Mid Sweden University

This paper presents a lab test system for studying pressure dependence of NDIR gas sensors in accuracy demanding applications. The test system consists of a hardware test bench, user software (LabVIEW based) and a calculation algorithm. The lab test bench provides highly accurate test environment for collecting characterization data. The software and the calculation algorithm process the data to derive accurate compensation parameters for each specific design of NDIR sensors. The compensation parameters are used to normalize the measured concentration to the values valid for standard atmospheric pressure.

Measurements of atmospheric gas concentrations using of NDIR gas sensors requires compensation of ambient pressure variations to achieve reliable result. The extensively used general correction method is based on collecting data for varying pressures for a single reference concentration. This one-dimensional compensation approach is valid for measurements carried out in gas concentrations close to reference concentration but will introduce significant errors for concentrations further away from the calibration point. For applications, requiring high accuracy, collecting, and storing calibration data at several reference concentrations can reduce the error. However, this method will cause higher demands on memory capacity and computational power, which is problematic for cost sensitive applications. We present here an advanced, but practical, algorithm for compensation of environmental pressure variations for relatively low-cost/high resolution NDIR systems. The algorithm consists of a two-dimensional compensation procedure, which widens the valid pressure and concentrations range but with a minimal need to store calibration data, compared to the general one-dimensional compensation method based on a single reference concentration. The implementation of the presented two-dimensional algorithm was verified at two independent concentrations. The results show a reduction in the compensation error from 5.1% and 7.3%, for the one-dimensional method, to −0.02% and 0.83% for the two-dimensional algorithm. In addition, the presented two-dimensional algorithm only requires calibration in four reference gases and the storing of four sets of polynomial coefficients used for calculations. 

A multispectral nondispersive infrared (NDIR) sensor was developed for simultaneous detection of methane and water vapor in air. The NDIR sensor is capable of measuring optic transmission in the CH4 absorption spectra at 3.375 μm and the H2O absorption spectra at 2.7 μm. Data from a third channel, 3.95 μm, is used as reference value for 'zero-level' calibration. The actual CH4 concentration is retrieved by adjusting the data obtained in the CH4 spectra with respect to the concentration sensed in the H2O spectra. A calibration procedure was developed and tested, which involves matching of the absorbed light energy in the CH4 and the H2O spectrum in humid reference environments. A compensation algorithm for elimination of humidity impact was developed and validated in environments with variable CH4 and H2O concentrations. By implementing the multispectral approach, and the developed algorithm, an uncertainty of 15-25 ppm relative the reference concentrations was achieved. For a concentration range valid for environmental monitoring applications this should be compared to an uncertainty of 180-200 ppm for the non-corrected CH4 concentration. 

A selective algorithm for nondispersive infrared (NDIR) sensing of gases with overlapping absorption spectra was developed and evaluated in modified multichannel NDIR sensor. Measurements in the optic channel with the spectral band where two gas species (target and secondary gas) have overlapping absorption lines are complemented by additional measurements in second channel where spectral absorption for only one gas (secondary gas) is present. The real concentration for the target gas is retrieved by adjusting the absorption data obtained in the overlapping gas spectra's optic channel, with respect to the absorption data retrieved in the second optic channel that has sensitivity only for the secondary gas. An implementation example is performed by obtaining the true concentration of CH4 (as target gas) in a mixture with H2O vapor. The channel for the target gas is equipped by an optic filter with spectra at 3.375 μm where both CH4 and H2O have absorption lines. The complementary second channel provides sensing in spectra at 2.7 μm where only H2O have absorption. Data from a third channel, at 3.95 μm, is used as reference value for 'zero-level' calibration. A calibration procedure was developed and tested, which involves matching of the absorbed light energy in target and secondary channels in humid reference environments. A selective algorithm for sensing of CH4 with elimination of spectral impact from H2O was validated in environments with variable CH4 and H2O concentrations. By implementing the multispectral approach and the developed algorithm, an uncertainties of 5-10 ppm relative the reference concentrations were achieved. For the environments where selective algorithm was validated this should be compared to an uncertainty of 70-90 ppm for the non-corrected CH4 concentration. 

A system for the measurement of methane concentration in water is presented. The system is a stand- alone device using a high-resolution NDIR (Non-Dispersive Infra-Red) gas sensor. The NDIR sensor is configured to measure methane, water vapor, and carbon dioxide in the air. It is mounted in a housing with a stabilized environment and includes cross-sensitivity compensation. An equilibrator is used to transfer the methane concentration from the water into a circulating gas flow that is analyzed by the NDIR gas sensor. The equilibrator consists of a vertical plastic tube filled with 2,000 glass marbles, where the water runs from top to bottom on the surface of the glass marbles, in contact with a circulating air flow, exchanging gas. The system is stand- alone, including power supply and logging features for 72 hours of operation. The system performance was evaluated in a field test, measuring the methane content of seawater at a fiber bank in Sundsvall, Sweden. This fiber bank consists of remaining waste from an old paper industry from before 1970 and is known to produce methane. The detection limit of the tested system is below 1.4 nmol/L in water, corresponding to 1 ppm methane concentration in the air. The settling time of the system in its current configuration, including the equilibrator and gas sensor housing, is 30 minutes.