Guide values for CO2-concentration:
• ~ 40,000ppm Proportion in exhaled human breath (20l CO2/h)
• = 5,000ppm Limit of CO2-concentration at the workplace
• > 1,000ppm Fatigue and reduced concentration
• = 1,000ppm Recommended CO2 level of indoor air
• = 400ppm Fresh, natural ambient air
Measuring methods
CO2 measurements in HVAC applications are based exclusively on the Infrared (IR) absorption principle.
There are two methods to measure CO2 concentration with the IR absorption method:
- Non-Dispersive Infrared (NDIR) absorption sensor
- Photo-acoustic sensor
E+E Elektronik has decided for the NDIR absorption sensor principle. Compared with the photo-acoustic sensors, NDIR sensors offer the following advantages:
- less sensitive to pressure variations
- less sensitive to vibrations and acoustic interference
- compact design
The E+E’s NDIR system uses a two-source two-beam procedure to detect a certain wavelength of the infrared light.
A patented auto-calibration procedure uses two IR-sources with distinct operation cycles:
One IR-source operates to measure the CO2 concentration and generates an IR signal every 30 seconds. The second IR-source, the reference source, is used for the auto-calibration only. This source is activated twice every 24h, which leads to virtually no aging and therefore to negligible drift. The quasi drift-free signal of the reference source is used to offset eventual drifts of the measuring source.
The advantages if the E+E’s NDIR CO2 sensing are:
- easy drift compensation with stable IR-reference source
- use of simple IR-filter. Other methods use two IR-filters or complicated switchable IR-filters • only one IR-detector required
- simple and reliable design
