Page 47 - EE Times Europe Magazine | June2020

EE|Times EUROPE — Sensors Insert 47
Monitor Carbon Dioxide Level by Photoacoustic Sensing
Figure 2: SCD30 technology (Image: Sensirion)
Figure 3: NDIR principle (Image: Sensirion)
Figure 4: Size comparison of the SCD30, based on NDIR, and the SCD40, based on
PASens Technology (Image: Sensirion)
Public awareness of the negative cogni-
tive effects of CO 2 pollution is growing, said
Marco Gysel, product manager for CO 2 sensors
at Sensirion (Figure 1). As a result, “there
is an increasing number of initiatives from
the public and private sector to monitor and
counteract high CO 2 concentrations,” he
said. “Most initiatives focus on classrooms,
universities, and commercial office buildings,
but there is an increasing demand for CO 2
sensing for residential apartments as well.”
PHOTOACOUSTIC DETECTION
VERSUS NDIR
Sensirion’s previous-generation sensor, the
SCD30, was based on the non-dispersive
infrared (NDIR) optical detection principle.
The size and cost of NDIR sensors have
restricted their use to a few applications.
For the SCD40, Sensirion used the photo-
acoustic detection principle to reduce the
size of the optical cavity without compromis-
ing performance. The new device is smaller
than its predecessor by a factor of 7. The
company is betting that its miniaturized CO 2
sensor will create the basis for a range of
new sensing applications.
NDIR-type sensors are frequently used
in gas analysis. The main components are
the infrared source with a wavelength filter,
a sample gas chamber, and an IR detector
(Figures 2 and 3). By illuminating an infra-
red beam through a sample cell containing
CO 2 and measuring the amount of infrared
absorbed by the sample at the required
wavelength, an NDIR detector is able to
measure the volumetric concentration of
CO 2 in the sample.
The sensitivity of a sensor based on the
NDIR principle is directly proportional to
the optical beam path. A large reduction
of the path leads to a compromise of its
performance, which limits the miniaturiza-
tion potential of this technology. The size,
structure, and large number of discrete com-
ponents required for NDIR sensors result in
a bill-of-materials (BOM) cost that is beyond
the reach of many applications.
“Sensirion is always aiming at disrupt-
ing sensor markets by making components
smaller and more price-effective without
compromising performance,” said Gysel. “For
CO 2 sensing, we identified photoacoustic
technology as the most promising approach.
In addition to reducing the size and the cost
of CO 2 sensors, this technology allows for
SMT [surface-mount technology] assembly
to replace arduous through-hole soldering.
These three factors combined have the poten-
tial to open up new CO 2 -sensing markets.
Personally, I believe that photoacoustic
technology has the potential to replace NDIR
as the standard CO 2 -sensing technology over
the next five to 10 years.”
www.eetimes.eu | JUNE 2020