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Gas Chromatograph (LGC)

NSSDCA ID: 1978-078D-04

Mission Name: Pioneer Venus Large Probe
Principal Investigator:Mr. Vance I. Oyama

Description

The objective of this experiment was to determine the composition of Venus' lower atmosphere. From these measurements, deductions were made of the gaseous sources of infrared opacity, the degree of differentiation of Venus' interior, the degree of similarity between the solid bodies of Earth and Venus, and evolution of Venus' atmosphere. Two gas chromatograph columns were used to analyze samples of the atmosphere during probe descent.

The Gas Chromatograph sampled the lower atmosphere three times during its descent. It was constrained to take the three samples within 20 minutes, so the sample acquisitions were done about 10 minutes apart, and could not be done in the lower 20 km or so of the atmosphere because of the extreme temperature. An inlet port would open, allowing the atmospheric sample in, where it was swept by a stream of helium gas into two chromatograph column assemblies, a long column and a short column. The instrument occupied 0.03 cubic meters, had a mass of 6.3 kg, and used 42 W.

The long column assembly consisted of two 1585-cm long packed columns bifilarly wound (wound in parallel in a spiral shape) on a cylindrical aluminum housing with the detector located inside the column housing. The columns were made of tubing with 1.1-mm internal diameter. A kapton heater was bonded to the housing, and phase change n-hexadecane material was in a shell surrounding the column to keep the temperature at 18.3 degrees C. Each column was packed with a polystyrene (Porapak N) 100/120 mesh, after flowing through the columns the gas flowed into thermal conductivity detectors housed inside the aluminum housing. The atmospheric constituents were identified by the time it took each to flow through the columns. The long columns were used for gases with masses between those of neon and carbon dioxide.

The short column assembly consisting of a pair of bifilarly wound 213-cm columns (also tubes with 1.1 mm internal diameter) that contained a mixture of polymer spheres (80% polydivinyl benzene, 20% ethylvinylbenzene) in an 180/220 mesh, kept at an operating temperature of 62.0 degrees C by kapton wire heaters. The short column was set up and operated similarly to the long column assembly. These short columns were used for gases in the mass range from carbon dioxide to sulfur dioxide.

Prior to entry into the Venusian atmosphere the columns were primed with 1% by volume 02 and SO2 in helium, followed by a helium purge. The first sample was collected 9 minutes, 5 seconds after entry, at an altitude of 51.6 km. The second sample was taken at 18:35 at 41.7 km and third at 29:05 at 21.6 km. Just before the third sample, A mixture of 10% Freon 14, 10% Freon 22, and 80% helium was introduced as a calibration reference.

The columns were not purged before the second and third sample collections. Some constituents of the atmosphere could have had retention times in excess of 10 min, these could be distinguished by the peak broadening caused by diffusion in the gas phase. Analysis and transmission continued all the way to the surface.

A reference gas was passed by one thermistor while the reference gas plus the unknown sample was passed by the other. A difference amplifier monitored the thermistor voltage difference. An A/D converter handled signals from 0 to 5 V with a 2.4-mV resolution and operates at 262 kHz. A buffer memory was capable of storing 2048 bits of data. The data transmitted are the thermistor voltage differences. Seven 8-bit words were returned every 2 seconds, a total of about 12000 8-bit words.

Alternate Names

  • LGC
  • PioneerVenusLargeProbe/LGC
  • urn:nasa:pds:context:instrument:pvmp.lp.lgc

Facts in Brief

Mass: 3 kg
Power (avg): 4.6 W

Funding Agency

  • NASA-Office of Space Science (United States)

Discipline

  • Planetary Science: Atmospheres

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. James B. PollackOther InvestigatorNASA Ames Research Center
Mr. Glen C. CarleOther InvestigatorNASA Ames Research Center
Dr. Fritz H. WoellerOther InvestigatorNASA Ames Research Center
Mr. Vance I. OyamaPrincipal InvestigatorNASA Ames Research Center

Selected References

  • Oyama, V. I., et al., Pioneer Venus sounder probe gas chromatograph, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 85-92, doi:10.1109/TGRS.1980.350287, Jan. 1980.
  • Oyama, V. I., et al., Venus lower atmosphere composition: Analysis by gas chromatography, Science, 203, No. 4382, 802-805, doi:10.1126/science.203.4382.802, Feb. 1979.
  • Oyama, V. I., et al., Pioneer Venus gas chromatography of the lower atmosphere of Venus, J. Geophys. Res., 85, No. A13, 7891-7902, doi:10.1029/JA085iA13p07891, Dec. 1980.
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