Monitoring & metering

Figure 1: Wireless CO2 Sensor

monitoring of CO2 levels is a good proxy for fresh air, CO2 sensors can be used to check if there is enough ventilation in the building and if not, to trigger a response. At its simplest, this can

be as simple as setting a CO2 alarm level to prompt opening a window in the room. Most high-performance ambient level CO2

sensors use a measurement method called Non-dispersive Infrared (NDIR), where the

CO2 level is determined using the Beer- Lambert law. Beer-Lambert’s law states that the loss of light intensity when it propagates in a medium is directly proportional to intensity

and path length. CO2 molecules absorb infrared radiation at a wavelength of around 4.25 microns. CO2 monitoring systems often need to be

installed in locations where access to mains power is limited, or its provision is costly. The

ability to be able to power the CO2 sensor for long periods of time from a battery or from energy generated using harvesting techniques is highly desirable. To reduce maintenance

parts per million (ppm). CO2 is a by-product of normal human activity and is removed from

the body via the lungs in the exhaled air. Unless an indoor space is adequately

ventilated, CO2 will naturally build up over time. CO2 levels in a well managed indoor space are generally 350-1,000ppm. Above

1,000ppm and most people will begin to complain about the stuffy atmosphere or poor

air quality. High levels of CO2 indoors are also associated with headaches, sleepiness, poor concentration, and loss of attention. In

extremely high concentrations, CO2 is harmful to life due to oxygen deprivation. CO2 sensors along with temperature and

humidity sensing are often used as part of automatic ventilation control systems. But what if the building or school does not have such a sophisticated environmental control setup? Ample natural ventilation is considered to

be the best method to prevent the SARS- CoV-2 virus from spreading indoors. The amount of fresh air that needs to be supplied is a matter of debate, but good practice is to ensure

ventilation is capable of keeping CO2 levels below 1,000 ppm or even lower. Assuming

Instrumentation Monthly November 2020 Figure 2: CozIR-Blink CO2 Sensor

costs, users want the ability for the CO2 sensor to operate autonomously for many years without user intervention. Conventional CO2 sensors use an

incandescent light source. However, these mid- IR light sources consume lots of power during a lengthy warm-up phase and during operation, making them unattractive, especially for retrospective installations where there is often a lack of an easily accessible power source. All GSS sensors use an in-house designed

ultra-efficient LED light source. LEDs are much more efficient in converting electrical power into light than conventional light sources and they do not need the long warm up times

suffered by incandescent light sources. The length of time the light source is active is a major contributor to how much power is consumed by the sensor. In a power sensitive

application, a GSS CO2 sensor is typically pulsed on and off to minimise overall power consumption. Depending on installation requirements, a

CO2 monitor can range from a simple display on the wall with a programmable alarm

through to sophisticated systems with wireless interfaces sending data up to the cloud. The latest GSS sensors such as the CozIR-

Blink are designed to operate in battery powered units so they can be easily installed and deployed. They are designed to be power cycled, where the whole device is powered

down after a CO2 reading has been made. A typical installation might be pre-programmed to take one reading every few minutes.

Depending on the required CO2 measurement accuracy, if the sensor is configured to take a reading every minute, the power consumed by the CozIR-Blink can be as low as 26uW per reading. Whilst obviously dependent on what

other electronics are in the sensor, CO2 monitors using the CozIR-Blink are often designed to last for two or more years on a single battery charge. All GSS sensors can also be pre-

programmed to run an automated background

‘reference-setting’ routine where CO2 levels are monitored over time. The reference value is the lowest concentration to which the sensor is exposed over an extended period such as a week and is typically considered to be the fresh air minimum ambient level. This scheme allows users to set an alarm threshold that is relative to a fresh air reference value, which takes account of changing outdoor

ambient CO2 levels. The sensor programable alarm can easily be used to drive a “traffic light alert” indicating it is time to open the window Ultra-low power sensors such as the CozIR-

Blink open-up new installation possibilities in a wide range of offices, workplaces and schools.

Used correctly, this type of CO2 sensor can be employed as a simple and cost-effective tool to help avoid catching the virus indoors.

Gas Sensing Solutions


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