Short communication‘Envilution™’ chamber for performance evaluation of low-cost sensors
Graphical abstract
Introduction
Unlike more expensive instruments used for regulatory purposes, there has been an emerging trend for using low-cost sensors (LCSs), due to the rising demand for affordable air quality monitors (Morawska et al., 2018; Rai et al., 2017). The affordability, flexibility and portability associated with the LCSs, which are generally small in size, with low power consumption and offering the possibility for more spatial data and higher temporal frequency whilst easing the data transmission/collection and recovery process (Austin et al., 2015; Kumar et al., 2015; Snyder et al., 2013), make LCSs available to the public. In addition, deploying a network of LCSs under known atmospheric conditions and emission sources can assist in (i) detecting pollution hotspots; (ii) assessing real-time exposure for designing mitigation strategies; and (iii) creating pollutant bottom-up emission inventories (Kumar et al., 2015). As a result of this emerging technology, new projects have been funded to establish LCS networks for trial purposes in both the European Union and the US (e.g., CITI-SENSE; iSCAPE; USEPA Air Pollution Monitoring for Communities Grants) (CITI-SENSE, 2016; iSCAPE, 2019; USEPA, 2016). Additionally, air pollution as a highly topical subject has attracted substantial interest from companies to innovate using these types of LCSs in the Internet of Things applications to create disruptive solutions for consumers and business. However, due to the lack of uncertainty, certifications and standards, users are unaware of the performance of LCS under different environmental conditions and pollutant concentrations. In fact, they have hardly any alternative options for calibration other than co-location against reference instruments, which is costly and covers only a limited range of variations (environments and concentrations) under uncontrolled/narrow conditions. Hence, evaluating the performance of LCS for both research and environmental awareness as a function of environmental parameters has been an emerging research topic in recent years.
In characterising performance, the cost and inflexibility of field experiments noted earlier (e.g., calibration using co-location) means that environmental-pollution (referred to as ‘Envilution™’) chamber, which can control both environmental conditions and pollutant concentrations, can be instrumental for evaluating the performance of LCSs. Envilution™ chamber, hereafter referred in this manuscript to as ‘Chamber’, can generate controlled environments for ambient temperature, relative humidity (RH), and other parameters (e.g., atmospheric pressure, air pollutant concentrations), become an increasingly useful tool for this space. Chambers, as a state-of-the-art tool, offer the ability for studying/testing a single or a range of meteorological conditions and known concentrations of different pollutants during LCS testing (Austin et al., 2015; Papapostolou et al., 2017; Wang et al., 2015).
Although global market of LCSs is growing by more than $3 billion annually towards the year of 2027 (Pugh, 2016), this expected growth in sensor technology is due to the interest of agencies or citizens who are unable to track individual's exposure under different microenvironment conditions (Williams, 2018). Current research studies revealed the importance of test chambers for performance validation due to poor to modest performance of LCSs compared with reference instruments employed by regulatory agencies. In addition, to cover the global market and end-user prospective, companies need to cover a wide range of testing to minimise the risk and time to market before developing products based on LCSs. Hence, the ability to use a chamber to control single environmental variable at a time is perceived to offer a faster rate of development, through the ability to quickly test components, iterate and validate to solve issues before moving into the more complex field environment. The laboratory chamber designed by Air Quality Sensor Performance Evaluation Centre at South Coast Air Quality Management District in US (Papapostolou et al., 2017) and the chamber used by the National Physical Laboratory in UK (NPL, 2019) are the most comprehensive and the most recent state-of-the-art testing units for this purpose around the world. However, the high associated costs with a chamber of this purpose mean that this approach becomes prohibitive for most research laboratories around the world. Hence, simpler designs for limited studies, e.g. Wang et al. (2015) for particle testing, Jayaratne et al. (2018) for RH testing, and Ren and Cao (2020) with a transparent chamber equipped with a visualisation device for indoor experimental and modelling studies on CO2 concentration have likewise been developed.
Compared to the available simple and detailed chambers in the market (Alpert et al., 2017; Bernard et al., 2016; NPL, 2019; Papapostolou et al., 2017; Weiss Technik), a chamber has been designed and manufactured at the Global Centre for Clean Air Research (GCARE), University of Surrey, UK, to generate required environmental conditions (temperature/RH), various pollutant types and concentrations (fine particulate matter (PM2.5; fraction of particles with an aerodynamic diameter ≤ 2.5 μm), nitrogen dioxide (NO2), nitric oxide (NO), sulphur dioxide (SO2), and carbon monoxide (CO) – see Supplementary Information (SI) Table S1) for LCSs' testing with unique features, such as state-of-the-art 3D printed components, lightweight and small size, affordable cost (around £10k, see SI Table S2), low maintenance/operational costs and automated bringing ease in operation.
The chamber has been designed to operate with a number of pollutants, such as PM2.5, CO, NO, NO2, and SO2 pollutants due to the rising demands of their high spatiotemporal resolution monitoring in local environments, which can aggravate pre-existing conditions such as asthma and cause sufferers to be admitted to hospitals as well as the mortality risk from cardiovascular diseases (Morawska et al., 2018; Northcross et al., 2013; Sousan et al., 2016a). This paper describes design details and performance assessment of the chamber, while preliminary assessment of six LCSs in terms of two environmental parameters (temperature and RH) and PM2.5 concentration is included for demonstration.
Section snippets
Chamber
Fig. 1 shows a schematic diagram of the chamber with a list of major components. This 125L chamber (interior dimensions of 50 cm × 50 cm × 50 cm) was made of acrylic sheets sprayed with Teflon, which is isolated by 100 mm of Styrofoam. The sheets were used to minimise surface reactions for gaseous and aerosol experiments (Liu et al., 2016). The interior edges of the chamber were sealed with rubber strips as well as silicone sealant to prevent leakage. The chamber has been designed to be able to
Results and discussion
The capability of the chamber is demonstrated in a series of experiments on both temperature/RH control and PM2.5 concentrations as elaborated below.
Conclusion
Generating and maintaining different environmental conditions and pollutant concentrations by using an affordable Envilution™ chamber is a highly useful facility to evaluate the performance of LCSs. Here, we describe the engineering design, specifications and performance of the chamber. A set of experimental studies was designed and conducted to demonstrate its performance in maintaining stable conditions including meteorological parameters and PM2.5 pollutant. The results demonstrated the
CRediT authorship contribution statement
Hamid Omidvarborna: Formal analysis, Methodology, Writing - original draft. Prashant Kumar: Conceptualization, Funding acquisition, Resources, Methodology, Supervision, Writing - review & editing. Arvind Tiwari: Data curation, Validation, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work has been carried out as a part of an Innovate UK funded project 'Pollution Guardian' under the Technology Strategy Board File Reference: 104572. We thank Dr Thor-Bjorn Ottosen from the Global Centre for Clean Air Research (GCARE) at the University of Surrey for his inputs during the early design phases of the chamber. We also acknowledge the co-operation and comments from Robert Alfonsi and John Byrne from All about the Product Ltd. on the manuscript. AT and PK thanks the University
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