Dr. phil. nat. Christof Huber, Chief Technologist, TrueDyne Sensors AG @ the 4th conference on microfluidic handling systems – MFHS Enschede – October 2019
MULTIPARAMETR GAS MONITORING SYSTEM COMBINING A FUNCTIONALIZED AND A NON-FUNCTIONALIZED Microcantilever
¹TrueDyne Sensors AG, Reinach BL, Switzerland, ²University of Zaragoza, Nanoscience Institute of Aragon, Zaragoza, Spain
We present a novel online gas monitoring system combining a pressure and a temperature sensor with two resonating microcantilevers from which one is functionalized with a highly sensitive hydrophilic material. By simultaneously observing eigen-frequencies and quality factors of the cantilevers gas density and viscosity can be determined as well as gas humidity in a sub-permille range. With this sensor information we can deduce molar concentrations of binary or even ternary gas mixtures in % together with the gas vapor ratio in a low ppm range.
The aim of the presented sensor project is to develop a compact, robust and maintenance free gas monitoring system for industrial use in the field of inert gases e.g. welding or modified atmospheric packaging gas mixing application. In such applications typically binary or ternary mixtures of argon, helium, nitrogen, carbon dioxide, oxygen or hydrogen are used. Gas concentrations must be controlled with accuracies in the % range. In most cases, the humidity must also be monitored in these applications. Typical limit values are between 200 and 40 ppm . The state of the art is the use of several different sensors in a sequence, e.g. thermal conductivity, together with specific optical absorption and dew point sensors. Such installations are bulky and require frequent recalibration. To bring the same functionality in one multiparameter sensor system we combine two resonating microcantilevers together with pressure and temperature sensors on one PCB board that is exposed to the process gas.
Commercially available silicon microcantilevers initially designated for atomic force microscopy are used (see Fig. 1). One cantilever is used to measure the density and the viscosity of the gas as reported by Badarlis et al.  or Huber et al.  and the other to measure water vapor in low concentration by adding a functionalized hydrophilic surface layer as reported for example by Urbiztondo et al .
Figure 1: Sensor PCB with the a first microcantilever in front of a permanent magnet. The second cantilever is placed on the backside of the PCB. Actuation occurs by sending a small AC current over the metal coil on the cantilever tip.
 ISO 14175:2008, “Welding consumables — Gases and gas mixtures for fusion welding and allied processes”
 A. Badarlis, A. Pfau, A. Kalfas., “Measurement and Evaluation of the Gas Density and Viscosity of Pure Gases and Mixtures Using a Micro-Cantilever Beam”, Sensors, 15(9), pp. 24318-24342, 2015
 C. Huber, P. Reith, A. Badarlis, “Gas Density and Viscosity Measurement with a Micro-cantilever for Online Combustion Gas Monitoring”, 19.GMA/ITG-Fachtagung Sensoren und Messsysteme, Nürnberg Germany, 2018
 M. A. Urbiztondo, A. Peralta, I. Pellejero, J. Sesé, M.P. Pina, I. Dufour, J. Santamaría, “Detection of organic vapours with Si cantilevers coated with inorganic (zeolites) or organic (polymer) layers”, Sens. Actuators B, Chem., v. 171–172, pp. 822–831, 2012.