Metrology for humidity at high temperatures and transient conditions

It has been estimated that the drying of products costs European industry around 30 B€ per year in associated energy costs, and to enhance European competitiveness improvements in efficiency need to be made. For every 0.1 % improvement in efficiency through better process controls 30 M€/year could be saved. Monitoring humidity under transient conditions and at temperatures above 100 C is an essential factor in controlling drying processes in the food industry, the second largest manufacturing sector in the EU with a turnover of over 900 B€. Currently humidity sensors, such as hygrometers, are calibrated at temperatures of 80 C or less, thus if a sensor is used at 100 C or above, there is no guarantee in the traceability of its measurements. The pharmaceutical, food and feed industries need improved humidity measurements at these high temperatures to enable better process control. One parameter controlling the quality of food is water activity (a_w) which describes the effect water in the environment has on a product. In order to determine a_w, samples are taken and measured using laboratory analysers. During this process a significant quantity of material is wasted, and energy consumption for this process is not optimised due to sampling errors and transient conditions.

This project addressed the most pressing needs for industry in this area. A high-temperature mass fraction water vapour/steam generator and two new facilities were built for the calibration of sensors, allowing the development of traceable humidity standards for temperatures up to 180 C and pressures up to 600 kPa. A new modular calibration setup for measuring humidity under transient conditions for fixed temperatures and temperatures from 100 C to 180 C was validated, demonstrating a 40% reduction in calibration time compared to existing methods. Three field humidity calibrator prototypes were designed and tested, and a new type of hygrometer based on direct Tuneable Diode Laser Absorption Spectroscopy (dTDLAS) was developed for process environments up to 180 C. This instrument was then validated at ambient pressure and temperatures between 40 C to 100 C. A new SI traceable in-line water activity measurement method was validated and a sample-based system for sorption isotherm measurements, which is the relationship between water content and the equilibrium relative humidity of a material, was developed, tested and launched as a service.

To demonstrate the new methods and instrumentation they were then applied in relevant industrial applications. The humidity calibrators were successfully used in a dairy manufacturing process and at a pharmaceutical plant and the a_w methodology was demonstrated at a pet-food manufacturer and for in-process monitoring during hazelnut drying. The new methodologies, calibration services and instrumentation delivered will support sectors such as pharmaceutical, food and feed industries which rely on accurate humidity measurements, helping them reduce energy consumption and waste. Moreover, through more efficient and reliable quality assurance and new products, they will enable a wide range of industrial enterprises in the EU to enhance their competitiveness.