A meteorological station inside a cave for CO2 monitoring

 

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Motivation

Karstic origin caves are very heterogeneous environments with complex hydrological and atmospheric processes. Water plays a crucial role braking the limestone base, eroding cracks and transporting CO2 which after crystallization form the well know structures of this kind of caves. The main source of CO2 in this environment is related with the interaction of water with the organic matter present in soil. The concentration of CO2 vary spatially and temporally inside the cave which is significantly higher than in the outer atmosphere.

From a thermal point of view, caves are also complex environments. Temperature is fairly constant inside but shows little fluctuations related with the outside daily cycle. The analysis of the dynamic relationships between these two can give valuable climatic signals.

Rain is an excellent indicator of climate variability. Rainfall monitoring outside the cave along with drops measurements inside the cave might also give valuable information about the climate fluctuations occurred in the past and fixed in the cave through its formations.

Objective

The objective of this project was to obtain reliable measurements of rain, temperature and relative humidity outside a karstic cave. Along with this outside meteorological station the objective was to obtain scientific quality measurements of temperature, humidity and CO2 concentrations inside the cave.

Methodology

The outer station was solved using a multiparameter Vaisala probe WXT520 connected to a CR800 Campbell Scientific data logger. The advantage of using such sensor is price along with reliability and compact design. The location of this cave is very popular and receives visitors, so vandalism was minimized. Communications are solved with GPRS as gateway using dynamic IP and interMET own DNS.

Regarding measurements inside the cave, a portable solution was made using a rugged suitcase for easy relocation if wanted. Logging was solved with a Campbell Scientific CR1000 connected to a Vaisala GMP343 CO2 probe, using RS232 protocol for communications since logger and probe were some meters distant. For temperature and humidity we first used a Vaisala HMP155 with fast external temperature sensor. The probe is heated to avoid condensation/evaporation interferences.

For powering the whole system a UPS using gel batteries and a 220/12 V charger was installed. This is to prevent punctual power failures and over voltage.

Regarding the siting criteria:

  • Maximum representativity: well mixed CO2 concentrations, away from micro-atmospheres and interference (like humans visiting the cave)
  • Minimum impact: hiding cables, probes, cabinets since the cave is regularly visited, minimum civil work
  • Regarding installation: access, minimum risks for installation and maintenance.

Results

The outside weather station has been operative for years, with excellent data completeness. The cave weather station results have been also excellent. Power and data logging systems performed as expected and CO2 sensor has been re calibrated in order to guarantee accuracy. An long time series on variables and parameters related with CO2 is now available for research.

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