Impact on metrological and scientific communities

The ongoing EURAMET Project 1400 – Moisture metrology development brings together European institutes working in physical and chemical metrology. Active since 2016, it supports measurement comparisons, the dissemination of measurement traceability, the development of calibration and measurement capabilities, laboratory accreditation, knowledge exchange and capacity building.

Within this wider framework, SoMMet developed soil-specific reference methods, uncertainty approaches, transfer standards and sampling procedures. This strengthened the basis for traceable laboratory calibration of point-scale soil moisture sensors and for transferring metrological traceability towards outdoor measurement conditions.

The project developed and evaluated primary-level reference methods and transfer standards for measuring soil water and moisture content. TÜBİTAK UME developed a primary-level Evolved Water Vapour System (EWS) that combined the direct measurement of extracted water vapour with flow-based loss-on-drying. This allowed soil water content and total moisture content to be assessed from the same sample. DTI adapted its existing evolved-water-vapour primary standard for soil measurements, while CMI applied an oven-coupled volumetric Karl Fischer titration method for the selective determination of water in soil samples.

The primary measurement standards of DTI, TÜBİTAK UME and CMI were evaluated alongside that of LNE-CETIAT in EURAMET Pilot Study P1685. Measurements of organic and inorganic soil samples showed bilateral agreement between the participating laboratories, supporting the comparability of their reference methods.

At the transfer-standard level, DTI developed a portable system based on flow-based loss-on-drying and validated it using both organic and inorganic soils. TÜBİTAK UME developed a portable visible and near-infrared spectroscopic system for rapid, non-contact and non-destructive soil moisture measurements. The spectroscopic system was calibrated with SI traceability using the TÜBİTAK UME primary-level reference system and was tested under both laboratory and field conditions. Together with the calibration facilities available at CMI, these methods and instruments supported new or improved capabilities for calibrating and evaluating point-scale soil moisture sensors.

For cosmic-ray neutron sensing, the consortium characterised CRNS detector responses in reference neutron fields, evaluated the SI traceability and uncertainties of neutron response functions, and benchmarked neutron-transport models and digital twins. It also developed calibration and validation practices for applying CRNS under outdoor conditions.

Field campaigns at the SoMMet test sites compared point-scale sensors, CRNS systems and remote-sensing products. This work led to good-practice guides covering CRNS field calibration, the harmonisation of soil moisture measurement methods and the fusion of soil moisture observations across spatial and temporal scales.

The results provide a foundation for further work by National Metrology Institutes, Designated Institutes, calibration laboratories, soil moisture researchers and observation networks. They can support more consistent uncertainty evaluation, new or improved calibration capabilities and greater comparability between measurements made using different methods and at different scales.

Some measurement approaches developed during SoMMet were subsequently taken up in the GrainMet project, extending traceable moisture and water-content measurements to plant-origin bulk materials and food-related applications.

SoMMet's work also fell within the broader remit of EURAMET's Technical Committee for Thermometry, which encompasses humidity and moisture metrology, including relevant standards and reference materials.