National Physical Laboratory

Measurement of moisture content in solids and liquids

Thermogravimetric moisture analyser
Thermogravimetric moisture analyser,
balance with incorporated infrared
heating element

NPL has established a facility for studying the moisture content of materials, now available for measurement services, consultancies and instrument assessments. This includes a range of instruments capable of measuring the bulk moisture content of materials through physical means using gravimetric, coulometric and microwave resonance principles.

The techniques currently available in the facility include:

Gravimetric moisture content determination

A separate drying oven and analytical balance (10 µg resolution) are used in order to make loss-on-drying measurement with traceability to NPL mass standards. Samples are weighed before and after oven drying and the mass fraction lost is expressed as the resultant percentage moisture content.

Also available is a thermogravimetric moisture analyser (bench-top integrated heater/balance) which displays real time values of moisture content from calculations made 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 1 mg resolution.

Coulometric analysis of evolved vapour

Capillary water fraction
Capillary water fraction resolved from
results of coulometric analysis of
evolved vapour from polyethylene
terephthalate (PET) sample

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

Microwave Resonance

Rapid, non-destructive measurements of the bulk moisture content of samples are possible once a material specific calibration against a suitable reference method has been performed. The microwave resonance in the cavity of this instrument is perturbed when samples containing water are inserted. Calibration data relating the shift in resonance to moisture or water content of a sample, is provided from traceable measurements using the other instruments in the facility.

Examples of Practical Measurements

The drying temperatures and durations required for powders to reach required moisture contents for more efficient industrial processing have been assessed.

Moisture uptake in materials has been assessed, including progressive non-destructive measurements of changes in moisture content using the microwave resonance instrument.

References

    Effect of inserting filled applicator into instrument cavity
    Effect of inserting filled applicator into instrument
    cavity on resonance frequency and bandwidth
    (Source: Sartorius)
  • 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.

    These 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

Customer Service tel: +44 20 8943 8681
E-mail: moisture_enquiries@npl.co.uk

Last Updated: 5 Oct 2016
Created: 14 Jul 2010

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