Welcome to the 2nd instalment of our Indoor Air Quality Monitoring (iAQM) discussion. Today we discuss the technology behind an iAQM sensing system.
In the 3rd and final instalment, we’ll do a review on an indoor air quality sensor we developed for a client – Grosvenor Engineering.
The technology used to sense iAQM parameters is quite advanced, so let’s have a look at some of the sensors in an indoor air quality monitoring product.
- CO2 sensing can be electrochemical (cheap, inaccurate and with significant long-term drift), versus infra-red which accurately analyses the air and determines the CO2 concentration, to an accuracy of 50ppm or better.
- TVOC sensors use a metal oxide (MOX) element to detect a wide variety of VOCs and calculate a rough estimate across the range of gases. The gases react with the MOX element and generate a voltage. They don’t detect a specific gas (unless the iAQM product has a specific gas sensor), however they do detect the most common ones including formaldehyde and easily detect abnormal or dangerous levels of VOCs
- Particulate sensors use a laser beam to measure the level of scattering through the air passing through a test chamber. The more scattering, the greater the concentration of particles and the higher the PM value usually expressed in micrograms per m3. They also include a fan to draw the air through the test chamber and maintain a certain air velocity to ensure accurate measurements.
- People sensors (PIR) are also very beneficial as they can detect the occupancy level in an area of a building, and can tune the sampling rate. The busier the area, the higher the sampling rate. In the evening when the building is empty, the sampling rate is backed off to reduce network traffic and improve battery life, if operating from batteries.
- Lower cost and more common sensors such as light sensors measure in Lux. Similarly, sound sensors measure in dBA. Temperature and humidity are found in every indoor air quality product.
Wired and Wireless
The data transfer from an iAQM system to the portal dashboard through a gateway can be implemented in a number of ways. Wired systems are possible (usually RS485), but not popular, as they increase installation cost. That leaves wireless solutions and there are many choices including Wi-Fi, Zigbee, cellular (NB-IoT), LoRaWAN and Sigfox.
Wi-Fi and LoRaWAN have no operating costs to use the network, but reliability can suffer. For instance the Wi-Fi SSID or network password can be changed by the IT department rendering the iAQM system non-functional (until it’s updated). Similarly, if a LoRaWAN gateway is taken off air on a community network, the devices will go off air. Cellular NB-IoT, Cat-M1 and Sigfox are much more reliable as they are operated by enterprise grade vendors – but come at a cost.
For these reasons, it’s up to the individual user to determine the best solution. If the user is confident of their Wi-Fi system and in-house IT have full control, use this as the mechanism, especially if IT are always aware of them and will maintain them. If however there are only one or two sensors, the cost to use cellular is minimal, and hence it a better option with increased reliability as it ensures 100% uptime.
LoRaWAN is the preferred choice if you reside a region with coverage which is sometimes the case when a local government has funded a network, as it’s free and reliable. In Australia there are examples on the Gold Coast and Newcastle.
Sigfox is the least preferred solution as the packet sizes are very small and infrequent, hence your system won’t have the bandwidth to support a reasonable number of sensors at reasonable sampling rates. If you’re sampling only temperature and humidity however, it will be fine.
Unfortunately, some of the sensors used in an iAQM system consume substantial power and hence make battery usage impractical. These include the CO2 sensor and the Particulate sensor (which often runs a fan and consumes a lot of power). There are techniques that can stretch battery life such as super slow sampling and “heads up sampling”, but invariably an iAQM system will require external power. If battery power is used, ensure the batteries are readily available, preferably AA, AAAs etc and can be easily replaced without requiring a technician to do it.
The supplier of the iAQM system may guarantee a minimum battery lifetime. Be careful however as such a guarantee will come with caveats. Invariably they will stipulate a maximum (fastest) sampling rate perhaps as slow as one hour, and a proximity to an RF transmitter. The faster you sample, the lower the battery life. The closer the iAQM system is to the transmitter – be it cellular, LoRaWAN, Wi-Fi etc, the less transmit power it requires and the longer the battery life. Your location may be further away than their specification and hence use more power.
If external power is used, ideally it should be hard wired as this prevents accidental or intentional disconnection. For instance, USB seems like a great idea … until someone needs to charge their phone and disconnects the power source taking the sensor off air. Worse still, the “offender” will usually forget to re-connect it, exacerbating the issue.
No iAQM article is complete without mentioning COVID-19. iAQM sensors can’t detect COVID-19. That’s very clear despite what some suppliers will claim. They can however detect environments that are stuffy, lack air circulation and have a high concentration of people in a specific area – all of which increase the risk of COVID-19 spreading. For these reasons, they are very useful in an indoor environment.