KD2 Pro 7 KD2 PRO THEORY
7 KD2 Pro Theory
Researchers have used transient line heat source methods to measure
thermal conductivity of porous materials for over 50 years. Typically
a probe for this measurement consists of a needle with a heater and
temperature sensor inside. A current passes through the heater and
the system monitors the temperature of the sensor over time. Analy-
sis of the sensor temperature determines thermal conductivity. More
recently the heater and temperature sensors have been placed in sep-
arate needles. In the dual probe the analysis of the temperature vs
time relationship for the separated probes yields information on dif-
fusivity and heat capacity as well as conductivity.
An ideal sensor has very small diameter and a length perhaps 100
times its diameter. It would be in intimate contact with the sur-
rounding material and would measure the temperature of the mate-
rial during heating and cooling. Ideally, the temperature and com-
position of the material in question would not change during the
measurement.
Real sensors fall short of these ideals in several ways. A sensor small
enough to be ideal would be too fragile for most applications. Mea-
surements in outdoor environments involve changing temperatures;
the ambient temperature generally is not constant. Heating moist,
unsaturated soil causes water to move away from the heat source,
thus altering the water content in the region of measurement, and
the hole made for the probe often disturbs the material around it
causing a contact resistance between the sensor and the material.
It is a challenge to design a sensor that gives accurate measurements
under all conditions. If the sensor is too small it is fragile, and the
contact resistance can be high in dry, porous materials. Large sensors
require a long heating time, but the long heating time drives water
away from the sensor and can cause free convection in liquid samples,
thus altering the reading. A high heating rate makes temperature
changes easier to read and less susceptible to temperature drift er-
rors, but results in water movement out of the measuring region and
free convection in liquids. We recommend long heating times to min-
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