Differential Diode Laser Sensor for High-Purity Oxygen

A compact portable sensor for determining the purity of oxygen concentrations near 100 percent is proposed based on differential absorption of two beams from a diode laser. One beam passes through a cell containing the sample of gas to be analyzed and the second beam passes through a reference cell containing a known concentration of high-purity oxygen. An autobalanced detection system will be used for measuring the difference in photocurrents of the transmitted beams. Common mode noise such as laser intensity noise will be rejected to a high degree. The system should not be subject to drift because it would be possible to lock the laser wavelength to the oxygen line using the reference cell. We estimate that the proposed sensor concept should enable an accuracy of 0.05 percent to be achieved with a cell length of less than 10 cm. The sensor can be made rugged with a small footprint using microelectronics for laser control and signal processing. The proposed effort will test the feasibility of this sensor concept by seeking to demonstrate measurements of the desired accuracy using a breadboard system.

A compact portable high-purity sensor for oxygen would be useful to gas suppliers for verifying purity in remote locations such as storehouses, delivery vehicles, and onsite at customer locations without having to bring the sample to the lab. High purity oxygen applications that could benefit include academic and governmental research, industrial cutting and welding, and municipal wastewater treatment. The sensor can serve as a complementary method to more complex laboratory analyses to check the purity of oxygen sources in the field. We will market commercial versions of the sensor to gas supply companies, research labs, and anywhere high purity oxygen is used. The sensor technique can also be adapted to other gases that have near infrared absorption lines, such as NH3, CH4, and CO2, which may find use in research and process quality control.

NASA applications of the proposed sensor include on-orbit monitoring of oxygen from storage tanks to determine long-term structural stability as affected by chemical attack of tank and seal materials, monitoring of oxygen streams in wastewater aeration processes, and monitoring for leaks in oxygen storage systems. In addition, the sensor may find use in land based NASA activities such as monitoring of oxygen during purification processes and testing of oxygen purity at remote locations.

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