Abstract
Mobile applications based on gas sensing present new opportunities for low-cost air quality monitoring, safety, and healthcare. Metal oxide semiconductor (MOX) gas sensors represent the most prominent technology for integration into portable devices, such as smartphones and wearables. Traditionally, MOX sensors have been continuously powered to increase the stability of the sensing layer. However, continuous power is not feasible in many battery-operated applications due to power consumption limitations or the intended intermittent device operation. This work benchmarks two low-power, duty-cycling, and on-demand modes against the continuous power one. The duty-cycling mode periodically turns the sensors on and off and represents a trade-off between power consumption and stability. On-demand operation achieves the lowest power consumption by powering the sensors only while taking a measurement. Twelve thermally modulated SB-500-12 (FIS Inc. Jacksonville, FL, USA) sensors were exposed to low concentrations of carbon monoxide (0–9 ppm) with environmental conditions, such as ambient humidity (15–75% relative humidity) and temperature (21–27 °C), varying within the indicated ranges. Partial Least Squares (PLS) models were built using calibration data, and the prediction error in external validation samples was evaluated during the two weeks following calibration. We found that on-demand operation produced a deformation of the sensor conductance patterns, which led to an increase in the prediction error by almost a factor of 5 as compared to continuous operation (2.2 versus 0.45 ppm). Applying a 10% duty-cycling operation of 10-min periods reduced this prediction error to a factor of 2 (0.9 versus 0.45 ppm). The proposed duty-cycling powering scheme saved up to 90% energy as compared to the continuous operating mode. This low-power mode may be advantageous for applications that do not require continuous and periodic measurements, and which can tolerate slightly higher prediction errors.
Highlights
Market forecasts [1] indicate that the number of portable air quality devices will increase with respect to fixed air quality stations over the few years
The response of the sensors to the experimental conditions described in Figure 3 was recorded for two weeks
The patterns of the continuously operated sensors showed no relevant variations between the calibration and validation samples
Summary
Market forecasts [1] indicate that the number of portable air quality devices will increase with respect to fixed air quality stations over the few years. The high market penetration of smartphones [2] and wearables has opened a market for mobile applications based on gas sensing. Adding a chemical analysis capability to smartphones targets the in situ detection of Volatile Organic. Very low levels of VOCs have been found to contribute to the “sick building syndrome”, which degrades workers’ health and decreases productivity [3]. At higher concentrations, both short- and long-term exposure to VOCs are known to create health risks [4].
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