Condensation as a microclimate process: measurement, numerical simulation and prediction in the Glowworm Cave, New Zealand

Abstract

The study examines condensation as a microclimate process. It focuses first on finding a reliable method for measuring condensation and then on testing a numerical model for predicting condensation rates. The study site is the Glowworm Cave, a heavily used tourist cave in New Zealand. Preservation of the cave and its management as a sustainable tourist resource are high priorities. Here, as in other caves, condensation in carbon dioxide enriched air can lead to corrosion of calcite features. Specially constructed electronic sensors for measuring on-going condensation, as well as evaporation of the condensate, are tested. Measurements of condensation made over a year are used to test a physical model of condensation in the cave defined as a function of the vapour gradient between the cave air and condensation surface and a convection transfer coefficient. The results show that the amount and rate of condensation can be accurately measured and predicted. Air exchange with the outside can increase or decrease condensation rates, but the results show that the convection transfer coefficient remains constant. Temporal patterns of condensation in the cave are identified, as well as factors that influence these. Short-term and longer-term temporal variations of condensation rates are observed and patterns explained. Seasonal changes are large, with higher condensation rates occurring in the warmer months and lower rates during the cooler months. It is shown that controlling air exchange between the cave and the outside can influence condensation. This and other aspects of cave management are discussed.

Studies of condensation/evaporation processes in the Glowworm Cave, New Zealand

“…Moreover, the sensitivity of C to changes in airflow is small compared to the effect of changes in other variables shown in equation (6). De Freitas & Schmekal (2003) confirm this and show empirically that k v = 3.7 g m -2 s -1 in equation (1) fits well with observations of C regardless of the location within the cave and gives good results regardless of season. A summary of results are shown in Table 1 and Fig.…”

“…The drawback is that this method does not measure the flux of moisture. By overcoming this, the approach was adapted for use in a cave environment by de Freitas & Schmekal (2003) who devised a novel method for measuring this exchange of moisture to and from a surface using what they called "condensation sensors". They are simple to construct and their size can be customised so it is possible to install them on uneven surfaces such as a cave wall.…”

“…To do this the sensor are weighed when dry and the conductivity reading set at zero. Using an atomiser, very fine drops of water were sprayed onto the sensor in stages and weighed at each step (de Freitas & Schmekal, 2003). The sensors showed no influence of ambient temperature over the range tested (10 to 20°C).…”

“…A large part of the problem is difficulties in measuring condensation in order to study it. Recently, however, de Freitas & Schmekal (2003) devised a reliable method for measuring condensation and evaporation as part of a single continuous process of water vapour flux. The aim here is to report on this research to cave scientists, specifically on the method for measuring condensation rates on cave rock surfaces.…”

A qualitative assessment of microclimatic perturbations in a tunnel

“…An understanding of microclimate dynamics in underground cavities is important for applications ranging from the design of waste disposal facilities to the preservation of artifacts or fauna in caves (De Freitas and Schmekal, 2003). In such environments, which typically have high humidity and contain partially saturated rocks, the microclimate is dominated by advection of heat and moisture (De Freitas and Littlejohn, 1987), with the thermal state being defined by the geothermal flux, thermal diffusion from the surface, water circulation, and air exchange with the outside atmosphere (Crouzeix et al, 2003).…”