VOC Sensors
Sensors that detect volatile organic compounds
(VOCs) come in various types that can detect
alcohols, aldehydes, methane, carbon monoxide,
and other gases or vapors.A VOC sensor is
sometimes called an eCO
2
sensor because it
estimates an equivalent calculated CO
2
level
using an algorithm based on the presence of
other gases.
The Bosch BME688 sensor, available from
Adafruit on a Stemma-format board, is $20.For
our purposes, VOC sensors don’t effectively
measure CO
2
but they’re great for calculating the
indoor air quality (IAQ) index, as VOCs are emitted
by paint, carpets, hand sanitizers, etc.
Figure
G
shows a plot of a day in the life of
Guido Burger, as measured by a CO
2
sensor and
a VOC sensor.The orange graph on top follows
the CO
2
sensor readings while Guido was sitting
in his office. Around noontime, he has a phone
call and starts talking, and CO
2
levels rise to
about 1000ppm, and then he opens the window
and the value drops. The purple graph shows the
VOC sensor, which doesn’t change with the rise
of CO
2
in the orange graph.At times there seems
to be a relation between the two, but they’re each
measuring different things and both can be worth
looking at.For instance, the VOC sensor detects
changes in air quality while Guido is sleeping even
though the CO
2
levels in the room remain steady.
A Warning System
A CO
2
device could simply display the ppm
reading from the CO
2
sensor, but that doesn’t
provide much context for what the reading
means.Guido used three colors of LEDs to create
a CO
2
Traffic Light (Ampel in German):“So green
light means everything is okay,said Guido.
Sensirion, Guido Burger, WinSen
“Yellow means you should open the window. And
red light is the ultimate warning.”
Red = Above 2000ppm
Yellow = Between 1000 and 2000ppm
Green = Below 1000ppm
Consider changing the threshold values if you
wish to be cautious and provide an earlier
warning for rising levels.For instance, you might
trigger red above 1000ppm, yellow between 800
and 1000ppm, and green below 800ppm.
While individual readings create a simple
indicator, we might want to visualize the data
over time on a graph, which can show the rate
of change.In Figure
H
, Guido added a matrix
display that shows the data as a curve.Now
we can see the readings change over time and
begin to see how quickly the exchange of fresh
air comes in and changes the reading.“This
allows us to see ventilation as a dynamic system,
explained Guido.
Carter Nelson’s RGB Matrix Portal Room CO
2
Monitor (learn.adafruit.com/matrix-portal-room-
co2-monitor) used a large display that anyone in
a room could read and understand. The device
presents a concise public-health message using
29
make.co
Comparison of the graphs for a CO
2
sensor (orange)
and a VOC sensor (purple).
Guido Burger’s CO
2
Traffic Light, with optional graphical display of CO
2
readings over time.
G
H
M78_024-35_CO2Monitor_F1.indd 29M78_024-35_CO2Monitor_F1.indd 29 7/13/21 9:49 AM7/13/21 9:49 AM
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