Traceable measurements of moisture or water content
NPL makes measurements of material bulk moisture content, or of water content specifically, using various physical methods. A range of instruments utilising gravimetric, coulometric and microwave resonance principles are available. Measurement traceability comes from certified reference materials and traceability of supporting weighings to national standards of mass.
A wide variety of materials and sample types can be measured in a range down to trace amounts of water. Please enquire for details and uncertainties achievable.
Gravimetric moisture measurement: mass loss on drying
Measurements of mass loss on drying are used to evaluate moisture content. We offer this type of measurement, using a drying oven and an analytical balance with traceability to NPL mass standards. A thermogravimetric moisture analyser (bench-top integrated heater and balance) is also available.
In classic loss-on-drying technique, samples are weighed before and after oven drying, according to a protocol of temperature, dwell time and end-point determination. The protocol usually depends on the material or application and is sometimes documented in published standards. The resultant mass fraction lost is reported as the moisture content. This value of 'moisture content' is not necessarily the same as 'water content', since other volatile substances can be evolved during the drying process, contributing to mass loss.
NPL can make water-specific measurements using two techniques:
1. Coulometric analysis of evolved vapour
Where a measurement of (specifically) water content is needed, measurements can be made using an evolved vapour analyser. In this approach, a sample is heated, and the evolved water vapour is released into a dry gas stream which carries the vapour to a water-specific coulometric sensor. Water fraction of the sample is evaluated from electrolysis current together with traceable weighing of the sample. The technique is suited to small samples and allows resolution of a few nanograms of water. Suitable heating profiles can be selected to study water release as a function of temperature, which is related to the level of physical or chemical binding of water in the matrix material (known as free water, capillary water, water of crystallisation, and others). This gives valuable insight for product drying and other heat treatment processes.
2. Microwave resonance
Rapid, non-destructive measurements of the bulk water content of samples can be made using a microwave resonance technique. This utilises the interaction of water molecules (free water) with a low-power microwave field. Material-specific calibration is provided from traceable measurements using other instruments in the facility. The microwave technique is rapid and non-destructive, so that real-time changes in material water content can be observed.
Examples of practical measurements
Measurement services are highly customised to address particular applications. Examples of past work include:
- surface water content of metal powders for additive manufacturing
- residual water in powders dried under different conditions, to identify target temperature and duration for industrial drying
- water content of adhesives
- water content as a marker of breakdown of bioabsorbable polymers used for medical implants
- soil moisture measurement
Find out more about Calibration of humidity salts