TRANSDUCERS, TRANSMITTERS & SENSORS


The CARBOCAP CO2 sensors can thrive in a variety of environments


but this does not provide the ability to detect poor ventilation issues in specific spaces. Temperature is undeniably the most


important control parameter in occupied spaces. Some systems also measure and control humidity to maintain a level of 40-60% RH. Temperature measurements do not generally suffer from drift, but traditional humidity sensors do, so Vaisala’s HUMICAP sensors are preferable because of their long- term stability and insensitivity to interferences such as dust and condensation. These thin- film capacitive humidity sensors have become the industry standard in a variety of applications where long-term accurate, reliable, maintenance-free humidity measurements are required. Increased humidity levels can be an


indication of human activity and poor ventilation. However, humidity varies as a result of external factors (e.g. freezing dry conditions or rainy humid conditions) rather than as a result of human exhalation. Temperature and humidity monitoring play


an important role in a BMS, but where facility managers need to take into account the occupancy of people and reduce human- generated pollution in spaces, CO2 is the ideal additional parameter for automatic ventilation control. Carbon Dioxide (CO2) is exhaled by people


as they breathe, so an accumulation of CO2 indicates that (a) people are in the room and (b) the ventilation is insufficient, so a good ventilation system should be able to detect this and automatically apply the correct amount of ventilation. The system must be automatic, and it must be able to ventilate individual spaces, so that each space is ventilated optimally and energy is not wasted. The ASHRAE Green Standard 189.1(USA) and


the European standard FprEN 16798-3 recommend using demand controlled ventilation (DCV) to reduce energy usage while promoting healthy indoor air. From an HVAC design perspective, CO2 is an


ideal proxy for indoor air quality where the building is predominantly occupied by people. Humidity would be either better or at least a useful additional parameter. Typically, outdoor air contains 250 to 400


ppm CO2. In contrast, exhaled breath contains around 50,000ppm CO2which represents a 100 fold increase over inhaled gas, so without adequate ventilation, when people are indoors, CO2 levels will gradually rise. Occupied spaces with good air exchange


may contain 350-1,000 ppm CO2, but anything above this can induce drowsiness, with levels above 2,000 ppm causing headaches, sleepiness, poor concentration, loss of attention, increased heart rate and slight


a building owner/facility manager. For the former, the system must work perfectly immediately, and for at least the period of the warranty, but for the latter the requirement is more long-term. The cost of a good sensor fades into


insignificance in comparison with the benefits that it provides. Energy savings from accurate, need-based controls can be considerable, but even more importantly, the health and well- being of people inside of the building are protected and indoor conditions improve workplace performance. The ideal solution is therefore to opt for


nausea. Exposure to very high levels (from oil/gas burners or gas leaks) can even result in fatalities from asphyxiation. Recommended minimum ventilation rates


are provided for a wide variety of indoor spaces in ANSI/ASHRAE Standard 62.1-2019 Ventilation for Acceptable Indoor Air Quality. Several studies have evaluated the effects


of CO2 concentration on cognitive function. For example, Allen et al (2016)9


found that, on


average, a 400-ppm increase in CO2was associated with a 21% decrease in a typical participant’s cognitive scores. DCV based on CO2measurements can therefore deliver improvements in well-being and productivity that outweigh the costs of the system itself. It is important to resist the temptation to


purchase the cheapest sensors that meet the required specification. This is because, whilst accuracy and range are important; the ongoing performance of the BMS will rely on the stability of the sensors. Suppliers of HVAC systems will naturally


prefer sensors that you can ‘fit and forget’. Consequently, it is necessary to select sensors that do not require frequent recalibration to prevent drift. However, the selection process is further complicated by sensors that claim to compensate for drift by implementing a software solution which assumes that the lowest measured readings are the same as the average outdoor concentration of CO2. The danger with this type of algorithm is that small errors are compounded as time passes; leading to significant errors in the longer term. As an attempt to avoid true calibration, these software algorithm sensors are not applicable in spaces that are continuously occupied, and can also be fooled by building automation systems that aggressively ramp down fresh- air intake during off-peak hours. In some cases even the concrete in the walls may absorb CO2 and thereby ‘trick’ the algorithm and create further inaccuracy. There is potential for a slight conflict of interest between a BMS supplier/installer and


Vaisala CARBOCAP CO2 sensors. This is because they employ dual-wavelength NDIR technology capable of thriving in a variety of environments and are able to conduct true self-calibration with an internal reference. The cost of this technology is insignificant compared with the energy costs of a BMS that is not efficient or with the cost of maintenance when low-cost sensors drift or fail. It is not uncommon for Vaisala’s sensors to


operate trouble-free for up to 15 years. This stability and reliability has been recognised around the world… and beyond. In summary, disease prevention measures


can be re-enforced by smart ventilation with reliable CO2measurements. Furthermore, good indoor air quality can have a significant positive impact on the health and well-being of people inside buildings.


References:


1. Kampf, G. et al. (2020) Persistence of coronaviruses on inanimate surfaces and their inactivation with biocidal agents. Journal of Hospital Infection. 2. Ratnesar-Shumate S, et al. (2020). Simulated sunlight rapidly inactivates SARS-CoV-2 on surfaces. The Journal of Infectious Diseases


3. World Health Organization: https://www.who.int/news- room/q-a-detail/coronavirus-disease-covid-19-how-is-it-tra nsmitted


4. Greenhalgh. T. et al (2021). Ten scientific reasons in support of airborne transmission of SARS-CoV-2. THE LANCET. https://doi.org/10.1016/S0140-6736(21)00869-2 5. www.gov.uk/government/news/new-film-shows- importance-of-ventilation-to-reduce-spread-of-covid-19#:~ :text=Coronavirus%20is%20spread%20through%20the,vi rus%20transmissions%20happen%20indoors. 6. https://ricochet.media/en/3423/there-is-still-time-to- address-aerosol-transmission-of-covid-19 7. Fennelly, K.P., (2020). Particle sizes of infectious aerosols: implications for infection control. THE LANCET, Respiratory Medicine, VOLUME 8, ISSUE 9, P914-924. 8. Kudo.E. et al (2019) Low ambient humidity impairs barrier function and innate resistance against influenza infection. Proceedings of the National Academy of Sciences, 116 (22). 9. Allen J.G. et al. (2016) Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments. Environmental Health Perspectives 124:6 CID: https://doi.org/10.1289/ehp.1510037


Vaisala www.vaisala.com


JUNE 2021 | PROCESS & CONTROL 31


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