GAS CYLINDER FILLING SCALE FROM SETRA ENABLES NASA TO SAFELY PREPARE ACCURATE GAS MIXTURES FOR SHUTTLE EXPERIMENTSWhen Space Shuttle flight STS-83 is launched into space on April 4, 1997, it will carry NASA's Spacelab module and a crew of seven for a scientific mission of up to 18 days. Inside the Spacelab will be a number of experiments for studying topics such as crystal growth, orbital acceleration, and combustion. A special Combustion Module (CM-1) is being constructed to help scientists gain an improved understanding of combustion and the mechanisms that cause flames to propagate in low-gravity environments.
As part of the CM-1's SOFBALL (Structure of Flame Balls at Low Lewis Number) experiment, scientists on the ground will have to prepare a number of highly-accurate gas mixtures. In space, the mixtures will be ignited and burned in the Microgravity Science Lab. To study the combustion process, the resulting flame propagation for various mixtures will be monitored using imaging, temperature, pressure, and radiant emission measurements.
The Right Mix
To prepare mixtures -- consisting of combustible hydrogen (or methane), oxygen, and a diluent such as nitrogen or carbon dioxide -- on the ground, NASA needed a scale that offered high-accuracy weighing of gases and could operate in an explosion-proof environment. High accuracy is needed to prepare 15 unique mixtures (e.g. 4% fuel, 20% oxygen, 76% nitrogen) within an accuracy of up to 2 percent.
At the same time, depending on the amount of hydrogen in the test sample, the mixtures can be very dangerous if not handled properly. That's why NASA could not acquire and transport "ready-mixed" supplier-prepared mixtures from off-site. It's also why NASA ultimately targeted a solution that could meet strict explosion-proof requirements.
According to Beth Roncace, ground-support engineer at NASA's Lewis Research Center in Cleveland, the experiment team first tried partial pressure techniques to prepare mixtures, but discovered that this option was not accurate enough. At this point, the team looked at a number of vendors who could supply high-accuracy gas filling scales. After careful analysis of several systems and a competitive bidding process, the team chose the GF 100 gas cylinder filling scale from Setra Systems, a leading developer of electronic weighing instruments.
The GF 100 incorporates Setra's patented ceramic weight sensor, a unique design that applies variable capacitance technology to achieve outstanding performance and reliability -- at a reasonable cost. The scale has a capacity of 100,000 grams and a sensitivity of 0.2 grams throughout its entire range. It also meets all of NASA's explosion-proof requirements.
The GF 100 features a keyboard that provides selectable levels of readability and display update settings. The two-piece design consists of a stainless steel base and a separate controller with an LED display and keyboard. A 10 foot cable connects each piece. With its advanced sensor, the Model GF 100 can accurately weigh gases with a nonlinearity of only ±0.5 grams, which decreases to less than ±0.2 grams when less than 25,000 grams of gas is added.
A Balanced Solution
With the GF 100, NASA is now able to safely and cost-effectively prepare accurate gas mixtures for the SMALL experiment. In operation, a 44 liter "K bottle" gas cylinder is positioned on the scale, and then pulled to vacuum using a vacuum pump. After the scale is zeroed, the bottle is filled with a purge gas to remove remaining contaminants. After being emptied, the scale is tared again, and the cylinder is ready to prepare a mixture.
To prepare a hydrogen/oxygen/nitrogen mixture, the cylinder is first filled with a pre-mixed nitrogen/hydrogen mixture to the proper weight. The scale may be tared at this point. Then, to complete the mixture, the cylinder is filled with oxygen to the total desired weight. To reduce vibrations that could cause rocking of the cylinder (and measurement errors), the scale's base and bottle being filled are shielded from air currents by an aluminum enclosure.
Currently, in laboratory tests, NASA scientists have been able to achieve 3.3 percent accuracy mixtures, and are working toward their goal of 2 percent accuracy. According to Roncace, "The GF 100 works really well in this application. In fact, the project scientist liked the scales so much, she purchased more for another project."
An edited version of this was seen in SENSORS, October 1995.
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