Power Electronics

Soda-straw-like Tubes Solve Sensing Problems

Sending weak beams of light through inexpensive glass tubes that resemble soda straws, Sandia National Laboratories researcher Jonathan Weiss can inexpensively solve problems ranging from the migration of waste through a landfill to detecting when an automobile battery soon will be too weak to start a car.

Similar sensors also could tell oil field operators when to stop pumping oil from their tanks before the pumps pick up water that accompanies oil from the ground. The oil/water interface sensor is the subject of a pending Sandia patent application and a research agreement with Custom Electronics, an electronics company in upstate New York. The company is partnering with Sandia, a National Nuclear Security Administration (NNSA) lab, to develop a prototype device from the current bench-top demonstration.

The car battery solution awaits a visionary entrepreneur to put this cheap, safe, patented solution in the hands of the public. A turkey-baster-like device inserted into a popped-open port has been the traditional way for a driver to test the amount of acid in a car battery (and possibly splash sulfuric acid on his or her fingertips). Weiss’s simple invention requires no direct human intervention under the hood. His procedure: Factory-inject sulfuric acid or even, possibly, sugared water into a clear glass tube smaller than a soda straw and immerse the tube in the battery’s acid. Glass is inert in acid and should have ample longevity, he says.

Next, using LEDs, send light through the tube and measure the amount that returns. The amount of light that stays in the tube depends on the refractive index of the surrounding solution. If the refractive indices are identical, light would just as soon escape from the sides of the tube as stay within it. That is the case when the tube is filled with sulfuric acid at maximum charge. The refractive index is at first the same as that of the battery acid surrounding it (1.38). But over time, the battery’s acid weakens and becomes more like water (1.33). Its lessening refractive index attracts less light from the tube. The exchange rate, in a manner of speaking, is worsening and light remaining in the tube (as reflected by a tiny piece of metal placed at the tube’s far end) increases.

A simple solid-state light detector—a photodiode—at the tube’s near end registers more light, therefore, as the battery deteriorates. The detector easily could be wired to activate a dashboard alarm light similar to ones that notify a driver that a seat belt is unclasped.

Sugar water inserted in glass tubing also works well, Weiss says, because the refractive index of water can be adjusted upward by dissolving sugar in it. “Quite a substantial change can be produced, far exceeding that needed for this application,” he says.

While the glass of the tubing does have an effect on light leakage, says Weiss, “The liquid core and liquid cladding are dominant.” The tube is a millimeter in diameter, 2-in. to 3-in. long and inexpensive: 200 tubes set Weiss back $10 for his experiments. Mass production would drive costs down even lower.

Measuring battery deterioration will become increasingly important as more hybrid electric/gas vehicles, with their high reliance on batteries, take to the highways, he says. Another possible use for the device is for cheap, continual monitoring of battery banks maintained by local phone companies. The batteries are used for back-up power to keep home phones working when wall-current electricity fails due to an outage.

For more information, visit www.sandia.gov/news-center/news-releases/2005/optics-lasers/lightsimple.html.

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