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Mass Flow Control for Gas Sensor Characterization and Calibration

Tuesday, April 7, 2015

This month's news article looks at a local application with Cambridge CMOS Sensors who are an industry leader in sensor technology. Their patented CMOS MEMS gas sensors use Micro-Hotplates technology to provide lower power consumption and faster response times for monitoring air quality and hazardous gases. A test platform was required for characterization and calibration of the sensors, during which carbon monoxide, ethanol or volatile organic compounds (VOCs) would be pumped into a test chamber with each sensor.

Because their own sensors are so fast, Cambridge CMOS Sensors needed a mass flow control system that also responded rapidly to generate varying gas concentrations in the test chamber. Each test would require different concentrations of the gases at flow rates ranging from 0.1 ml/min to 500 ml/min. Fast flow stabilization was also necessary to gather transient data and to conduct leak testing of the test chamber.

We responded to these requirements with a responsive and flexible Alicat mass flow control system. The 50-ms valve response times for setpoint changes ensure that test gas concentrations are achieved rapidly and remain stable for the duration of each test. Alicat’s on-board multiple gas calibrations and wide 200:1 turndown ratio significantly reduced the equipment costs for setting up the sensor test platform. Each 0.1-500 ml/min flow range could be covered by just two Alicat mass flow controllers (500-ml/min and 10-ml/min models), with local pipework tees allowing high or low flows to be selected.

Two pairs of mass flow controllers were selected: one pair for carbon monoxide and ethanol, and a separate pair with anti-corrosive 316L construction for the volatile organic compounds. These four controllers were paired with a fifth controller for dilution into clean air, and an anti-corrosive mass flow meter to monitor leak rates from the test chamber. All six instruments feature Alicat’s Gas Select system, which provides gas properties data for 30 or more gases and gas mixtures. This makes the devices “future-proofed” and ready to accommodate additional testing with new gases.

All six instruments receive power and communicate through Alicat’s BB9-USB distribution box, which can connect up to nine mass flow or pressure instruments to a computer via a single USB connection and power supply. The fact that Cambridge CMOS Sensors can operate the controllers locally using the touchpad interface means they were able to complete preliminary testing without having to program the units through a computer. Now that their automated test platform is in place, they can control all the units over multi-drop RS-232 using their proprietary software control system.

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