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Thermal needle probe measurement

Measuring ground conductivity for horizontal ground loops

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A thermal needle probe can be used to measures the thermal conductivity of the ground at shallow depths where a horizontal ground collector is proposed.
The thermal needle probe is inserted into the soil at the usual depth of the ground collectors - typically around 1 metre - having prepared a narrow hole with an auger.  Once a stabilised ground temperature has been measured heat is introduced in to the probe.  The temperature increase over a period of time is measured and together with the power input this is used to calculate the thermal conductivity of the soil.  A number of readings are taken in each hole and appropriately averaged ignoring any readings with a large standard deviation.
On most sites readings would be taken at a number of different locations within the area proposed for the ground collector. Having obtained a number of determinations of conductivity on a site, statistical techniques such as geometric mean would then be used to provide a value of bulk conductivity representative of the site as a whole.
The conductivity value can then be used on smaller schemes to size ground collectors using published tables.  On larger schemes the conductivity is further modelled in ground loop design modelling software together with the heating load profile of the building to determine the amount of ground collector pipe required.
A different technique called a thermal response test is used for measuring conductivity at depth for boreholes.
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