Since Joseph Black, a Scottish chemist and physician first identified carbon dioxide in the 1750s, scientists have looked for a way to measure this common gas. The ability to accurately measure CO2 is critical for hundreds of applications from indoor air quality to horticulture to medicine to underwater breathing systems and more.
Fortunately, today we have access to low-cost, reliable CO2 sensors that can easily be integrated into virtually any device.
Early CO2 Measurement Devices
Early CO2 measurements were taken using mercury manometers developed around the turn of the century. Manometers use a U-shaped glass tube filled with mercury to measure gas pressure. If temperature, pressure and volume of a dry gas sample containing CO2 molecules are known, the moles of CO2 can be calculated using the Ideal gas law (PV=nRT).
While mercury manometers can be very accurate, the procedure for measuring CO2 levels in air samples can take hours. That’s why when Charles Keeling was asked by the US Weather Bureau to begin taking hourly atmospheric CO2 measurements on the Mauna Loa volcano in Hawaii he used an early infrared (IR) gas analyzer calibrated against his manometer. The original Applied Physics Corp. Infrared Gas Analyzer operated on Mauna Loa from 1958 until 2006.
Like every IR gas sensor, the analyzer at Mauna Loa used the same basic principle for measuring CO2. It has an infrared light radiation source at one end of a gas sample tube and an IR detector at the other. The 4.2 micron band of infrared radiation is very close to the 4.26 micron absorption band of CO2. As a result, the amount of light radiation that is absorbed by the CO2 molecules is proportional to the amount of carbon dioxide in the gas sample. However, since low levels of CO2 do not absorb much light, a long tube is needed before the effect can be measured.
While the original IR gas analyzer was accurate, it was bulky. The sample tube alone was 40cm (16 inches) long.
The challenge is getting the right balance. Engineers use a longer optical path to measure lower levels of CO2 more accurately - which means a larger gas sample chamber. On the other hand, in ambient air environments - such as schools, offices and homes - the demand is for ever-smaller sensors to fit neatly inside compact devices.
Modern CO2 Sensors
While sensors continued to get smaller, an engineering breakthrough occurred in 1993 when SenseAir AB patented a design for small-footprint CO2 sensors. These sensors solved the size problem by using folded optics and metallized molded plastic to reflect the light through a curved shape (refered to as a 'waveguide') that was longer than the footprint of the sensor module. The highly reflective coating insured that the CO2 molecules inside the gas sample chamber would absorb the same amount of light as a traditional straight-path design.
By using advanced optics, new waveguide designs allowed for progressively smaller sensors with increased sensitivity. For example, in 2003 SenseAir's K20 CO2 sensor (shown here) used a "banana" waveguide design. This sensor was used for many years in OEM consumer safety products.
Today's newest generation of CO2 sensors have even more optimized waveguides, allowing a longer optical light path to be folded into an even smaller 8 mm x 33 mm x 20 mm footprint. An example is the SenseAir S8 sensor shown here. Combined with ultra-low power IR elements, these sensors can take measurements for weeks on battery or solar power alone.
Beyond the challenge of optimizing waveguides, IR light sources and IR detectors, the fabrication of CO2 sensors resembles that of smart phones or any other state-of-the-art electronics. High-speed robotic assembly takes place in specially designed clean rooms. The modules are then batch-tested and calibrated before shipment. SenseAir has a promotional video where you can see sensors being assembled at their factory in Sweden here.
What is in store for the future? For years, high-end gas analyzers have been produced based on photo-acoustic spectroscopy, or PAS. PAS is was discovered by Alexander Graham Bell in the 1880’s. He noted that strobed sunlight shown on different materials produced an audible sound. Since PAS systems do not rely on the length of the waveguide, an even smaller CO2 sensor using a pulsed MEMS (Micro Electro Mechanical System) mirror and MEMs microphone are theoretically possible. When they go into production, we'll tell you about them here.
Images courtesy of SenseAir® AB