Sensors & transducers


spaceX aBB sensor to detect greenhouse


gas emissions Figure 5: Arterial carbon dioxide pressure (PaCO2) waveform


breath cycle. The shape of the waveform can change quite dramatically depending on patient condition. Accurately and stable sampling the waveform over time can represent a significant


challenge for the CO2 sensing system. The number of breaths is normally in the


range of 12-20 per minute so the average sampling rate of the system does not need to be


very high. Capturing the ETCO2 level as represented by the peak CO2 in the above waveform is straightforward.


However, as discussed previously, new diagnostic requirements are driving the need for


improvements in CO2 waveform morphology. This means the waveform must be captured in near real time, and at high precision. In order to achieve this, a number of obstacles need to be overcome. For the sensor to perform correctly, the gas that


has been measured has to be removed from the system before the new gas can be sampled. In most side-stream applications, the expired gas is pumped into the sensing system and it is helpful to minimise the overall gas volume in the sensor and connecting pipes. GSS sensors are designed to have a small gas measurement chamber, which only requires approximately 2.8ml of gas for a single measurement. The sensor sampling rate is an essential


component in ensuring the waveform is captured with high fidelity. In addition to the basic amplitude of the waveform, the spatial frequency content is critical in providing more accurate waveform morphology. Harry Nyquist was helpful in defining the Nyquist Sampling Theorem. It states that a band-limited continuous-time signal can be sampled and


perfectly reconstructed from its samples if the waveform is sampled over twice as fast as the highest frequency component. GSS has two sensors that are specifically


designed for high-speed sampling of CO2 gas. The SprintIR6S samples at 20Hz and the SprintIR-R at


50Hz. This means that in the case of the SprintIR- R, and assuming the gas flow rate can support it, the sensor can resolve spatial frequency components of up to 25Hz at 12-bit resolution. This makes it ideal to support a new generation of capnography monitoring applications.


Digital Filtering


The signal from the LED is detected by the photo-diode within the sensor. This signal is intrinsically noisy and is typically filtered by the sensor before it is read out by the user.


However, time dependent indicators in the CO2 waveform can be corrupted or removed entirely by this digital filtering process. For most CO2 sensing applications, the time


domain signature is not that important, and the user is generally only concerned with the slow evolution of concentration levels over extended periods of time. Note that the sampling speed is not the same as responsiveness. There are many


applications where the CO2 sensor must be able to respond very rapidly to the change in concentration but where the time domain signature is not critical. However, accurate CO2 morphology for


patient diagnostics is entirely dependent on both the fidelity of the waveform in time as well as in amplitude. All GSS sensors have the ability to output unfiltered data. This allows the user to deploy sophisticated digital filtering techniques without the need to compromise responsiveness, sampling speed or signal fidelity.


COnClUSiOn


GSS offers a number of technologies that are well matched to the emerging requirements of


next generation CO2 monitoring applications. Its range of SprintIR sensors are low power, compact and compatible with pumped gas analysis systems. Combined with their accurate, high speed and resolution sampling features, they provide the user with the ability


to analyse the CO2 waveform with unmatched fidelity. Gas Sensing Solutions


www.gassensing.co.uk 46 A


n optical sensor manufactured by ABB was deployed with the successful launch of satellite Hugo from GHGSat,


the emerging leader in greenhouse gas sensing services in space. The ABB supplied optical sensor can map


methane emissions from space at a resolution that is 100 times higher than any other sensors. Whilst previously only larger regions could be surveyed, for the first time the new greater granularity now allows the identification of the source of emissions. An additional nine units are currently under manufacture at ABB to be launched by the end of 2022 ready to be on-board across the first private satellite constellation dedicated to emission measurement. Space offers the ideal location to freely


monitor emissions across jurisdictions and quantitatively report on improvements. The ABB sensors will provide valuable insights which will enable governments and industries around the world to meet their emission reduction targets and reduce the negative impact on global warming. “We selected ABB for its ability to deliver


world-class instruments while meeting the challenges of a new space company like ours.“ said Stephane Germain, CEO of GHGSat. “We strive to innovate for the needs of the future, and we’re excited to work with ABB to achieve that.”. “ABB shares GHGSat’s goal of reducing


emissions through the creation of their greenhouse gas sensing constellation. Our selection as the manufacturer for these advanced sensors demonstrates our competitiveness and strong fit with the private space sector requirements.” said Marc Corriveau, general manager ABB Measurement & Analytics Canada. “The space revolution is well underway and


ABB with its heritage of unique space instruments and serial production of advanced measurement sensors for industrial applications is extremely well positioned to serve this emerging sector.”


ABB www.abb.com February 2021 Instrumentation Monthly


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