A device for monitoring shallow ground temperatures

Gonçalo Teles Vieira*,  Carla Mora* and  Miguel Ramos**

* Centro de Estudos Geográficos, Universidade de Lisboa, Faculdade de Letras, Alameda da Universidade, 1600-214 Lisboa, Portugal; E-mail: ; gtvieira@ceg.ul.pt / cmora@ceg.ul.pt

** Departamento de Física, Universidade de Alcalá de Henares, Facultad de Ciencias, Ctra Madrid-Barcelona, km 33,600, 28871 Alcalá de Henares (Madrid), Espanha; E-mail: miguel.ramos@uah.es

Introduction

In marginal periglacial areas like the medium altitude Mediterranean mountains, the High altitude Tropical mountains or the Subantarctic Islands, a daily frost regime with shallow penetration of the freezing front in the ground controls most of the geocryological processes. Many geomorphological features are originated by shallow frost action in the ground, like miniature sorted nets and stripes, needle-ice raked ground, nubbins, shallow solifluction lobes, etc. Monitoring the ground temperatures at shallow depths contributes to characterize and better understand these marginal processes. In this note we describe the design and construction of a low-cost device used for that purpose in the Serra da Estrela range (Portugal).

View of the data logger after modification of the thermistor setting. The arrow indicates the original position of the thermistor. In the picture it is possible to see the wires and a mechanical protection made of “melted plastic glue”.

Design and construction of the device

The device is based on miniature data loggers from Gemini. In order to minimize costs, Tiny Talk II® without protection cases were chosen. This model also has the advantage of a small size (34 mm of diameter and 52 mm of length). A detailed description of Gemini data loggers can be found elsewhere in this field book or at www.geminidataloggers.com. Other companies provide similar data loggers, which may probably be used following this approach.

Arrangement of the 4 data loggers inside the IP68 case.

a) Data logger setting

Tiny Talk II data loggers are compact electronic devices, with one built-in NTC100 thermistor. Our design is based on 4 data loggers recording independently at 4 depths. The data loggers are protected inside a standard IP68 case (water and dust protection), which will be totally buried in the ground. This proved to be a very efficient protection from people and animals.

In order to obtain an optimal contact with the ground, we extracted the thermistors from the data loggers and mounted them on external electrical wires. The sensor must be detached from the data logger with special care in order to do not cause damage to the circuits and to the thermistor itself. It is however a very simple procedure made using a good soldering iron.

View of the device with the 4 external sensors.

b) Protection case

The 8 wires (each thermistor has two) pass to the exterior of the case through 8 small holes, which are sealed using epoxy glue, both on the inside and outside parts of the case. It is important that the wires have a circular section and that there is not a cotton infill between the metal and the plastic protection of the wire. That kind of material could easily deform the wire section, making it easier for moisture to migrate into the case.

The case that we used opens from above and is sealed with 4 screws. It proved very efficient and we never had problems with moisture. It is especially important in this kind of approach to use an IP68 case.

Case opening for data download in the field.

c) Sensors

For maximizing the contact with ground particles and to obtain an average value of the temperature at the same depth, the external thermistors were glued (epoxy) to high-diffusivity aluminium plates. In our study sites, near to the surface, the material is coarser and finer at depth. Therefore, the shallowest thermistor, which was buried 1 cm deep, was glued into a 15 x 15 x 0.5 cm aluminium plate, and the other three (5, 10 and 15 cm) were glued into very thin aluminium sheets with 2 x 2 cm.

d) Field setting

In the field, the case is buried together with the sensors. The case should stay at an intermediate level between the lower and the upper sensor, in order to minimize the possible effect of heat conduction through the sensors wires. Depending on the ground texture and climate regime, frost-heave may disturb the position of the sensors. Either their position needs to be checked frequently, or better, they can be fixed to some kind of anchoring device fixed deeper in the ground. In our experiment the sensors did not show detectable movement.

For the data transfer, the case must be opened (but not necessarily removed from the ground) to connect the RS232 cable from the computer to the data loggers. Our experience shows that it is difficult to clean the case properly before opening it, especially when the ground is moist. We therefore suggest covering the case with a plastic bag, before burying it, in such a way that it can be easily opened and kept clean, without the need to remove it from the ground.

Example of data collected at the Cântaro Gordo ridge (Serra da Estrela, Portugal).

Final note

Prices, capacity and models of data loggers available in the market are becoming more competitive as time passes, and simpler systems are for sure, becoming more frequently available at reasonable prices. Therefore, the intention of this note is only to contribute with some ideas for those who are interested in field monitoring of periglacial processes.

Acknowledgments

The data loggers were constructed and used in the framework of the Project “ESTRELA – Geomorphological and biophysical processes and landscape units in Mediterranean mountains. The case-study of Serra da Estrela” (PRAXIS/P/CTE/11153/98) – http://www.ceg.ul.pt/proj_estrela, funded by the Fundação para a Ciência e a Tecnologia.

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