Gas Sensor Characterization Apparatus

A measuring apparatus was designed for testing the gas sensors (Figure 5). The apparatus allows for the precise adjustment of gas concentrations using Bronkhorst mass flow controllers MFC1 to MFC3 (F-201DV-AAD-22-K in the range 10 mL–500 mL, F-201EV-AAD-22-K in the range 40 mL–2000 mL). Synthetic air (SA: 21% O₂ and the rest N₂) is used as a carrier gas, which also serves as a purging gas. The exact concentration of the test gas can be obtained by mixing it with the carrier gas or by diluting the saturated vapor of the desired solvent from the bubbler with dry air. This second method is mainly used to generate different concentrations of humidity and volatile compounds (ethanol, methanol, acetone, toluene, cyclohexane, etc.). The resulting mixture is then injected into the test chamber with the tested sensor. The volume of the test chamber is 50 mL. A Keithley 2400 sourcemeter was used to measure the resistance. A relay multiplexer is used to switch individual sensors, which is controlled by a National Instruments USB-6351 DAQ (Data Acquisition) device (16 analog inputs, 24 digital inputs/outputs, 2 analog outputs, maximum sampling rate 125 MS/s). The resistance values of the sensors are measured every 250 ms. The connection of the sensors between the multiplexer and the sourcemeter is performed using a coaxial cable to reduce interference from external electromagnetic fields. The VICI EUTA-4VL4MWE2 four-port two-way valve is used to switch the flow of gasses (test gas–purge gas) to ensure a rapid increase to the target concentration of the test gas, and to avoid the influence of overflows during switching. The apparatus and measurement process are controlled by the LabView application.
The change in sensor resistance (∆R/R₀) as a function of exposure time was investigated. The response of the sensor is given by the change in relative resistance,

where R_g represents the resistances upon exposure to a specific gas and R₀ is the reference resistances in synthetic gas. The fabricated sensors were used for the detection of ammonia (NH₃), carbon dioxide (CO₂), nitrogen dioxide (NO₂), acetone (CH₃COCH₃), toluene (C₆H₅CH₃), and humid air (RH) under various concentrations at room temperature.