Heat Flow Analysis for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 503

There is no need for a calibration constant if the heat
input to the needle is well controlled. The calibration con-
stant remains a constant value and the thermal needle probe
can be used to measure the thermal conductivity of other
materials, λ , through use of the following equation from
ASTM (2008b) D5334-08:

Temperature monitoring
element

Thermal

Thermal
readout
unit

Heater wires

C n Q
4 πS

λ = (10.33)

Constant
current
source

where:
Q

(a)

= heat input, W/m,

C n = calibration factor, and

Heater element no. 26 manganin
or nichrome no. 30 gauge wire

S t = slope of the temperature change versus natural log-

arithm of elapsed time.

Thermocouple
Jack

Copper no. 30 (0.25 mm)

Figure 10.19 shows typical results of temperature change
versus the natural logarithm of elapsed time, the slope of
which is the variable S t . It is also possible to use the mea-
surement of the drop in temperature as the needle probe is
allowed to cool when the application of the constant current
is terminated.

Constantan no. 30 (0.25 mm)

CU CN

Bakelite head

5-12 V DC heat source, 1 A

Epoxy Filled

10.5.2 Measurement of Volumetric Heat Capacity
and Specific Heat of Soils
Heat capacity controls the amount of temperature change
that will occur as heat is absorbed or released from a mate-
rial. It is possible to measure the specific heat of the con-
stituents of a soil as well as the volumetric heat capacity of
the combined soil-water system. It is possible to estimate
the volumetric heat capacity of a soil-water system if the
specific heat of each of the constituents of a soil is known.
Specific heat is usually defined with respect to mass while
volumetric heat capacity is defined with respect to volume.
It is quite common to refer to the specific heat of each of the
constituents of a mixture while referring to the volumetric
heat capacity of a mixture.

Hypodermic tubing
1.8 mm (0.072 in.)
1.4 mm (0.054 in.)

50 mm
(2 in.)

Thermocouple junction

100 mm
(4 in.)

Epoxy tip

(b)

Figure 10.18 Details of needle probe used to measure thermal
conductivity of soils: (a) thermal probe setup; (b) components of
needle probe (ASTM D5334-08, 2008b).

25.60

Heating data
y = 0.0124 x + 25.548

probe can be calibrated against a material with a known
thermal conductivity such as water (i.e., thermal conduc-
tivity of 0.607 W/m/ ◦ Kat25 ◦ C) or glycerine (i.e., thermal
conductivity of 0.292 W/m/ ◦ K). The calibration constant for
the needle, C n , can be calculated as follows:

25.59

25.58

λ material
λ measured

C n = (10.32)

25.57

where:

25.56

1.5 2 2.5 3 3.5

λ material = thermal conductivity of the known material

and

In (elapsed time), s

λ measured = thermal conductivity measured with the needle

probe on the calibration material.

Figure 10.19 Data obtained during heating cycle with thermal

needle probe.