Functionality NDIR sensor
Description of the mode of operation for the technically interested
An NDIR sensor is a non-dispersive infrared sensor consisting of an IR diode as light source, an optical wavelength filter and an IR detector. Ambient air diffuses through a membrane into a chamber. Each gas has its own wavelength in which it optimally absorbs light in the infrared spectrum. The extent is measured electro-optically, providing a reference for the concentration present. The optical filter ensures that only the narrow spectrum of interest is evaluated. A pure air mixture is enclosed in a second chamber.
Now the same measurement is performed alternately in both chambers at a low pulse frequency of just under 1 Hz. External influences such as temperatures affect both chambers and are thus compensated for as far as possible, since 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. Here a first filtering takes place, which checks the signals for plausibility and smoothes strong fast, possibly unusual signal changes and replaces them with intermediate values. The voltage values read in are now related to a concentration that can be represented in ppm according to an internal reference and output as a pulse width modulated continuous signal.
By means of an analog low-pass filter, the pulses are converted back into a modulated DC voltage, whereby approximately 1 mV per ppm is achieved here. Fresh air contains approx. 400 ppm CO2, so the sensor should output approx. 0.4 volts when the air is fresh, which is about what appears in the display of the GX-A1 warning device. Since the GX-A1 in particular has another signal filter for smoothing and avoiding false alarms, displays of 0.2 - 0.6 volts are possible.
All gas detectors from Elektrotechnik Schabus are in fact warning devices and not measuring devices, therefore such a deviation is okay and does not represent a fault. The values are displayed to show a tendency to determine whether, for example, ventilation measures achieve the desired effect. Gas detectors from ES switch from 2 volts to the pre-alarm, if this is provided for in the device, and from 2.5 volts to the main alarm. Here also possibly existing relays switch and inserted Piezotöner emit strong acoustic warning tones. These thresholds correspond to about 2000 and 2500 ppm. These values are far below the limits at which it becomes dangerous for humans. For example, the maximum workplace concentration MAK in Germany is 5000 ppm CO2 for an exposure of 8 hours. Thus, a warning device with the sensor GX-DS is an early warning system.
