NEW MEMS METER CAN GO WITH THE (LOW) FLOW

FAYETTEVILLE, Ark. - Microelectromechanical systems (MEMS) are too small to be seen with the naked eye, but one could save your life today. The microscopic devices are rapidly replacing the bulky electronic assemblies used to inflate air bags in automobiles. But as products become smaller, the tools used to monitor and regulate them must also become smaller. University of Arkansas researcher Steve Tung has collaborated with Umachines to fabricate and test a MEMS low-flow meter that can accurately monitor gas flow in very small systems.

Tung, assistant professor of mechanical engineering, worked with Edward Chiu of Umachines and graduate students Jason Sheppard and Jason Clendenin. He presented their results yesterday at the 2002 American Society of Mechanical Engineers International Mechanical Engineering Congress in New Orleans.

Most devices or industrial processes that require gas or other fluid to move from one place to another are designed for it to flow at a specific rate. If the flow rate is too high or too low, the process does not work correctly. This can result in a range of problems from decreased efficiency to serious damage. To prevent these situations, the flow rate is monitored by a flow meter.

Of the several types of flow meters, two - the differential pressure flow meter and the thermal flow meter - can produce accurate measurement at a very small size. But each of these flow meters has limitations, and the problems get larger as the size of the device gets smaller. The researchers first explored which flow meter was more accurate at the micro scale.

"We incorporated both pressure and thermal sensors into a flow meter so that the two designs could be tested side-by-side under identical flow conditions," explained Tung.

The researchers designed and fabricated a multi-sensor chip that has three sensor clusters spaced evenly apart. Each cluster has three different MEMS sensors designed to measure the temperature, pressure and surface shear stress of the gas flow on the surface of the sensor chip. The multi-sensor chip was incorporated into double-sided printed circuit board so that it formed the bottom of the channel through which the gas flowed.

The resulting flow meter was tested in a temperature-controlled low-flow calibration facility using nitrogen as the fluid medium. Flow rates were varied incrementally from 0.5 to 10 standard cubic centimeters per minute. Temperature was adjusted to different levels from 30 to 50 C and pressure was increased successively from 0 to 35 pounds per square inch.

Results from each of the three types of sensors were analyzed. The researchers were surprised to find that the shear stress sensors were more accurate than the pressure sensors and compared well with the temperature sensors.

"Based on the calibration results, we determined that the shear stress sensors in the flow meter could provide a more accurate and repeatable measurement than the pressure sensors in a small gas flow situation," Tung said.