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| Titel |
Continuous multichannel monitoring of cave air carbon dioxide using a pumped non-dispersive infrared analyser |
| VerfasserIn |
D. Mattey |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070963
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| Zusammenfassung |
| The concentration of CO2 in cave air is one of the main controls on the rate of degassing of
dripwater and on the kinetics of calcite precipitation forming speleothem deposits.
Measurements of cave air CO2reveal great complexity in the spatial distribution among
interconnected cave chambers and temporal changes on synoptic to seasonal time scales.
The rock of Gibraltar hosts a large number of caves distributed over a 300 meter
range in altitude and monthly sampling and analysis of air and water combined
with continuous logging of temperature, humidity and drip discharge rates since
2004 reveals the importance of density-driven seasonal ventilation which drives
large-scale advection of CO2-rich air though the cave systems. Since 2008 we have
deployed automatic CO2 monitoring systems that regularly sample cave air from
up to 8 locations distributed laterally and vertically in St Michaels Cave located
near the top of the rock at 275m asl and Ragged Staff Cave located in the heart of
the rock near sea level. The logging system is controlled by a Campbell Scientific
CR1000 programmable datalogger which controls an 8 port manifold connected to
sampling lines leading to different parts of the cave over a distance of up to 250
meters. The manifold is pumped at a rate of 5l per minute drawing air through
6mm or 8mm id polythene tubing via a 1m Nafion loop to reduce humidity to local
ambient conditions. The outlet of the primary pump leads to an open split which is
sampled by a second low flow pump which delivers air at 100ml/minute to a Licor 820
CO2 analyser. The software selects the port to be sampled, flushes the line for 2
minutes and CO2 analysed as a set of 5 measurements averaged over 10 second
intervals. The system then switches to the next port and when complete shuts down to
conserve power after using 20 watts over a 30 minute period of analysis. In the
absence of local mains power (eg from the show cave lighting system) two 12v
car batteries will power the system for analysis at 4h intervals for about 1 month.
Two logging systems sampling cave air from 13 locations over a vertical range of
275m have run continuously for up to 5 years and return a very detailed picture of
cave ventilation patterns and their responses to local weather and seasonal change. |
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