Carbon Dioxide (CO₂) Monitoring for Indoor Air Quality
Introduction to CO₂ Monitoring
Accurate control of critical indoor environmental parameters by a carbon dioxide monitor is not only essential for the well-being and comfort of building occupants, but also has a major impact on energy efficiency. A multitude of parameters can affect indoor air quality (IAQ) from gases such as carbon dioxide, carbon monoxide and volatile organic compounds to particulates, humidity and bacteria such as legionella. Carbon dioxide is one of the greatest variables affecting indoor environmental quality since it is produced by people occupying the building and is also a good indication of the general level of the other pollutants within buildings.
Carbon dioxide production is a function of the number, size and activity levels of the people present in the building. Local concentrations can therefore vary dramatically – for example when a meeting room is occupied, or while workers are on lunch or coffee breaks. Excess carbon dioxide levels can lead to tiredness and a lack of concentration and can contribute to the symptoms of Sick Building Syndrome such as headaches, eye, nose and throat irritation, itchy skin and nausea. CO₂ monitoring to determine carbon dioxide levels is therefore an intrinsic part of indoor environmental quality strategies such as demand control ventilation.
IAQ Control Methods
Cost effective IAQ strategies aim to control building ventilation to maintain indoor air quality as a function of varying ambient conditions, thus minimising energy consumption. Demand control ventilation based on carbon dioxide measurements is a popular method which can adjust ventilation rates according to the actual carbon dioxide levels present, rather than using pre-determined ventilation rates based on maximum occupancy. Other systems are available which adjust ventilation rates based on the measurement of a number of other ambient parameters as well as carbon dioxide.
CO₂ Monitoring Sampling Methods
Air can be sampled either by pumped or a combination of diffusion/convection sampling. Pumped systems use an integral pump to draw air through the detector for continuous sampling, and are particularly useful when the sensor cannot be conveniently mounted at the required sampling point. Diffusion/convection sampling methods use convection to circulate gas between the sensor head and a diffusion cell with gas exchange occurring, by diffusion, at the surface of the diffusion membrane, giving a fast response time. Since the gas in the cell circulates within a closed system, all optics are protected from the ingress of dust.
Self-contained systems for CO₂ monitoring
Self-contained CO₂ monitoring systems can be used in demand ventilation control applications. The “Plug & Play” Guardian NG series of pumped aspirated wall-mounted NDIR gas monitors can detect CO2 from sampling points up to 30 metres away, using the integral pump. These versatile monitors offer near-analyser quality continuous sampling, measurement and display of target gas concentrations.
Carbon dioxide concentration ranges of 0 – 3000 ppm cover typical target carbon dioxide levels for IAQ, while models covering wider ranges are also available. Standard temperature and pressure compensation is provided to minimize drift. Housed in a robust IP54-rated enclosure with a liquid crystal display, the monitors provide user-selectable alarm set-point controls.
OEM carbon dioxide sensors
Manufacturers of carbon dioxide-based demand ventilation control systems and multi-parameter ventilation control systems need to build carbon dioxide sensors into their products. Edinburgh Sensors’ Gascard® NG pumped aspiration dual wavelength NDIR carbon dioxide sensors feature automatic temperature and pressure corrections for real-time, reliable measurements of true concentration levels in the 1000, 2000, 3000 and 5000 ppm ranges. The sensor head and electronics are mounted on a Eurocard PCB with a number of bit-switches which enable the user to control various aspects of the sensor’s behaviour including analogue output and filter type selection. With true RS232 communications for control and data logging, along with the option of TCP/iP communications protocol, the Gascard® NG can be easily integrated into a wide range of proprietary gas detection systems and multiple sensor architectures. Gascard® sensors have been incorporated into IAQ systems installed in many prestigious buildings around the world. All our gas sensing products can be used in a wide range of applications including HVAC.