NDIR Technology fastLine sensors
Measurement principle
NDIR stands for None-Diffractive Infrared and is a photometric, non-invasive method of measuring gas concentrations.
NDIR technology measures the absorption of infrared (IR) light by a gas in a specific optical spectrum. It make use of the physical property of spectral absorption of the gas: Each gas only absorbs light in certain spectral ranges, otherwise gases are transparent.
The higher the gas concentration in the measuring cell, the greater the absorption of the IR light in the beam path through the measuring cell [1].i
Spectral infrared absorption of gases
Many gases absorb light in the infrared range between 2 and 20 µm wavelength, each gas in its own typical spectral range [1]. The absorption spectrum is like a fingerprint for a gas. The absorption spectra of various gases in the infrared range are shown as in an overview diagram below [2].
In detail, the gas molecules absorb the energy from the infrared spectrum by changing their vibrational and rotational movements. However, the transition from one rotational state to the next is quantised, i.e. the rotation of the molecule changes ‘ jump-like’ [2][3]. This is clearly visible as absorption lines in the light spectrum on the receiving side of the measuring cell.
As an example of the detailed absorption spectrum of carbon monoxide (CO) in the spectral range of 4.6 µm at low pressure.ell.
Measurement channels, single-beam and dual-beam technique and measurement cell
The gas is measured in a measuring cell (cuvette). With fast gas sensors, a gas stream flows through the measuring cell, whereas with fastLine sensors the gas is typically extracted as a side-stream by a pump and pushed through the cell (side-stream sensors). The IR light source radiates through the optical measuring path filled with gas in the measuring cell to the IR receiver. This measures the received light intensity in one, two or more spectral ranges, whereby the spectral ranges are defined by optical filters.
With dual-beam sensors, a reference beam is placed in a spectrum in which no gas absorbs light in the measuring cell. This reference beam can therefore be used as a zero reference level. Sensors with dual-beam technology are stable over the long term and can largely compensate for ageing effects of the IR components and some optical changes.
Sensors with single-beam techique do not have a reference beam, but typically have a higher sensitivity at the expense of long-term stability.
Customized measurement cells of fastLine sensors
There are different goSense measuring cells depending on the gas to be measured. The respective length of the optical path is optimised for the gas concentration to be measured.
Each cell can be used for single, dual or multi-beam techniques.
Sources
[1] Jacob Y. Wong, Roy L. Anderson, „Non-Dispersive Infrared Gas Measurement“, Int. Frequency Sensor Association Publishing, 2012, ISBN-13: 978–84-615–97321‑1
[2] I. E. Gordon, L. S. Rothman, R. J. Hargreaves, R. Hashemi, E. V. Karlovets, F. M. Skinner, et al., „The HITRAN2020 molecular spectroscopic database“, J. Quant. Spectrosc. Radiat. Transfer 277, 107949 (2022). [doi:10.1016/j.jqsrt.2021.107949]
[3] Planck, Max. „Zur Theorie des Gesetzes der Energieverteilung im Normalspectrum“. Vorgetragen in der Sitzung vom 14. December 1900. JA Barth, 1900.