NPL has facilities for measurement of moisture content in a variety of materials and sample types with services offered including measurements, research studies, training, consultancies and instrument assessments.

Measurements of the bulk moisture content of materials are possible in the facility through various physics-based methods. A range of instruments utilising gravimetric, coulometric and microwave resonance principles are available. Measurement traceability comes from moisture certified reference materials and traceability of supporting weighings to national standards of mass.

Gravimetric moisture measurement - mass loss on drying

Measurements of mass loss on drying are used as a means of evaluating moisture content.

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.

A drying oven and an analytical balance (10 µg resolution) are used in order to make loss-on-drying measurements with traceability to NPL mass standards.

Also available is a thermogravimetric moisture analyser (bench-top integrated heater and balance), which displays real-time values of moisture content calculated from changes in the sample mass measured during drying. This portable instrument incorporates a radiative heating element in a lid above a top-pan balance with a 1 mg resolution.

Coulometric analysis of evolved vapour

Often, a measurement of (specifically) water content is needed, rather than moisture content. Water-specific measurements can be made using an evolved vapour analyser.

A water specific coulometric sensor measures the total amount of evolved vapour released into a dry gas stream that passes over samples heated inside the instrument's oven. The mass of water released is compared to the initial mass of the sample and the resultant mass fraction is reported as water content.

Suitable temperature profiles for heating can be selected to enable fractions of free water, capillary water and chemically bound water in the sample under test to be resolved with a resolution of the order of a few nanograms of water.

Microwave resonance

Rapid, non-destructive measurements of the bulk moisture content of samples can be made using a microwave resonance technique.

The microwave resonance in the cavity of this instrument is perturbed when samples containing water are inserted. The resulting shift and broadening of the resonance peak depends on the amount of water present. Material-specific calibration is provided from traceable measurements using the other instruments in the facility. Real-time changes in material moisture content can be monitored.

Examples of practical measurements

Drying of particular powders has been studied, with the aim of optimising an industrial drying process. By measuring samples dried under varying conditions, target drying temperatures and durations were identified to obtain moisture content within given limits.

Moisture uptake in materials has been assessed, including progressive non-destructive measurements of changes in moisture content of individual samples.

Real time, non-destructive measurements have also been made of the variations in moisture content of granular bulk materials when exposed to air at several well-defined humidities.

Current Work

Certified reference materials for moisture or water content

NPL has a project to investigate the development of certified reference materials for water content (or moisture content), where needs exist in industry. We welcome input on requirements or solutions in the areas of practical calibration, measurement traceability, and certified reference materials (CRMs) for either water content or moisture content.

References

• NPL Report Eng 7: Needs for NMS support for measurements of moisture in materials, P A Carroll and S A Bell. Unrestricted report. May 2008.
  
  
• NPL Report Eng 21: Calibration issues for measurement of moisture in materials , P A Carroll, S A Bell and M Stevens. Unrestricted report. January 2010.
  
  The above are available as free downloads.
  
  
• Non-destructive Moisture Content Measurement of Bioabsorbable Polymers used in Medical Implants. P A Carroll, S A Bell, A S Maxwell and P E Tomlins. Paper published online in the International Journal of Thermophysics, May 2011.

Contact

For further information, please contact Paul Carroll