ABB’s Sensi+ NG is a compact natural gas contaminant analyzer based on a unique tunable diode laser (TDLAS) technology known as Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS). The technology accurately, reliably and simultaneously measures corrosive substances such as hydrogen sulfide, carbon dioxide, water and oxygen in real time in complex and time-varying natural gas streams.
While many installations need to identify oxygen upsets above thresholds of 1000 ppm, some have thresholds much lower than that. Sensi+ NG is uniquely capable of sensitive measurements across both ranges – in the single-digit ppm range as well as at higher concentrations.
This standout capability is due to the fact that OA-ICOS is a cavity-enhanced absorption spectroscopy (CEAS) technique. It features laser light that is reflected back and forth between highly reflective mirrors and absorbed by contaminant gases over kilometers of effective path length within a physically small gas cell. For a given amount of contaminant gas, the absorption strength scales with the effective path length, which is a function of the mirror reflectivity and the amount of light absorbed by gases. As the concentration of a contaminant gas increases in the stream, it absorbs more light, thereby reducing the effective path length.
This relationship between the amount of contaminant, the effective path length and the absorption strength means that the measurable range is truly dynamic, resulting in a much larger range than is possible with traditional TDLAS analyzers that have fixed path lengths. As a result, Sensi+ NG is capable of high dynamic-range measurements without compromising performance at the low end of the range.
As an example of the high dynamic range, Sensi+ NG measured natural gas mixtures containing up to 4800 ppm oxygen with large step changes, as well as mixtures with less than 20 ppm oxygen with step-change magnitudes near the specified detection limit. To generate the mixtures, oxygen was blended with methane at various ratios using mass flow controllers by varying the relative flow rates of each gas. The resulting mixture was then sampled by the Sensi+ NG analyzer and compared to the set point.
For the high range, 4800 ppm oxygen was diluted with methane. The set point first decreased from 4000 ppm to 0 in 400 ppm step changes every 7 minutes, then increased using the same steps. Next, the set point alternated between 800 ppm and 4000 ppm five times. Finally, each condition was measured for 15 minutes.
Sensi+ NG responded quickly and accurately to each set-point step change. Over this range, the precision of the measurement was much smaller than the step-change magnitude as shown in Figure 1. The average measured value of each step is compared to the set point in Figure 2, showing that the measurement is linear and accurate within 2% of the set point for this high range.
01 Measured oxygen in natural gas at discrete set points between 0 and 4800 ppm, where the oxygen set point was defined by the flow rates of the mass flow controllers.
02 Comparison of the average oxygen concentration measured at each step to the set point. The right axis shows the same data plotted as the percent difference between the measured values and the set points.
For the low range, 200 ppm oxygen was diluted with methane. In this test, the set point varied from 0 to 20 ppm with step changes ranging from 2 ppm to 20 ppm, both increasing and decreasing. In this case, the step changes were within an order of magnitude of the instrument repeatability, as seen in Figure 3, where the random variations within a step are clearly visible in contrast to the higher range of Figure 1.
This illustrates typical measurements near the limit of detection where Sensi+ NG still responds quickly and accurately to each set-point step change. During the zero step, the mass flow controller for the oxygen source was closed. When it opened for the next step, there was an initial increase in oxygen before it stabilized at the desired level based on the set flow rates. The Sensi+NG measurement was able to capture this upset quickly, as well as track the stabilization to the set-point value.
03 Measured oxygen in natural gas at discrete set points near the limit of detection between 0 and 20 ppm, where the oxygen set point was defined by the flow rates of the mass flow controllers. The spike after the 0 ppm step reflects a transient increase that occurred after opening the mass flow controller for the gas containing oxygen.
Figure 4 shows the average measured value of each step compared to the set point, demonstrating that the measurement remains linear and accurate in this low range. Note that to display the differences using range appropriate units, the right axis of the high-range dataset in Figure 2 shows the difference as a percent of the measurement, whereas the right axis of the low-range dataset in Figure 4 shows the difference as an absolute value in ppm.
These results show that while there is some uncertainty in this range due to measurement repeatability, single-digit ppm oxygen steps can still be clearly differentiated.
04 Comparison of the average oxygen concentration measured at each step to the set point. The right axis shows the same data plotted as the absolute difference between the measured values and the set points.
Sensi+ NG delivers reliable oxygen measurements across the entire range of contaminant detection needs – from trace ppm to high concentrations – ensuring that operations remain compliant and responsive to changing specifications. With its ability to monitor levels and detect upsets in real time, Sensi+ NG provides reliable performance as industry standards continue to tighten.
