Abstract
Abstract. A popular method for measuring the thermal conductivity of solid materials is the transient hot needle method. It allows the thermal conductivity of a solid or granular material to be evaluated simply by combining a temperature measurement with a well-defined electrical current flowing through a resistance wire enclosed in a long and thin needle. Standard laboratory sensors that are typically used in laboratory work consist of very thin steel needles with a large length-to-diameter ratio. This type of needle is convenient since it is mathematically easy to derive the thermal conductivity of a soft granular material from a simple temperature measurement. However, such a geometry often results in a mechanically weak sensor, which can bend or fail when inserted into a material that is harder than expected. For deploying such a sensor on a planetary surface, with often unknown soil properties, it is necessary to construct more rugged sensors. These requirements can lead to a design which differs substantially from the ideal geometry, and additional care must be taken in the calibration and data analysis. In this paper we present the performance of a prototype thermal conductivity sensor designed for planetary missions. The thermal conductivity of a suite of solid and granular materials was measured both by a standard needle sensor and by several customized sensors with non-ideal geometry. We thus obtained a calibration curve for the non-ideal sensors. The theory describing the temperature response of a sensor with such unfavorable length-to-diameter ratio is complicated and highly nonlinear. However, our measurements reveal that over a wide range of thermal conductivities there is an almost linear relationship between the result obtained by the standard sensor and the result derived from the customized, non-ideal sensors. This allows for the measurement of thermal conductivity values for harder soils, which are not easily accessible when using standard needle sensors.
Highlights
Thermal conductivity is one of the key parameters required for modeling the thermal evolution of a planetary body and the interaction between the solid surface and subsurface layethrsereanedxistht emaettmhoodsspthoedriectearnmSdinorealdtihidaetrimvEeaalepnravtrhiarmonemteernsto. fWa shuilreface layer by remote measurements, e.g. by analyzing the irradiation emitted from the surface, these methods usually demand “ground truth” measurements that have to be performed inside the material, i.e. by an in situ method to allow for proper sert a long eavnadlutahtiinoTnn.eheTedhleeCsinimrtoyptolheesst mpwahatyeertiroael dtoo this is to inbe measured and to heat this needle with a constant electrical power for a specified time
According to the classical hot needle theory, the temperature response of a needle inside a material which is heated by a constant power consists of two parts: an initial nonlinear phase which depends on conductivity and heat capacity and later on a phase where the temperature versus logarithm of time graph rises linearly and the inclination of the graph depends on heat conductivity only
Rather we show them here to illustrate the good repeatability of the performed measurements and the fact that both of the two ruggedized sensors give consistent www.geosci-instrum-method-data-syst.net/2/151/2013/
Summary
Discuss.: 3 September 2012 Revised: 13 March 2013 – Accepted: 22 March 2013 – Published: 12 April 2013
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