A meteorological station inside a cave for CO2 monitoring
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 that after crystallization form the well know structures of this kind of caves. The main source of CO2 in this environments is related with the interacting of water and the organic matter present in soil. The concentration of CO2 vary spatially and temporally inside the cave and are significantly higher than in outer atmosphere.
From a thermal point of view, caves are also complex environments. Temperature is fairly constant inside but also suffers of certain fluctuations related with fluctuations in the outside. The analysis of the dynamic relantionships between these two can give climatic signal very valuable.
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.
The objective of this project was to obtain reliable measurements of rain, temperature and relative humidity outside a karstic cave. Along with this outside regular meteorological station it was necessary to measure temperature, humidity and CO2 concentrations inside the cave.
The outer station was solved with a multiparametric Vaisla probe WXT520. The advantage of using this sensor was price along with reliability and compactness considering this was a very visited area with some risk of vandalism. Communications are solved with GPRS as gateway using dynamic IP and our own DNS.
Regarding inside cave, the client wanted a portable solution that they could transport if decided. Datalogger and SAI was integrated inside a ruggered suitcase with weels. 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. This probe was heated and this effect take out of the measurements. Condentation was a problem also for relliable temperature measurements.
For powering the unit we prepared a UPS using gel batteries and a Victron charger, since we had reliable 220VA power.
We had to take on the siting criteria, so we went for:
- Maximum representativity: well mixed CO2 concentrations, away from micro-atmospheres and interferences (like humans visiting the cave)
- Mínimum impact: hiding cables, probes, cabinets since the cave is regurlary visited, miminum civil work
- Regarding installation: access, minimum risks for installation and maintenance.
The whole system, outside and inside the cave monitoring stations, have been running without problems for years. It is soon yet for the client to come with scientific conclussions, but it will surely help in the knoledge of the dynamics of CO2 in caves and in the understanding of climate.