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How do CO2 Sensors work?

co2 sensor by Aranet

You may have recently read or heard on the news about the use of CO2 sensors to monitor ventilation levels indoors to enable action to be taken to reduce transmission of covid, improve health and concentration.  Increasingly, the likes of theatres, nightclubs, schools, workplaces, retail outlets, pubs and restaurants and other indoor public spaces are installing CO2 sensors to monitor air exchange.  High CO2 levels indicate insufficient ventilation and enables corrective action to be taken through the opening of windows or switching on a fan that draws in fresh air. If you have ever wondered how the measurement is derived, the following article will hopefully provide a simple explanation.

CO2 Sensors are used to measure the levels of carbon dioxide (CO2). It is important to monitor this gas as it is an indicator of CO2 emission levels, ventilation quality and combustion industrial processes. As CO2 has no colour or smell it is impossible to detect its presence and monitor levels without a dedicated CO2 gas sensor.

  • CO2 sensors are used to monitor fermentation, respiration, photosynthesis, and other carbon dioxide consuming or producing processes. They can be used for HVAC applications to monitor air quality.
  • There are wider applications for CO2 sensors in the agricultural, food, pharmaceutical, refrigeration and brewing sectors.

The most common type of CO2 gas sensors are the nondispersive infrared (NDIR) sensors.

They deliver excellent performance and accuracy across a wide range of volume. In simple terms, NDIR sensors use explicit wavelengths of light to measure the concentration of CO2 in the atmosphere.

NDIR sensors are an excellent choice for portable and fixed accurate CO2 monitoring.

NDIR CO2 Sensors: Now for the Technical bit or information!

An NDIR sensor is a spectroscopic sensor that can detect and monitor gas using infrared light (IR) via an infrared light source, a sample chamber, infrared filters, and an infrared detector. The more gas there is present, the greater the light absorbed.  The drop in signal is proportional to the concentration of the gas being measured.

Infrared measurement can be used as a method of detecting CO2 gas owing to the principle that heterogeneous molecules absorb infrared energy dependent upon their structure. Different molecular species have unique absorption wavelengths. This enables measurement of CO2 within a mixture of other gases.

NDIR sensors all work on the same basic principle described above but can differ in their configuration.  A single beam single detector sensor design will generate a measurement prone to drift as there is no background correction.  The optical bench and electronics have long since evolved to generate a stable measurement with minimal recalibration involved using ratioed measurements. A dual beam dual detector NDIR sensor comprises a single IR light source and two infrared filters: a measurement wavelength and non-absorbing reference wavelength narrow band optical filter and two detectors.

An NDIR CO2 sensor works on either a diffusion or flow principle; the gas either diffuses or is pumped into the sample tube. The IR light will pass through the length of the tube and any CO2 gas molecules will absorb the waveband of 4.2 µm but allow other wavelengths to pass through. Any non-absorbed light reaches the optical filters, the measurement wavelength filter only transmits non absorbed energy at 4.2 microns, the reference filter will transmit non- absorbed energy at 3.9 microns, this will be greater since it is not selectively absorbed by CO2.

How is the level of CO2 calculated?

The two absorbance readings from the reference and measurement filters are ratioed and the logarithmic output is directly proportional to CO2 concentration

Internal electronics will measure the absorption by converting the light absorption into an electrical output. Results are shown in parts per million (ppm) or as a percentage of volume.

The measurements are first output as analogue micro-voltages. Some sensors then convert this analogue voltage to a digital output.

Benefits of an NDIR CO2 Gas Sensor

Measurement range and Response times

NDIR sensors and monitors can be configured to detect CO2 over several concentration ranges by using sample cells of different path length and filters with different band width and transmission properties, so measurement is possible over a range as low as 0-100 ppm or as high as 0-100%. Measurement is possible at well below 1000 ppm which is lower than required safety levels and before any negative impact on wellbeing.

CO2 NDIR Analyser modules can measure down to ppm levels with response times as low as 1.1 seconds.

Longevity

NDIR sensors can last for up to 15 years dependent upon the gas mix and pre-treatment.

Cross-sensitivity

CO2 NDIR sensors are designed to minimise cross-sensitivity to other gas species and humidity through specific wavelength selection for CO2.  Other types of sensors, for example semiconductors and electrochemical are less selective by comparison but aren’t even applicable to CO2 measurement.

Drift

Compared to other types of sensors, NDIR sensors do not suffer from drift. They provide reliable readings with no deterioration in performance over time.

Why choose a CO2 Sensor from Omni Sensors and Transmitters?

At OMNI Sensors and Transmitters Ltd we offer a wide range of CO2 Sensors that can be used for different applications and can advise on the most suitable to meet your specific needs.

Working with manufacturers E+E Elektronik, SmartGAS and Aranet we have NDIR monitors, transmitters/probes, sensors, and modules that suit a variety of budgets and needs.  We can provide a range of fixed, portable, or handheld standalone monitors; Wireless, Bluetooth and wired. Transmitters and probes for refrigerant leak detection, HVAC systems and process control and both sensors and sensor modules for OEMs.

NDIR sensors offer high accuracy and a fast response time.  Minimally affected by the presence of other gases, they are the most reliable way of detecting the presence of carbon dioxide gas in a variety of settings.

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