Spreading salt on roadways is still the most cost-effective tool for preventing death, injury, and property damage from winter traffic accidents around the world.
In 2011, highway departments in the U.S. alone spread 19.6 million tons (1.8 billion kilograms) of salt on icy roads. They pay anywhere from $37 to $130 a ton for rock salt, so the total price tag is in the $1 billion to $2 billion range. But that’s just the beginning of the cost. Overall, for every dollar spent on the salt itself, there may be three dollars in damage—to roads, vehicles, and the environment, as salt eats away at bridge decks, corrodes rebar, blights neighboring vegetation, and drains off into surface and groundwater.
Spreaders lay down salt repeatedly during the icy season. Sometimes they may throw down more rock salt on areas that already carry far more than the optimum 15 grams per square meter; at other times, they may not drop enough. The Salt Institute says that roads departments could cut their salt requirements in half through careful controls.
Accurate, real-time measurement of the amount of residual salt on the pavement is vital to maintaining safety and reducing both direct costs and collateral mayhem. There are already sensors that collect water splashed back by a salt truck’s wheels and measure the changes in refractive index that accompany increasing salinity. These systems, first described a decade ago, work well…if the road is wet.
In the current issue of Sensors and Actuators B: Chemical, engineers at Spain’s Universidad Carlos III de Madrid demonstrate a simple, robust optical sensor that can strap onto a bumper and measure dry-pavement salt levels as the truck drives over dry pavement.
Salt glows orange in black light, and the brightness of the glow is proportional to the amount of salt present. More precisely, when illuminated at 273 nanometers, sodium chloride fluoresces at 610 nm (and also, though somewhat less usefully, at 310 nm).
The instrument the researchers describe includes an ultraviolet LED (much lower-powered than the lasers required by some other methods) and a photoreceptor, some filters for both emitter and receiver to tighten the ranges of stimulus and response, a power supply, A/D and D/A converters, a field programmable gate array (FPGA) to control the components and analyze the results, and a power supply that can modulate the LED output—all in a compact, rugged three-board package.
The researchers focused on salt’s 610 nm emission band: the wavelength is far enough away from the excitation wavelength to reduce backscatter effects, and this wavelength avoids the fluorescence of some of the biological material that collects on roads.
Engineers Marta Ruiz-Llata, Pedro Martín-Mateos, José R. López, and Pablo Acedo used a modulated UV LED to help them discriminate between reflections (from the UV and environmental sources) and the 610 nm fluorescence. It’s a balancing act: the slower the emitter fluctuation, the brighter the fluorescence, but the lower the ability to sort the fluorescence signal from reflections. (Fluorescence does decline very, very slightly with increasing temperature—but the change was small compared to other sources of variation, especially the naturally non-uniform distribution of salt broadcast over a hard surface.)
They adopted a 10 Hz excitation cycle (at this rate, the fluorescence persists for about 14 milliseconds). Tests showed that the device accurately measures the concentration of salt to within 10 percent over the critical range from 0 to 20 grams per square meter.
