KDS-1037
-
2
-
Measuring
CO
2
levels in classrooms. The air in a classroom during the lesson can reach as
high as 1000 to 1200 ppm CO
2
.
Determining the rate at which CO
2
diffuses through a gas diffusion tube.
Monitoring production of CO
2
during fermentation or respiration of sugars.
How it Works
The NDIR (Non Dispersive Infrared) type utilizes the phenomena that an asymmetry gas molecule,
consisting of 3 atoms or more, absorbs the energy from its inherence vibration selectively.
For example, H
2
O gives 1.4 absorbance at 1.9
㎛
wavelength, while SO
2
, CO
2
, CO, NO give a
maximum absorbance at 4.0
㎛
, 4.3
㎛
, 4.7
㎛
, 5.3
㎛
wavelengths, respectively.
CO
2
gas concentration is calculated from the electric signal that was transformed from the
absorbance that changes with the gas concentration. The absorbance is measured at the course of a
light with 4.3
㎛
wavelength, emitted from Infrared lamp, reaches gas sensor position.
This is the type that overcomes the shortcomings of a Single type and generally called as Dual type.
In this type, two sensors are packed in one case, so that one terminal is used for measuring standard
while the other is used for measuring infrared absorbance change that was affected by CO
2
concentration.
Standard value is the absorbance that was measured at the wavelength, in which the absorbance
does not affected by CO
2
and other gases.
Calibration
For many measurements it will not be necessary
to calibrate the CO
2
gas sensor. The sensor has
measurement range between 0 to 100,000 ppm
1
.
The output voltage is between 0 and 4 volt.
Most of the time this calibration is sufficient.
For more accurate measurements, however, the
sensor can be calibrated at one known CO
2
level
using the calibration button on the sensor box.
Default calibration graph of CO
2
gas sensor:
value (ppm) = 25,000 * Vout (V)
Coefficients of the calibration function:
a = 25,000; b = 0
Figure 2. Calibration of the CO
2
gas sensor.
1
The CO
2
sensor measures in units, parts per million, or ppm. In gaseous mixtures, 1 part per million
refers to 1 part by volume in 1 million volume units of the whole. This unit can be recalculated to
percent
by dividing a value in ppm by 10,000. 100,000 ppm is equal to 10 %.
The level of CO
2
in the earth’s troposphere has gradually increased from 317 ppm in 1960 to current
levels of nearly 370 ppm. Exhaled human breath has CO
2
concentration of about 50,000 ppm.
C
O
2 C
onc
en
tu
ra
tion
(p
pm
)
Sensor Output (V)
100,000 ppm
0 ppm
4.0V
0V
