When 'clean' isn't working, try an infrared gas sensor

Greensboro, NC -EPA regulations require that all passenger cars manufactured after 1999 be equipped with on-board refueling vapor recovery (ORVR) systems. The ORVR cleans vapors in the car's fuel tank by eliminating many of the gaseous hydrocarbons through a carbon filter. While this may sound like a great idea for the environment, it creates a problem at the pump.

Cars outfitted with an ORVR canister emit so few hydrocarbons that the VaporVac(R), a common vapor recovery system used by many gas stations, draws only air from a motorist's fuel tank. When this clean air is returned underground, the gasoline in the storage tank evaporates at a higher rate than it would with hydrocarbon-saturated vapor. Since the volume of the tank is fixed, this creates a great deal of pressure in the underground unit. The tank vents this pressure through the tank chimney, releasing hydrocarbons into the atmosphere, thereby defeating the purpose of the ORVR.

Engineers at Marconi Commerce Systems, manufacturers of VaporVac, decided they needed a more intelligent vapor recovery system-one that could determine clean air from "dirty" and respond accordingly. So they developed a non-dispersive infrared (NDIR) spectroscopic gas sensor that can be retrofitted into existing gasoline hoses. When hydrocarbons are present, the system acts normally. It creates a vacuum using a motor-driven vapor pump that pulls exiting vapors through the intake holes near the nozzle tip and directs them to the underground storage tank. When the air from a fuel tank is clean, meaning hydrocarbons are absent, the sensor signals the vacuum pump to shut down.

"Our lead engineer, Edward Payne, scoured the universe looking for a sensor to do what we needed it to do," says Steve Robertson, Vapor Recovery product manager at Marconi. "We looked at various optical sensors, but these couldn't withstand the harsh environment." Payne decided to try NDIR, which does not need to come into direct contact with the surroundings.

Most gases have a unique infrared absorption signature in the 2 to 14 micron region, says Brian Kinkade, vice president at Ion Optics. The uniqueness of each gas spectra or wavelength enables a sensor to identify chemicals in liquid and gas phase mixtures.

A simple NDIR sensor consists of an IR light source, a sample compartment of known optical length, an optical filter, and an IR detector with its associated electronics. The IR emitter produces broad band IR illumination. Spectral filters restrict the view of the IR detector to the desired wavelength.

One of the more formidable challenges for Marconi engineers was to meet the Underwriter Laboratories' (UL) requirements for flammable liquid dispensers. To help meet this, they choose the pulsIR from Ion Optics (Waltham, MA) as their IR source. "Prior to the pulsIR, an incandescently hot glow bar source was required, effectively precluding any possibility of UL certification due to the high temperatures involved," says Edward Payne of Marconi.

To ensure the pulsIR's cool operation, Ion Optic engineers treated the surface with ion milling. This creates random surface texture of sub-micron scale rods and cones, says Kinkade. This "tubeworm" texture modifies the reflection and absorption spectra relative to that for a flat filament of the same material. The high emissivity enables it to efficiently and rapidly cool via thermal radiation, while still providing significant in-band illumination over a desired waveband in the 2 to 20 micron region.

Marconi engineers also employed a dual-channel pyroelectric detector manufactured by Eltec Instruments, which contains integral precision bandpass filters-one at 3.4 microns whose wavelength is absorbed by the hydrogen-carbon bond, and a second at 4.0 microns which serves as a reference channel. As the target gas passes through the sampling chamber, the ratio of 3.4 micron and 4.0 micron energy is measured, yielding the concentration. "The combination of the Ion Optics source and Eltec Instruments detector greatly simplifies the design of an accurate and reliable NDIR measurement system," says Payne of Marconi.

Other applications for NDIRs include natural gas leak detection, non-invasive blood glucose monitoring, home indoor air quality, and personal/portable air quality monitors.

"We hope to get California Air Resources Board certification this fall," says Robertson.