Towards improved humidity measurements at high temperatures and transient conditions

Humidity is a key parameter in controlling drying processes and ambient conditions in many industrial manufacturing, storage and test applications. In many cases, improved humidity measurements increase productivity through better energy and/or material efficiency. In industrial dryers, the humidity of the high temperature exhaust air/gas (typically around 140 °C but can be up to 180 °C depending on the process) is measured. Ideally, higher inlet air temperatures reduce the process time but require careful and fast in-line temperature humidity vs. time control for early detection of the end-point in order to prevent deterioration of the material exiting the dryer. Similar process control challenges are associated with drying/baking in food processing, drug manufacturing, paper production, ceramic production, wood processing and textile production. However, humidity calibrations are not usually performed at temperatures above 100 °C and the calibration equipment that is commercially available for industry cannot be operated in this range. On the other hand, non-static conditions in processes and environmental tests form another type of challenge because different humidity sensors have different response times. Temporal and spatial variations of humidity in a process line make it also impossible to measure water activity, i.e. equilibrium relative humidity, directly although this is an important parameter in food processing. A European project HIT was started in 2015 to improve the accuracy of industrial humidity measurements at high temperatures up to 180 °C and non-static conditions by developing improved measurement and calibration techniques. These techniques will enable extending the temperature range of SI traceable humidity calibrations for industrial humidity sensors. Also, new measurement methods for transient humidity conditions and spatial variations and for measuring water activity in line will improve the control of humidity conditions in food processing, storage of pharmaceutics and testing electronic components. All the developments will be demonstrated and validated in industrial applications. In this presentation, we report developments of humidity calibration setups for temperatures up to 180 °C and absolute pressures up to 6 bar. Also, systems designed for laboratory and field calibrations with non-static calibration schemes are described. Developments of a new hygrometer based on direct Tunable Diode Laser Absorption Spectroscopy (dTDLAS) and a new in-line water activity measurements method are reported. In addition, a numerical tool for the adsoption/desorption analysis in transient regime has been developed and experimentally validated. The HIT project is co-funded within the EMPIR initiative by European Union’s Horizon 2020 programme and the EMPIR Participating States.