An electromagnetic measuring stick.

An electromagnetic measuring stick

The usual methods for measuring levels of contained liquids don't work well inside nuclear power plants; ordinary floats cannot withstand the heat and poisons inside the tanks, and methods that bounce ultrasonic signals to the top of the liquid are difficult to set up in a sealed, hostile environment. So scientists at Sandia National Laboratories in Albuquerque, N.M., have created a new liquid-level sensor that uses electromagnetic pulses.

The device sends an electromagnetic signal through two coaxial cables, one submersed in the liquid to be measured and another operating as a reference line. At the point where the first line reaches the liquid level, the signal automatically is reflected back to the source. The reference line is adjusted so that its signal bounces back from the exact same spot. Then, when the liquid level changes, the signals from the two lines differ, and the difference can be translated into measurements of the level change.

The device can measure changes as small as a millimeter, according to William Sullivan, one of the designers.

The sensor was designed to measure hot liquid sodium (about 1,475[deg.]F), which is used to cool the nuclear reactor core.

Knowing the level is critical, Sullivan says, because the core must be entirely submersed. "As a matter of fact," he says, "defective knowledge of the liquid level was one of the major contributors to [the accident at] Three Mile Island."