1.
Detection of CO2 Gas Using
Non-Dispersive Infrared (NDIR) Gas Sensor
As
A part of research internship
at
CSIR - Central Electronics Engineering Research Institute, Pilani
Submitted by:
Kotha Venkata Ramakrishna
M.Tech (Microelectronics & VLSI Design - NIT, Sikkim) Under the guidance of:
Rishav Pandey Dr. Vijay Chatterjee
B.Tech (Elect & Comm Engg - Jabalpur Engineering College) Sr. Scientist, CSIR-CEERI

2.
What is NDIR Gas Sensor?
• NDIR is an industry term for "non dispersive infrared" and is the most common type
of sensor used to measure the concentration of carbon dioxide, or CO2.
• It is called as Non- Dispersive because the IR light which passes through the gas
chamber is not pre-filtered.
• The operating principle of basic NDIR System is somewhat related to Beer-Lambert
Law.

3.
Beer-Lambert Law
• Lambert’s Law:
When a beam of monochromatic light passes through an absorbing
medium then its intensity decreases exponentially as the length of the
absorbing medium increases.
I=I0 e-kl or, A ∝ l
• Beer’s Law:
When a beam of monochromatic light passes through an absorbing
medium then its intensity decreases exponentially as the concentration of
the absorbing medium increases.
I=I0 e-k’c or, A ∝ c

4.
Continued….
• When both the laws are combined, we get Beer Lambert Law which is
given by:

5.
Continued….

6.
Continued….

7.
Operating Principle
• When infrared radiation interacts with gas molecules, infrared light is
absorbed by the gas molecules at a particular wavelength, causing
vibration of the gas molecules. NDIR gas sensors detects the decrease in
transmitted infrared light which is in proportion to gas concentration. This
transmittance, the ratio of transmitted radiation energy to the incident
energy, is dependent on target gas concentration.

8.
Continued….
• NDIR gas sensor consist of an infrared source, detector, optical filter, gas
cell, and electronics for signal processing. A single light source, dual
wavelength type gas sensor has two detectors and two optical filters of
different wavelengths which are placed in front of each detector. Infrared
light that is absorbed by a target gas passes through the active filter with a
particular bandwidth for the detection of the target gas. Infrared light that
does not interact with the target gas passes through the reference filter.
The difference between transmitted light intensities in these two
bandwidths is converted into gas concentration.

9.
NDIR Gas Sensing Circuit

10.
Circuit Components….
The following components are used to design a circuit diagram of basic NDIR system:
1. Lamp (An IR light source).
2. Active and Reference optical filters.
3. IR Detectors (consisting of Thermopiles and Thermistor)
4. Two AD8629 Operational Amplifiers.
5. One ADA4528-1 low noise amplifier.
6. One ADuCM360 analog microcontroller (consisting of ADCs, Muxes, etc)
7. One ADP7105 low drop regulator.

11.
Circuit Explanation….
• The printed circuit board (PCB) is designed in an Arduino shield form factor and interfaces to
the EVAL-ADICUP360 Arduino-compatible platform board. The signal conditioning is
implemented with the AD8629 and the ADA4528-1 low noise amplifiers and
the ADuCM360 precision analog microcontroller, which contains programmable gain
amplifiers, dual 24-bit Σ-Δ analog-to digital converters (ADCs).
• The thermopile sensor is composed of a large number of thermocouples connected usually in
series or, less commonly, in parallel. The output voltage of the series connected
thermocouples depends on the temperature difference between the thermocouple junctions
and the reference junctions.

12.
Continued….
• The circuit uses the AD8629 op amp to amplify the thermopile sensor output signals.
• The ADP7105 low dropout regulator generates a stable 5 V output voltage to drive the lamp
and is turned on and off by the ADuCM360.
• In the NDIR application, pulsed and filtered IR light is applied to the series connected active
junctions; the junctions are therefore heated, which in turn generates a small thermoelectric
voltage. The temperature of the reference junction is measured with a thermistor.

13.
In NDIR Gas Sensors, generally two main things happen:
1. Beer-Lambert Law
2. IR-Spectroscopy

14.
Different ways to detect an IR Radiation….
• Thermal Detectors
1. Thermopile
2. Thermistor
3. Pyroelectric Detector
4. Bolometer
5. Golay Cell
• Photodetectors (or Photon Detectors)
1. Photoconductivity Cell
2. Semiconductor Detectors

15.
NDIR Gas Sensor Using Thermopile Detector

16.
Cont….
• Since, in case of a thermopile detector ( usually, series connection of thermocouples), the temperature
difference between the active and the reference junction creates a potential difference, and if the later
quantity is measured with a voltmeter and if we measure the temperature of reference junction using
thermistor, we can calculate the concentration of the target gas.
• Ex- If IR light is applied to a dual thermopile detector fitted with a pair of optical filters so that one filter is
centered on 4260 nm and the other on 3910 nm, the concentration of carbon dioxide can be measured from
the ratios of the two thermopile voltages.

17.
Driver Circuit for Thermopile Detector

18.
NDIR Gas Sensor Using Thermopile and Temperature Sensor

19.
NDIR Gas Sensor Using Pyroelectric Detector

20.
Cont….
Fig. 1a shows a schematic of the ScAlN-based pyroelectric detector that we use. The pyroelectric
detector consists of a pyroelectric sensing layer with area ~ 500 μm x 500 μm. We use 12 %-doped
ScAlN with thickness of ~1 μm as the pyroelectric sensing layer. On top and below the ScAlN sensing
layer is the top and bottom electrodes respectively. Titanium nitride (TiN) is used as the top electrode
and molybdenum (Mo) as the bottom electrode. Above TiN top electrode is a dielectric stack of
silicon dioxide – silicon nitride – silicon dioxide (SiO2-SiN-SiO2) which acts as the absorber to help
enhance light absorption into the device. This absorber stack is used to help broaden the absorption
bandwidth with 3 layers of dielectric films and create destructive light wave interference in the stack
for more efficient absorption.

21.
Cont….
• The top and bottom electrodes are connected to aluminium metal pads which act as metal contacts
for electrical connections. Below the bottom electrode is a ~1 μm thick SiO2 layer with waffle-like
structures. The SiO2 material helps to thermally isolate the thermal energy received by the
pyroelectric detector sensing layer, slowing down thermal conduction to the medium below. In
addition, the Si substrate is released from the backside to form an air cavity area under the
pyroelectric sensing region to further reduce thermal losses as air is a poor thermal conductor. The
SiO2 ribs help to increase the mechanical stiffness of the pyroelectric sensing region which is in
membrane form.

22.
Driver Circuit for Pyroelectric Detector

23.
NDIR Gas Sensor Using Bolometer Detector

24.
Driver Circuit for Bolometer Detector

25.
Absorption Spectra of Different IR Active Gases

26.
Optical Window & Optical Filters
• Since there are different ways to detect an infrared radiation, i.e., an IR light in basic NDIR System
can be detected by various detectors like bolometer, thermistor, thermocouples, and pyroelectric
detector. So, we have different architecture in each of the aforementioned detection methods.
• We found that if we are using an optical window to filter the infrared light (of wavelength 0.7 μm to
1 mm) to a particular wavelength of around 4.26 μm then we shall not use active and reference
filter near the detector side.
• However, if there is no optical window, then IR light of all possible wavelengths will pass through the
gas cylinder. Some of the IR radiation will be absorbed by CO2 gas molecules while other will be
passed unabsorbed. In this case it’s important to use active and reference filter to calculate the
overall decrease in IR radiation.

27.
Cont….
• Infrared light that is absorbed by CO2 gas molecules passes through the active filter with
a particular wavelength. Infrared light that does not interact with CO2 gas molecules
passes through the reference filter. The difference between transmitted light intensities
is converted into gas concentration.

28.
Ideal Length Of NDIR Gas Cylinder
• The most ideal dimension of gas chamber as per industry standard is a cylinder with
20mm Diameter x 16.5mm Length.
• With this length of cylinder there is a negligible decrease in the intensity of IR
radiation.
• Since, as we increase the length of the gas chamber intensity of IR radiation
decreases exponentially as per the below mentioned Lambert Law.
I=I0 e-kl or, A ∝ l

29.
How to prevent loss of intensity due to multiple reflection?
• In NDIR Gas Sensor, the intensity of IR radiation also decreases because of
continuous reflection within the gas chamber.
• So, in order to prevent this decrease in intensity, the gas chamber is coated with a
layer of gold, to absorb minimum IR radiation.

30.
Noise Equivalent Power:
NEP is the minimum optical power required for an output signal-to-noise ratio (SNR) of 1.
Basically, this represents the threshold above which a signal can be detected.
Detectivity:
The detectivity (D) of a detector is just the inverse of its Noise Equivalent Power. The larger the detectivity of a
detector, the more it is suitable for detecting weak signals which compete with detector’s noise. Its unit is
Hz1/2/W
Specific Detectivity:
The specific detectivity (D*) is the detectivity normalized to a unit detector area (1 m2) and detection
bandwidth (1 Hz); one can calculate it by multiplying the detectivity with the square root of the detector area
(in square centimeters). Its unit is cm Hz1/2/W

31.
Calculation of NEP:

32.
Calculation of Pmin:

33.
References….
• https://www.co2meter.com/
• https://en.gassensor.com.cn/
• https://www.analog.com/
• https://www.researchgate.net/
• https://www.sciencedirect.com/
• https://www.nature.com/
• https://www.semanticscholar.org/
• https://core.ac.uk/