Sensor Technology 13 stages of CO2 composition (200 ± 10 ppm to 500 ± 10 ppm


ppm) under a constant temperature of 20 ± 1 ºC, constant water vapour concentration of 12,000 ± 500 ppm and constant pressure of 101.3 ± 1 kPa. To demonstrate the multipoint calibration results, Figure 2 shows the time series of CO2 H2


and O compositions (ppm) measured by a LI-COR 7815 reference


grade gas analyser sampling from one of the MACE exposure chambers. It can be observed that there is a high degree of stability of the generated conditions inside the chamber, as well as a good adherence to the target gas concentrations.


The second example shows a multipoint calibration of H2


O under constant CO2 composition of 405 ± 10 ppm and


environmental conditions similar to the previously described example. This assessment had the objective of establishing traceable calibration factors for the humidity effect on the sensors’ response to CO2


O compositions (ppm) measured by a reference grade gas analyser sampling from one of the MACE exposure chambers. Once again, good stability and adherence to the target levels can be observed.


calibration results, Figure 3 shows the time series of CO2 H2


Authors:


Nick Martin is a Principal Research Scientist and the Science Area Leader of the Air Quality and Aerosol Metrology Group at NPL. He is a Member of the Royal Society of Chemistry, a Chartered Chemist, Chartered Scientist (MRSC CChem CSci) and an Associate of King’s College London (AKC). He joined NPL in 1990, having completed his BSc in Chemistry, AKC, and PhD in kinetic spectroscopy at King’s College London. This was followed by postdoctoral research at the Department of Physical Chemistry, University of Cambridge employing infrared lasers with molecular beams. At NPL he was fi rst involved in developing a ground-based laser heterodyne spectrometer to detect stratospheric molecules. More recently, Nick developed pumped and diffusive sampling methods for measurements of nitrogen dioxide, VOCs, and ammonia using a Controlled Atmosphere Test Facility (CATFAC). He was involved in the “Breathe London” pilot consortium, which implemented a new network of low-cost sensors for hyperlocal monitoring of air quality in London and projects concerned with measuring in-cabin air quality and bleed air on aircraft, and QA/QC for NO2


diffusion tube networks.


Nick is a member of DEFRA’s Air Quality Expert Group (AQEG) and is involved in CEN standardization committees including CEN TC264 WG42 for low-cost sensors.


. To demonstrate these multipoint H2 and


O


Gabriel Garcia graduated in chemistry (BSc) and conducted academic research in atmospheric chemistry to conclude his master’s and PhD degrees at the Sao Paulo State University (Brazil). During his studies he delivered projects for developing passive samplers for tropospheric ozone measurements and investigating the relevance of atmospheric deposition as sources of nitrogen and phosphorus for aquatic ecosystems. Gabriel joined NPL in 2019 as a Higher Research Scientist and is a Member of the Royal Society of Chemistry. More recently, he has been involved in laboratory and fi eld-based research projects and bespoke customer measurement services regarding gas sensors’ testing, ambient and indoor air quality assessments, and quality assurance for atmospheric measurements.


Correlating the sensors’ response to CO2 with these environmental


parameters should identify which factor(s) are most infl uential on the sensors performance (absolute or relative humidity) and calculating adjustments to the compensation algorithms.


Conclusions


The Multiple Atmosphere Controlled Environment (MACE) is a recently commissioned type testing facility for evaluating the performance of low cost gas sensors. It has been successfully deployed in metrological research projects that underpin product development for air quality monitoring purposes. The presented case study highlighted how the MACE capabilities can be deployed to validated novel carbon dioxide (CO2


O compositions (ppm) measured by a reference grade gas analyser sampling from one of the MACE exposure chambers.


Figure 3: Results of the H2 H2


Finally, the third example is a more complex experimental design, which had the objective of discriminating the cross- interference effects of absolute humidity and relative humidity on the sensors’ response to CO2


. To achieve this goal, the


exposure regime aimed at generating changes in relative humidity by alternating means: changing temperature and changing absolute humidity. The CO2


composition and pressure


were kept constant at 405 ± 10 ppm and 101.3 ± 1 kPa, respectively. Figure 4 shows the time series of temperature (ºC) and relative humidity (%) measured by a calibrated Michell Instruments WR293 reference hygrometer inside one of the MACE exposure chambers. Blue bars on the top of the graph indicate the periods in which the absolute humidity was kept constant. The vertical black lines indicate the moment where the absolute humidity was changed to a different value.


Author Contact Details Nick Martin • National Physical Laboratory • Address: Hampton Road, Teddington, Middlesex UK, TW11 0LW • Tel: 020 8977 3222 • Contact us at:www.npl.co.uk/contact • Web: www.npl.co.uk


Analogue humidity sensor for industrial applications


Sensirion’s SHT40I-Analog humidity sensor is designed for harsh industrial environments where high noise levels prevent the use of digital solutions. The sensor is ideal for applications such as air-conditioning and refrigeration, providing stable performance over the long-term. Like the digital version, the analogue sensor offers a 5 V supply voltage and robust housing to withstand harsh conditions. An optional variable power heater allows for operation in high-humidity and condensing atmospheres. Customised output characteristics and fi lter membrane options are available upon request.


“The SHT40I-Analog is built on the established foundation of our fourth-generation humidity sensor platform, enabling new possibilities in the industrial applications fi eld,” says Matthias Scharfe, Product Manager for Humidity and Temperature Sensors at Sensirion. “With this launch, we now offer a perfect solution for long-term stability in applications where system noise is a critical factor.”


60036pr@reply- direct.com


For More Info, email: email:


WWW.ENVIROTECH-ONLINE.COM Nick Martin Gabriel Garcia O multipoint calibration. Time series of CO2 and


Figure 4: Time series of temperature (ºC) and relative humidity (%) during the water vapour cross-interference test.


During the periods of constant H2 O concentration, the changes


in relative humidity were caused by the applied changes to the temperature. The events marked by vertical lines indicate the changes to relative humidity caused by increased absolute humidity. Additionally the test was set up to deliver three episodes of 30 ± 1 % relative humidity at different combinations of temperature and absolute humidity.


) low cost


sensor systems, which involved the generation of traceable gas compositions alongside the stringent control of environmental conditions such as humidity, temperature and pressure. These capabilities are directly applicable to the performance evaluations required by the CEN/TS 17660-1:2021 and NPL will soon obtain UKAS accreditation for providing this service.


Further details of other exciting projects and initiatives can be found at: www.npl.co.uk/commercial-services


For More Info, email: email:


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