Sampling Data Logger User Manual
CO2Meter
PAGE
10
of
16
Figure
7:
Filter
Installation
and
Orientation.
We
recommend
installing
the
included
humidity/particulate
filter
to
ensure
the
sensing
chamber
and
pump
baffle
stays
clear
and
corrosion
free.
Theory
of
Operation
The
CO2
sensor
inside
this
device
uses
non
‐
dispersive
infrared
technology
to
sense,
as
a
function
of
transmitted
light,
the
concentration
of
CO2
in
the
air.
It
has
been
factory
‐
calibrated
to
operate
within
the
specified
range
and
precision.
0
‐
1%
and
0
‐
30%
Units
Only
The
sensor
uses
an
algorithm
called
Automatic
Background
Calibration
(ABC)
to
continuously
adjust
the
zero
‐
point
to
ensure
accuracy
is
maintained.
This
is
required
to
compensate
for
sensor
drift,
which
occurs
normally
on
these
units.
The
primary
contributor
to
sensor
drift
is
the
normal
and
unavoidable
aging
of
the
infrared
light
source
within
the
sensor.
The
ABC
algorithm
allows
the
CO2
sensor
to
dynamically
shift
its
CO2
reading
by
a
constant.
It
works
via
storing
the
lowest
CO2
sample
taken
over
the
ABC
period
and
assuming
that
this
low
value
is
equal
to
a
known
value
(the
target
value).
It
then
adjusts
the
output
of
the
CO2
reading
by
the
delta
between
these
values.
This
algorithm
does
not
affect
the
linearization
of
the
output
signal.
For
example,
the
ABC
algorithm
is
enabled
by
default,
with
an
ABC
period
of
180
hours,
a
target
value
of
400ppm,
and
a
maximum
delta
of
30ppm.
This
operates
under
the
principle
that
a
CO2
level
in
ambient
outdoor
air
is
400ppm.
The
sensor
will
keep
track
of
the
lowest
CO2
reading
recorded
over
a
period
of
180
hours
and
then
adjust
the
zero
point
based
on
the
lowest
CO2
level
recorded,
up
to
30ppm
at
a
time,
towards
that
value.
