How does an NDIR work
How the NDIR sensor works
Description of the functionality for those interested in technology
An NDIR sensor is a non-dispersive infrared sensor consisting of an IR diode as the light source, an optical wavelength filter, and an IR detector. The ambient air diffuses through a membrane into a chamber. Each gas has its own wavelength in the infrared spectrum at which it optimally absorbs light. The extent is measured electro-optically and thus provides a reference for the existing concentration. The optical filter ensures that only the narrow spectrum of interest is evaluated. A pure air mixture is enclosed in a second chamber.
The same measurement is now performed alternately in the two chambers at a low pulse frequency of just under 1 Hz. External influences such as temperatures affect both chambers and are thus largely compensated for, as only the difference between the two chambers is evaluated. A powerful microcontroller system first converts the low detector currents into a voltage, which is then converted into digital signals by an AD converter. Initial filtering takes place here, checking the signals for plausibility and smoothing out strong, rapid, possibly unusual signal changes, replacing them with intermediate values. The read voltage values are then related to a concentration expressible in ppm according to an internal reference and output as a pulse-width modulated continuous signal.
Using an analog low-pass filter, the pulses are converted back into a modulated DC voltage, achieving approximately 1 mV per ppm. Fresh air contains approximately 400 ppm CO2, so the sensor should output approximately 0.4 volts in fresh air, which is roughly what the GX-A1 warning device displays. Because the GX-A1 specifically features an additional signal filter to smooth the signal and prevent false alarms, readings of 0.2–0.6 volts are possible.
All gas detectors from Elektrotechnik Schabus are actually warning devices and not measuring devices, so such a deviation is acceptable and does not constitute an error. The values are displayed to identify trends and to determine whether, for example, ventilation measures are having the desired effect. ES gas detectors switch to the pre-alarm at 2 volts, if this is provided for in the device, and to the main alarm at 2.5 volts. Any relays that may be present then switch on, and built-in piezoelectric sensors emit loud audible warning tones. These thresholds correspond to approximately 2000 and 2500 ppm. These values are well below the limits at which they become dangerous to humans. For example, the maximum workplace concentration (MAK) in Germany for an exposure of 8 hours is 5000 ppm CO2. Therefore, a detector with the GX-DS sensor is an early warning system.
