Instrument Development:

Raman Optical Sensors

SSI's Raman Hydrogen Monitor

Raman Sensors Can Solve Many Difficult Gas Measurement Problems

When rapid, real-time measurement is needed for difficult-to-detect gases, consider a Raman optical sensor. Many gases, including hydrogen and nitrogen which are "homonuclear," cannot be monitored using infrared spectroscopy and may be difficult to detect selectively by other means. In particular, current catalytic sensors for hydrogen do not have the speed, sensitivity, accuracy, or ruggedness needed for many critical applications in aerospace and chemical processing.

Motivated by the safety assurance requirements of NASA and the US Air Force, Spectral Sciences has developed custom Raman gas monitors which feature optimum detectivity and time response (under 1 second), extremely wide dynamic range, and rugged packaging at an affordable price. As with optical detection methods in general, there are no tapes, chemicals, or other consumables, and minimal maintenance is required.

How Raman Sensors Work

Raman scattering is a versatile optical detection method for solids, liquids, or gases. Visible light from a laser is passed through the sample; a small portion of the light is scattered by each molecular species at a shifted wavelength (color). The size of the wavelength shift is specific to the species, and the quantity of scattered light is proportional to the concentration. Spectral Sciences' patented high-efficiency optical design collects the maximum light signal with the minimum laser power. This allows ppm-level gas measurements with an affordable, turn-key instrument package.

NASA'S Hydrogen Leak Problem

After the Challenger explosion, NASA needed a fast, reliable sensor for detecting hydrogen leaks during fueling and launch of the Space Shuttle. The requirements of 2-second response, high sensitivity and stability (200 ppmv), and background gas-independent calibration ruled out conventional catalytic sensors, while mass spectrometric systems were extremely costly. NASA turned to Spectral Sciences, which designed and built the HLMS Raman-based hydrogen monitoring system.

HLMS is a complete gas sampling and sensing prototype system for monitoring hydrogen leaks at the launch pad of the Shuttle--or in a rugged industrial environment. It includes zero and span calibration gas inlets, dual-range analog outputs, analog and logical output diagnostics, and both local and remote-controlled operation. Linearity is within 2% over the full range of 200 ppm to 40%.

Other Applications

Building on the HLMS design, Spectral Sciences recently developed a Raman Hydrogen Monitor for the Arnold Engineering Development Center's wind tunnel tests of the National Aerospace Plane (shown in the photograph). Here the additional requirements are 0.2-second response time and operation at sub-atmospheric pressure with hot, moist air.

In addition to hydrogen, a variety of gases can be measured either singly or simultaneously with our Raman sensors, including nitrogen, oxygen, water vapor, methane, carbon monoxide, carbon dioxide, and nitrogen dioxide.

Spectral Sciences' technical staff will be pleased to discuss with you how a Raman sensor might benefit your particular monitoring application, please contact Dr. Steven Adler-Golden at sag(at)spectral.com.