Selecting IAQ Sensors

woman walking past interactive screen showing air quality in the workplace with wood panel wall and large window opposite

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Selecting Indoor Air Quality Monitors to Maintain Healthy Spaces

Air quality is a heightened concern around the globe as we prioritize the maintenance healthy spaces for our homes, as well as the places where we work, learn, shop, or play. From individual homeowners to facility managers, to campus directors of multi-building portfolios, people are considering how to monitor and communicate their air quality to keep occupants healthy and happy. But what they encounter in the process is a plethora of options in varying metrics and features which can become overwhelming and nearly impossible to compare. Ultimately, the quality of the product must be right-sized to meet their needs. The following information takes a look at what we consider with our clients when selecting a monitor.

Interactive indoor air quality data at your fingertips. Photo by Randy Braley.

In searching for an indoor air quality monitor, most devices will measure a combination of temperature, relative humidity, carbon dioxide, particulate matter, and total volatile organic compounds. Pursuing RESET certification requires an accredited monitor that senses temperature, relative humidity, TVOC, PM2.5 and CO2. WELL and LEED certifications vary slightly adding options from formaldehyde and PM10. Beyond meeting the sensed variable and聽,聽a number of other vital elements must be considered to make the wisest decisions in selecting a monitor.

Recalibration
When sensors go out of calibration, they start to report inaccurate measurements. This 鈥渟ensor drift,鈥� or the time taken for the sensor to go out of calibration, depends on the sensed variable. For instance, a CO2 sensor may have a lifetime of 15 years but a TVOC sensor needs calibration every year. When a sensor is out of calibration there is a risk of reporting a problem that may not exist, so it is paramount to consider your recalibration plan even before purchasing your device.

When dealing with a monitor that includes multiples sensors, the following are the most common recalibration options.

Factory Recalibration聽鈥� This involves sending the monitor back to the factory for cleaning and recalibration. This option is costly as it would involve removing installed devices from wall mountings, the need for a professional to tie off the power source, and the cost of shipping back and forth (likely overseas) not to mention the risk of import/export duty. If this is the only option for your device, it likely will be more economical to simply replace the device with the newer model, reducing unnecessary wasted time and sustainability concerns.
Remote Calibration聽– Many manufacturers will offer the option of remote calibration, which of course removes the inconvenience of shipping the monitor back to the manufacturer. This involves applying a compensation factor to the drifted sensed data which will bring the data artificially back to the correct range. In these cases, the device鈥檚 inner workings may be dirty and the sensor may be picking up inaccurate readings. This is a low-cost, no-cost solution but not a long term one.
Automatic Recalibration聽鈥� Some devices automatically recalibrate based on historical sensed data 鈥� which sounds great in theory but can artificially disregard a progressive build-up of toxins by identifying it as a calibration issue.
Modular Sensors聽鈥� The best-case scenario is to replace each sensor with the necessary frequency. Some newer monitors offer the ability to replace sensors in a modular way like the ink in a printer. This does mean more moving parts, inserting and removing sensors, which has a risk associated with it, but is the approach that produces the best results.

Data Backup
The purpose of permanent IoT installations is to gather a dataset over a long period of time, observe trends, and provide actionable insights to operators and occupants. If the monitor loses power or stops updates to the cloud because of a connectivity issue, there is a risk of losing part of a dataset forever. Worse yet is the potential lost certification if one is pursuing a performance-based standard. There are many causes for these types of failures. Some are as simple as someone unplugging a device, a power outage, changing a WiFi password, or modification of service IP address. Whatever the reason, the impact can be significant and, in some cases, cause an organization to miss their monitoring goals.

To prevent loss of data, select monitors with:

In-Unit Memory. Some monitors have data-logger capabilities so they can continue to gather and store IAQ data. Even if the monitor loses connection to the cloud, it will populate the database when connectivity is restored. This type of unit can also be used for applications when connecting to a network is not allowed due to security reasons.
Power. Most monitors powered by line voltage, or power over ethernet, are likely not available with battery-powered option. However, some monitors come with a backup battery as a stopgap measure if the device loses power for a short period of time. This has been a valuable backup option when power is lost. We anticipate seeing more battery-powered options on the market in the future.

Cost
This seems like an obvious consideration but the total cost of ownership for an indoor air quality monitor is much more than the off-the-shelf first cost. Evaluating the total cost of ownership of devices over a 10-year period allows owners to consider all of the unexpected costs that one might encounter. These include:

First Cost聽鈥� The expected cost for the hardware off the shelf. Monitor life expectancy should be considered as this first cost will be reoccurring for monitors with a shorter lifespan that do not support recalibration.
Annual Subscription Cost聽鈥� Many monitor companies have a low first-cost, and high reoccurring annual fee for feeding data to their cloud-based data visualization dashboard. One thing to consider is that even if an owner does not want to leverage the manufacturer-provided cloud dashboard, they still must pay for the subscription to avail the hardware API to pull the data into another dashboard.
Recalibration Frequency and Cost聽鈥� Typically calibration is required once per year for TVOCs. As noted in the Recalibration section above, if not considered up front, the monitor could be worthless in a year. Factory reconfiguration has proven to be cost prohibitive, with sensor replacement being the preferred method which typically costs $100-to-$200 per monitor per year.
Lifetime and Replacement Cost 鈥� Monitors typically last two-and-a-half to 10 years. When considering the total cost of ownership this replacement cost should be considered.