What is an NDIR Sensor

Joseph Black, a Scottish chemist and physician, first identified carbon dioxide in the 1750s. Since then, scientists have been exploring ways to measure this common gas. It is imperative to be able to accurately measure CO2 for a plethora of applications. These could range from indoor air quality, medicine, horticulture or underwater breathing systems, and more.

Luckily, nowadays we have access to low-cost, reliable CO2 sensors that are easily integrated into almost any device.

Initial CO2 Measurement Devices

Around the turn of the century, CO2 levels were measured using mercury manometers. Manometers involve a U-shaped glass tube that is filled with mercury in order to measure the pressure of gases. The Ideal gas law (PV=nRT) was used to calculate the moles of CO2 if temperature, pressure and volume of a dry gas sample was known and if it contained CO2 molecules.

Although mercury manometers have the ability to be very accurate, the method for measuring CO2 levels in air samples is time consuming. Due to this, Charles Keeling was approached by the US Weather Bureau and asked to record hourly atmospheric CO2 measurements on the Mauna Loa volcano, Hawaii.

He used an early infrared (IR) gas analyzer that he calibrated against his manometer. From 1958 until 2006, the original Applied Physics Corp. Infrared Gas Analyzer operated on Mauna Loa.

Similar to every IR gas sensor, the analyzer used at Mauna Loa used the same basic principle for measuring CO2. It involves an infrared light radiation source at one end of a gas sample tube and an IR detector at the other end.

The absorption band of CO2 is 4.26, alike to the 4.2 micron band of infrared radiation. Consequently, the quantity of light radiation that is absorbed by the CO2 molecules is proportional to the quantity of carbon dioxide in the gas sample. Nevertheless, due to the fact that low levels of CO2 do not absorb much light, a long tube is needed before the outcome can be measured.

Though the original IR gas analyzer was accurate, it was large and bulky, with the sample tube alone being 40 cm (16 inches) long. The challenge is finding the right balance. For examples, engineers use a longer optical path in order to measure lower levels of CO2 more accurately.

As a result, a greater gas sample chamber is created. However, in ambient air environments, like schools, offices and homes, there is demand for the ever-smaller sensors to fit precisely inside compact devices.

Modern CO2 Sensors

Sensors continued to get smaller, when in 1993, an engineering breakthrough occurred; SenseAir AB patented a design for CO2 sensors with a small-footprint. As a result, the size problem of the sensors was solved by using folded optics and metalized molded plastic. This was able to reflect the light through a curved shape ('waveguide') that was lengthier than the footprint of the sensor module. This highly-reflective coating ensured that the CO2 molecules inside the gas sample chamber would absorb the same amount of light as a traditional straight-path design.

Progressively smaller sensors increased sensitivity were achieved through using advanced optics with new waveguide designs. For instance, in 2003, SenseAir's K20 CO2 sensor used a "banana" waveguide design and was successful. Thus, this sensor was used for many years in OEM consumer safety products.

Nowadays, the newest generation of CO2 sensors have been further developed. They have even more enhanced waveguides, thus allowing a longer optical light path to be folded into an even smaller 8 mm x 33 mm x 20 mm footprint. For example, the SenseAir S8 sensor combines ultra-low power IR elements which can take measurements for weeks on battery or solar power alone.

Outside the difficulty of improving waveguides, IR light sources and IR detectors, the construction of CO2 sensors bears a resemblance to smartphones or other state-of-the-art electronics. High-speed robotic assembly is required, and is completed in specially designed clean rooms. Next, the modules are batch-tested and calibrated before shipping.

The question is, what does the future hold? For many years, high-end gas analyzers have been manufactured based on photo-acoustic spectroscopy (PAS). In the 1880s, Alexander Graham Bell discovered PAS and noted that strobed sunlight shown on different materials produced an audible sound. Due to PAS systems not relying on the length of the waveguide, an even smaller CO2 sensor is potentially possible, using a pulsed MEMS (Micro Electro Mechanical System) mirror and MEMs microphone.

This information has been sourced, reviewed and adapted from materials provided by CO2Meter.

For more information on this source, please visit CO2Meter.

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