating humidity generators for calibrating humidity sensors at elevated pressures and non-air gases. This will enable users to obtain calibrations that are appropriate to the conditions of use. There will be scope for study of the sensitivity of different sensor types to gas pressure and composition. Measurement and calibration in natural (fuel) gas is particularly of interest.
Moisture in materials
NPL is beginning new work on measurement of moisture content in materials.
Although air humidity is measured in many applications, very commonly humidity is only a 'proxy' for the property of real interest – material moisture content. Direct measurements of moisture content of materials are also widespread, using a variety of established methods. However traceable calibration is not common for many methods.
Ideally, moisture measurement traceability and calibrations would be provided through reference materials or by calibrations of instruments. Laboratory-based reference methods are established (Karl Fischer titration and oven drying), but both have their limitations, and the transfer of measurement traceability to many other methods is not straightforward or widely practiced.
There is interest in many moisture variables, including bulk moisture, surface moisture, moisture profile, moisture transport, and moisture action at interfaces. However a convenient and useful starting point is moisture in powders.Powders are high in surface area and moisture is critical to their processing. Some moisture measurements can only be applied to powders. Accordingly, powders are a promising area for initial study. NPL is developing a measurement capability for moisture in powders, aiming to apply this in various NPL and external projects, with the later goal of working on calibration methods.
Air temperature
Relative humidity varies strongly with air temperature. At room temperature a temperature change of 1 °C affects relative humidity by more than 6 percent of value. Thus accurate air temperature measurements are essential for reliable control of relative humidity - and nowhere more so than in humidity standards and calibrations. NPL is investigating the scope for improved calibration of air temperature sensors, and how this might benefit humidity science, and other users needing precise gas temperature measurements.