(Illustration: Mark Knorr)
by Jacques Gordon
When the first Ford Taurus reached the showroom, I was working as a lab tech at a company that makes catalytic converters. The job included driving cars on a chassis dyno while tailpipe samples were sent through a bank of gas analyzers known as a Horiba gas bench. The analyzers, 3 sets of five, fifteen in all, were mounted in six-foot-tall 19-inch rack cabinets and connected by yards of tubing to a multi-bag sample collection system. More yards of wiring connected the analyzers to a signal processor and data collection computer, and the whole system was housed in a semi climate-controlled room. It was state-of-the-art in 1986.
Today a gas analyzer and signal processor can be built as one assembly about the size of a box of crackers. Even so, the scientific principles behind them remain the same.
One of the most common types of gas analyzer utilizes on Non Dispersive Infrared (NDIR) detection. Infrared (IR) detectors are widely used in consumer products, scientific instruments and professional tools, including the non-contact thermometer in your toolbox. IR energy radiates in a frequency range below visible light (longer wavelength), and different materials absorb or radiate IR energy at different frequencies. Those frequencies are very specific, so that means we can build IR detectors to measure the concentration of one specific gas, such as the amount of R-134a refrigerant in 10 cubic centimeters.
An infrared gas analyzer has a sample chamber with an infrared Light Emitting Diode (LED) on one end and a detecting diode at the other. An optical filter tuned to the frequency of the gas we’re looking for is mounted in front of one of the diodes. In a Non Dispersive analyzer, the filter is placed in front of the detecting diode, so only IR light at the frequency of the gas we’re measuring reaches the detector. When a gas sample passes through the chamber, the IR light absorbed at that one frequency is absorbed by the gas. This reduces the amount of IR light reaching the detector and therefore reduces the detector’s output signal by the percentage of the gas in the sample.
While the concept is simple, it requires a high degree of precision to get repeatable readings. Still, modern electronics make it possible to build a reliable NDIR refrigerant identifier in a self-contained hand device.
Another common type of gas analyzer relies on chemiluminescence, which literally means ‘a chemical reaction that produces light.’ In the automotive world, this is used for measuring oxides of nitrogen called NOx (the small ‘x’ represents a variable number of oxygen atoms in the molecule). Normally nitrogen won’t chemically combine with anything, but in the extreme heat and pressure of a combustion chamber, it will bond with oxygen to form nitric oxide (NO). When NO is mixed with ozone (O3), it will react spontaneously to become O2 and NO2, and that reaction also produces tiny amounts of infrared light. Measuring the amount of IR light emitted provides a very reliable measurement of NO in the sample.
In most NOx analyzers, the light is measured with a photo multiplier tube (PMT), a type of vacuum tube that was originally developed for television cameras. The analyzer also has an ozone generator, a closed chamber that creates ozone with a device similar to a spark plug. Again, thanks to modern electronics, today’s chemiluminescent analyzers are small enough to fit on a shop cart, and they’re fast, sensitive and more reliable than the somewhat finicky rack-mounted version I used way back when.