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New calibrator measures flow rate of corrosive gases
OAK RIDGE, Tenn.,
Oct. 9, 1995
Measuring corrosive and toxic gas flow rates once took days, but a new calibrator developed by the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) needs only a few minutes to weigh corrosive gases accumulating at volume rates as low as a few cubic centimeters per minute.
The Gravimetric Gas Flow Calibrator is a new type of gas flow calibration device that measures how fast any type and quantity of gas is flowing through gas flow meters, said Carl Remenyik, the ORNL researcher who developed the device. By measuring the flow of gas in a minute, scientists can ensure that the gases, no matter how small an amount, are being discharged in precise doses during semiconductor manufacturing processes.
"The calibrator is significantly more accurate than other volumetric devices, many of which can operate only with non-reactive gases," said Remenyik, a member of the Instrumentation and Controls Division. "Strongly reactive gases eat away at most metals, but the calibrator is made of a stainless steel that will resist attacks. It will operate with almost any type of gas or vapor and its accuracy is not affected by chemical reactions, condensation or adsorption taking place inside."
"Accurately weighing small quantities of flowing gas is a very difficult thing to do," Remenyik said. Most methods determine the weight of gas by measuring it on a balance that requires a long time to collect samples or by deriving it from equations relating temperature, volume and pressure. Measuring temperature, volume and pressure are indirect mass measurement methods that will have some errors that accumulate in the calculations.
"Because this calibrator does not depend upon calculation of the gas density from pressure and temperature measurements, it is fundamentally more accurate than volumetric devices," Remenyik said.
Also, balances that can measure 50- to 100- pound objects, the weight range of most gas containers, cannot accurately measure a few tenths of an ounce of gas inside the container. Scientists usually must spend hours or days collecting gas heavy enough to be weighed. Chemical reactions and adsorption significantly reduce the accuracy of indirect methods that derive the gas weight.
In the semiconductor industry, high-quality wafer chips require accuracy in the control of gas flow. Dozens of gases in small doses are used to manufacture semiconductor chips. The chips may be as small as the tip of a finger and are found in electronic devices, such as computers, televisions and radios.
The calibrator, which won an R&D 100 Award from R&D Magazine this year, uses a patented technique to measure the weight of gas. From a load cell balance, scientists suspend an empty vessel submerged in water to balance its weight by buoyancy. While submerged, the 50-pound steel container feels like one-tenth of an ounce. The container seems lighter because water pushes against objects, causing them to float like a log floats in the river. With a lighter calibration container, scientists can use a load cell sensitive enough to measure fractions of one-thousandth of an ounce. The flow rate is then determined by subtracting the weight of the empty, submerged calibration container from the weight of a calibrator filled with gas for one minute.
"The key to accuracy is the type of balance used to measure weight," Remenyik said. "Balances are made for a particular range of weight. A balance for weighing hundreds of pounds will not accurately weigh a few ten-thousandths of an ounce of a gas. This is why our technique is important. With water taking away the bulk of weight, we can use the best balance available for accurate small measurements."
The calibration process begins and ends with the push of a button. In the first push, a valve seals off the normal path and reroutes the gas to an empty calibrator for a specific time interval, usually one minute. With the second push, the valve closes again and the gas continues to flow through the regular path. The test sample is then measured by the load cell. In the final push, the test sample is moved to an incinerator, where it is burned to harmless waste.
In addition to the semiconductor industry, gas flow calibrators can be used in standard laboratories and in other branches of the industry where gas flow controllers and meters are used. Funding for the gas flow calibrator was provided by ORNL's Metrology Program and the semiconductor industry.
ORNL, one of DOE's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.