Non-dispersive infrared sensor keeps pollution down at the pumps

By: Laurie Toupin

Contents
The problem
How it works
IR source

Vapor recovery systems on today's gas pumps have to be smarter than your average vacuum pump. So Marconi Commerce Systems (Greensboro, North Carolina) developed a non-dispersive infrared (NDIR) spectroscopic gas sensor that can be retrofitted into existing gasoline hoses. At the heart of the sensor lies the foil filament pulsIR light source from Waltham, Massachusetts based Ion Optics (see Figure 1).

Today, in order to eliminate hydrocarbon emissions while refueling, most pumps are equipped with a device that draws the exiting vapors from the car's fuel tank and directs them into the underground gasoline storage unit. One of the more common systems that customers see is the VaporVac. As a person refuels, a vacuum created by a motor-driven vapor pump pulls exiting vapors through the intake holes near the nozzle tip. The dislocated gaseous hydrocarbons flow up the hose and into the underground gasoline storage tank. Electronics control the speed of the vapor pump to ensure that the vapor travels to the storage tank at a rate proportional to the flow of fuel.

The problem(Back to Top)
A hitch arose when EPA required 100% of all model-year 2000 passenger cars to come equipped with on-board refueling vapor recovery (ORVR) systems. This device 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 recovery system draws only air. 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.

To offset this reaction, Marconi developed a sensor to detect when the air is "clean" and when it is "dirty." When hydrocarbons are present, the system acts normally. When absent, the sensor signals the vacuum pump to shut down.

How it works(Back to Top)
Marconi initially looked at various optical sensors but these couldn't withstand the harsh environment, says Steve Robertson, Vapor Recovery Product Manager at Marconi.. Then they decided to develop a non-dispersive infrared (NDIR) spectroscopic gas sensor, which does not need to come into direct contact with the environment they are testing.

Most gases have a unique infrared absorption signature in the 2 to 14 m region, says Brian Kinkade, vice president of marketing and sales at Ion Optics. This uniqueness of each gas spectra enables conclusive identification of 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.

In this case, engineers employed a dual-channel pyroelectric detector manufactured by Eltec Instruments, which contains integral precision bandpass filters--one at 3.4 m whose wavelength is absorbed by the hydrogen-carbon bond, and a second at 4.0 m which serves as a reference channel. As the target gas passes through the sampling chamber, the ratio of 3.4 µm and 4.0 µm 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 Lead Engineer Edward Payne of Marconi.

IR source(Back to Top)
The pulsIR from Ion Optics provides the IR source in the Marconi gas sensor system. Made from a thin foil filament, the pulsIR achieves greater than 82% intensity modulation for pulse rates under 2 Hz. This represents 700C of temperature modulation.

To improve the filament's infrared emission, engineers treat the surface with ion milling which creates random surface texture of sub-micron scale rods and cones (see Figure 2). This "tubeworm" texture modifies the reflection and absorption spectra relative to that for a flat filament of the same material.

The high emissivity of the ion-beam-treated radiator surface 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 m region. This design therefore allows the pulsIR to operate with package temperatures below the ignition levels of gasoline vapors, an important design consideration for Marconi Commerce Systems.

For wavelengths small compared to the feature sizes, the tube-like surface scatters most incoming light, therefore it has low reflectivity and the filaments appear visibly black. By Kirchoff's law, it must also have high emissivity, greater than 80%. For wavelengths long compared to the feature sizes, the surface still looks like flat metal and it therefore has low emissivity, characteristic of the flat metal (£ 0.1).

"We are going to test the device this September and hope to get California Air Resources Board (CARB) certification at that time," says Robertson. CARB is the national standard for vapor recovery systems. Once certified, the Marconi system will be retrofitted into existing gasoline service pump stations by breaking the vapor return retention line on either side of the gas pump. The company will also build new pumps already equipped with the IR sensor. "This is only the first in a series of requirements that will be coming on line for vapor recovery at gasoline pumps," says Robertson.

About the author…(Back to Top)
Laurie Toupin is a freelance writer based in Massachusetts.