Final Report: Portable Methane Flux Meter

EPA Contract Number: 68D99069
Title: Portable Methane Flux Meter
Investigators: Hovde, David Christian
Small Business: Southwest Sciences Inc.
EPA Contact:
Phase: I
Project Period: September 1, 1999 through March 1, 2000
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (1999) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , SBIR - Air Pollution , Small Business Innovation Research (SBIR)

Description:

This Phase I SBIR project investigated an approach to achieving a low power, portable system for measuring trace gas concentrations and fluxes. Improved instrumentation for measuring methane gas flux is needed both for atmospheric greenhouse gas research and for natural gas pipeline maintenance. Our approach uses diode lasers of the type developed for fiber optic communications, together with an advanced modulation method that can be implemented with low cost electronics.

The near infrared diode lasers at the heart of the instrument are small, rugged devices that operate at room temperature. These lasers run for years without maintenance. By combining them with spectroscopic modulation techniques, it is possible to measure trace gas concentrations in the parts-per-billion range with excellent specificity and without the need for consumable chemicals.

To determine gas fluxes from concentration measurements, a micro-meteorological technique called eddy correlation is used. Eddy correlation takes advantage of the turbulent mixing processes in the atmosphere to determine the flux. The measurement requires a rapid detector for the vertical wind speed and the trace gas concentration. Eddy correlation does not disturb the vegetation in any way.

Summary/Accomplishments (Outputs/Outcomes):

During the Phase I research, a computer model of the modulation and detection process was developed, the model was validated with laboratory experiments, the modulation waveform was optimized, and the sensitivity was estimated for detecting fluxes of methane, carbon dioxide, water vapor, ammonia, and nitrous oxide.

The computer model accurately predicted the line shapes for the demodulated methane signals, including distortions introduced by amplitude modulation.

The modeling effort led to two discoveries, both of which enhance the sensitivity of the method. Both discoveries then were verified in the lab. One discovery involved a simple method to reduce the effects of laser amplitude modulation on the detected signal. Amplitude modulation can otherwise lead to noise when trace gas concentrations are extracted from the data. The second invention was a method to reduce the effects of etalon fringes in the baseline. Usually such fringes limit the accuracy and sensitivity of laser instruments for detection trace gases. Southwest Sciences has a patent pending covering part of this technology, and plans to extend its intellectual property protection to cover these additional discoveries.

The modulation waveform could be generated using low cost electronics. A simple commercial platform was shown to be capable of operating for over a million cycles of the modulation. It provided flexibility and accuracy.

Excellent sensitivity was demonstrated in the lab for detecting methane. The modulation method yielded signals with a high signal/noise ratio. High sensitivity is needed for measuring concentrations and fluxes of trace gases. Sensitivity in the parts per million range was demonstrated with an 8 cm optical path. Higher sensitivity can be achieved by increasing the optical path length.

Conclusions:

The sensitivity that was attained in Phase I is sufficient, when scaled to a longer optical path length, for measurement of fluxes of methane, ammonia, carbon dioxide and water vapor at levels of interest to the scientific research community. We have previously demonstrated the use of long optical paths in field measurements. The combination of low costs for the electronics with improved sensitivity developed in this project will open up new markets for laser gas sensors.

In addition to the atmospheric research market, the natural gas pipeline industry requires instruments for monitoring the leak-tightness of the vast national network of delivery pipes and pumping stations. Existing instruments used by the industry are slow and require consumable chemicals. As a result, their low capital cost is offset by high recurring costs both to operate and to maintain the instruments. Our approach should have comparable capital costs and much lower recurring costs. A particular advantage of our instrument is that the response speed can be improved to permit detection of leaks at normal driving speeds.

Supplemental Keywords:

Greenhouse gas detection, flux measurement, CH4, CO2, H2O, N2O, NH3., Scientific Discipline, Air, Toxics, Ecosystem Protection/Environmental Exposure & Risk, air toxics, Chemistry, HAPS, Monitoring/Modeling, Engineering, Engineering, Chemistry, & Physics, monitoring, air pollutants, field portable systems, pollutant monitoring, detect, field monitoring, air pollution, methane, analyzer, continuous emissions monitoring, measurement